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155 Cards in this Set
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Alternative versions of a gene that produce distinguishable phenotypic effects.
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Alleles
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An observable heritable feature.
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Character
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An organism that is heterozygous with respect to two genes of interest. All the offspring from a cross between parents doubly homozygous for different alleles are dihybrids. For example, parents of genotypes AABB and aabb produce a dihybrid of genotype AaBb.
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Dihybrid
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An allele that is fully expressed in the phenotype of a heterozygote.
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Dominant allele
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The first filial, or hybrid, offspring in a series of genetic crosses.
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F1 generation
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Offspring resulting from interbreeding of the hybrid F1 generation.
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F2 generation
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The genetic makeup, or set of alleles, of an organism.
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Genotype
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Having two different alleles for a given gene.
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Heterozygous
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Having two identical alleles for a given gene.
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Homozygous
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In genetics, the mating, or crossing, of two true-breeding varieties.
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Hybridization
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Mendel’s second law, stating that each pair of alleles segregates independently during gamete formation; applies when genes for two characters are located on different pairs of homologous chromosomes.
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Law of independent assortment
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Mendel’s first law, stating that each allele in a pair separates into a different gamete during gamete formation.
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Law of segregation
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An organism that is heterozygous with respect to a single gene of interest. All the offspring from a cross between parents homozygous for different alleles are monohybrids. For example, parents of genotypes AA and aa produce a monohybrid of genotype Aa.
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Monohybrid
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The parent individuals from which offspring are derived in studies of inheritance; P stands for parental.
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P generation
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The physical and physiological traits of an organism, that are determined by its genetic makeup.
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Phenotype
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A diagram used in the study of inheritance to show the results of random fertilization in genetic crosses.
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Punnett square
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An allele whose phenotypic effect is not observed in a heterozygote.
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Recessive allele
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Breeding of an organism of unknown genotype with a homozygous recessive individual to determine the unknown genotype. The ratio of phenotypes in the offspring determines the unknown genotype.
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Testcross
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Any detectable variation in a genetic character.
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Trait
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Referring to plants that produce offspring of the same variety when they self-pollinate.
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True-breeding
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The situation in which the phenotypes of both alleles are exhibited in the heterozygote.
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Codominance
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The situation in which the phenotypes of the heterozygote and dominant homozygote are indistinguishable.
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Complete dominance
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A type of gene interaction in which one gene alters the phenotypic effects of another gene that is independently inherited.
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Epistasis
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The situation in which the phenotype of heterozygotes is intermediate between the phenotypes of individuals homozygous for either allele.
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Incomplete dominance
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Referring to a phenotypic character that is influenced by multiple genes and environmental factors.
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Multifactorial
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The range of phenotypes produced by a single genotype, due to environmental influences.
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Norm of reaction
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The ability of a single gene to have multiple effects.
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Pleiotropy
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An additive effect of two or more gene loci on a single phenotypic character.
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Polygenic inheritance
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A heritable feature that varies continuously over a range rather than in an either-or fashion.
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Quantitative character
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A human genetic disease caused by a recessive allele for a dysfunctional enzyme, leading to accumulation of certain lipids in the brain. Seizures, blindness, and degeneration of motor and mental performance usually become manifest a few months after birth.
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Tay-Sachs disease
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A technique of prenatal diagnosis in which amniotic fluid, obtained by aspiration from a needle inserted into the uterus, is analyzed to detect certain genetic and congenital defects in the fetus.
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Amniocentesis
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In genetics, an individual who is heterozygous at a given genetic locus, with one normal allele and one potentially harmful recessive allele. The heterozygote is phenotypically normal for the character determined by the gene but can pass on the harmful allele to offspring.
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Carrier
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A technique of prenatal diagnosis in which a small sample of the fetal portion of the placenta is removed and analyzed to detect certain genetic and congenital defects in the fetus.
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Chorionic villus sampling (CVS)
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A human genetic disorder caused by a recessive allele for a chloride channel protein; characterized by an excessive secretion of mucus and consequent vulnerability to infection; fatal if untreated.
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Cystic fibrosis
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A human genetic disease caused by a dominant allele; characterized by uncontrollable body movements and degeneration of the nervous system; usually fatal 10 to 20 years after the onset of symptoms.
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Huntington’s disease
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A diagram of a family tree showing the occurrence of heritable characters in parents and offspring over multiple generations.
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Pedigree
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A human genetic disease caused by a recessive allele that results in the substitution of a single amino acid in the hemoglobin protein; characterized by deformed red blood cells that can lead to numerous symptoms.
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Sickle-cell disease
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How do independent assortment, crossing over, and random fertilization work to produce genetic variability?
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Independent assortment, crossing over, and random fertilization work to produce genetic variability by increasing the possibilities of variation exponentially.
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Charles Darwin published “Origin of Species” in 1859. What are the four observations and hypotheses that he put forward to explain the variation he observed among species? Specifically, how is the term natural selection related to these observations and hypotheses?
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A. Populations produce more offspring than the environment can support.
B. Individuals with characteristics best fit to an environment will survive over less fit individuals C.Favorable, heritable characteristics accumulate in a population over time D.This accumulation produces adaptation of populations to their environment Natural selection is related to this because one could say nature selects what traits are needed to survive. |
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1. There was no accepted scientific explanation for how characteristics could be inherited in 1859. Although he did not know it at the time, Gregor Mendel was discovering how characteristics were inherited. Who was Gregor Mendel and when did he publish his treatise on inheritance in plants? What organism did Mendel use for his genetic studies? What three advantages did Mendel have in his studies over other investigators?
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Gregor Mendel was an Austrian Monk and he published his treatise in 1865. Mendel used the pea plant for his genetic studies.
He had three advantages: 1.Controlled crosses in garden peas. 2.Studies traits with qualitative differences 3.Started with true breading individuals |
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What are stamens and carpels in flowers? Peas can self-fertilize, what does that mean? Describe how the structure of the pea flower allowed Mendel to control his experimental crosses.
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Stamens are the male part of the flower and carpels are the female part of the flower. Peas can self fertilize because they have both male and female parts in them. The pea flowers structure allowed Mendel to control his experiment because he could remove the male part and transfer it to another plant.
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Mendel deduced an explanation for the results of his studies of garden pea crosses. What are the features of Mendel's explanation? Using his logic, explain his results or crosses of plants with purple flowers with plants possessing white flowers.
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A.Traits are governed by 2 alleles one completely dominant of the other
B.Alleles segregate in formation of gametes C.For 2 or more traits alleles assort independently during gamete formation 3 to 1 ratio dominant over receding |
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Define the terms phenotype and genotype. Define and give examples of homozygous and heterozygous genotypes.
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Phenotype-physical appearance;
Genotype- genetic makeup; alleles in an individual Homozygous- genotype consist of duplicates of the dominant or recessive alleles (PP or pp) Heterozygous-genotype consists of 2 different alleles (Pp) |
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What are the three parts of Mendel’s explanation for inheritance we described in class (sometimes referred to as the law of segregation)? Using the pea flower color trait studied by Mendel (purple and white flowers) give the genotype of the parental (P), true-breeding plants with purple and with white flowers. What kinds of gametes would each parent produce? What is the phenotype and genotype of the F1 offspring? If 2 F1 offspring are crossed to produce an F2 generation, what kind of gametes would each parent produce? What are the possible phenotypes and genotypes of the resulting F2 progeny and in what proportion would they be expected to be produced?
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Mendel’s Laws of segregation:
A.Traits (cooled for by genes) are governed by 2 alleles, one completely dominant of the other B.Alleles segregate in formation of gametes C.For 2 or more traits. Alleles assort independently during gamete formation |
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What is a Test Cross? What is the purpose of a test cross. Illustrate the Test Cross by explaining how you would determine the genotype of the pea plant that produces purple flowers.
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A test cross is when you don’t know the genotype of one trait. The purpose is to determine the genotype of the individual with a dominant phenotype. If you performed a test cross with a purple and white flower and all the offspring are purple the purple parent was PP. If half are purple and half are white the purple parent was Pp.
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Tall pea plants are dominant to dwarf pea plants and green pod color is dominant to yellow pod color in pea plants (note this is the opposite of seed color, Table 14.1). True-breeding tall pea plants producing green pods are crossed with dwarf pea plants producing yellow pods. Two resulting F1 individuals are crossed to produce an F2 generation. What are the possible phenotypes of the F1 and F2 generations?
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F2-1:2:1, CRCR: CRCW: CWCW
See... notes |
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You are given a tall pea plant that produces green pods but you do not know if it is true-breeding or not. How would you determine the genotype of the plant (see question 4)? Explain your answer
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I would determine the genotype by performing a test cross withy a true breeding recessive trait plant. That way I would be able to see from my results if the original plant was true breeding or not.
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In a trihybrid cross (3 traits), E is dominant to e, F is dominant to f and G is dominant to g. Calculate the phenotypic and genotypic probabilities for each trait in a cross of EeFfGg X EeFfGg? Considering all traits together, what will be the probability for producing offspring that have a dominant phenotype for each trait? What will be the probability for producing offspring that have the genotype EeFfGg?
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¾ x ¾ x ¾ = 27/64
½ x ½ x ½ = 1/8 (8/64) |
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Distinguish among Mendel’s dominance, incomplete dominance and codominance. A cross between snap dragon plants with red flowers and white flowers (P generation) produces pink flowers (F1 generation). Give the genotypes of the plants from these two generations. Two F1 individuals are then crossed. What gametes will be formed by the F1 individuals? What will be the phenotypes and genotypes of F2 individuals?
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Mendel’s dominance- there is one dominant trait
Incomplete dominance- heterozygous is intermediate between homozygous phenotypes Codominance- more than one type of allele is dominant |
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What are the alleles that control A-B blood groups in humans? Which alleles are dominant and which are recessive? What are the different possible phenotypes for these blood groups? What genotypes produce these phenotypes? What controls compatibility for transfusions among different blood groups? How does the control of A-B blood groups relate to the genetic term multiple alleles?
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The alleles are Ia,Ib, i. Ia and Ib are dominant and i is recessive. The phenotypes are A (IaIa / Iai), B ( IbIb / Ibi), AB (IaIb), and O( ii). The antibodies produced control the compatibilities among these blood groups. THE BLOOD GROUPS ARE DEPENDENT ON HAVING 3 ALLELES FOR THE TRAIT AND TWO OF THE ALLELES BEING CODOMINANT.
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1. A man with group A blood marries a woman with group B blood and they produce a child with type O blood. What are the genotypes of these three individuals? What other genotypes and in what frequencies would you expect in offspring from this marriage of the original man and woman?
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Man- Iai Woman- Ibi child- ii you could also expect IaIb, Iai, Ibi
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What is pleiotropy? Give an example of a pleiotropic trait. What is epistasis? Give an example of epistasis.
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Pleiotropy- a single gene has multiple pheonotype effects ex: sickle cell disease
Epistasis- one gene alters the expression of a second gene rhar is independently inherited ex: certain coat colors of mice and other mammals |
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What is polygenic inheritance? How are the phenotypes of polygenic traits thought to be determined? How is a Bell-Curve related to phenotype in polygenic inheritance?
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Polygenic inheritance- additive effect of 2 or more genes on a single phenotypic character
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Mendel found tall pea plants dominant to dwarf plants. His symbol for the dominant allele was ___?
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Mendel's traits were named for the dominant condition and the dominant allele is capitalized. Therefore for this trait the dominant allele would be T.
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Mendel crossed a true-breeding tall plant with a dwarf plant to produce F1 plants. What were the phenotypes and genotypes of the F1 plants?
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Phenotypes would be all tall. Genotypes would be Tt.
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Two F1 plants are crossed. what would be the expected phenotypes of the F2 plants and in what ratio?
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The phenotypes of the F2 plants would be tall and dwarf in the ratio of 3/4 to 1/4 (3:1).
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What would be the expected genotypes of the F2 plants and in what ratio?
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The genotypes of the F2 plants would be TT, Tt, and tt in a ratio of 1/4 to 1/2 to 1/4 (1:2:1)
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You examine 4 genes at once designated by the letters a, b, c, d. how many different gametes could and AaBbCcdd individual produce? If two
of these individuals were crossed, what would be the number of different combinations of offspring? |
There are 3 heterozygous combinations so the number would be 23 or 8 possible gametes. From two parents with 8 different gametes there
would be 64 combinations of offspring. |
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How many different alleles are there for A,B,O blood types and what are they?
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There are three alleles, IA, IB, and i.
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A basic principle in biology stating that genes are located on chromosomes and that the behavior of chromosomes during meiosis accounts for inheritance patterns.
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Chromosome theory of inheritance
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An individual with the normal (most common) phenotype.
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Wild type
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Chart of a chromosome that locates genes with respect to chromosomal features.
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Cytologenetic map
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The reciprocal exchange of genetic material between nonsister chromatids during prophase I of meiosis.
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Crossing over
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An ordered list of genetic loci (genes or other genetic markers) along a chromosome.
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Genetic map
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General term for the production of offspring that combine traits of the two parents.
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Genetic recombination
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A genetic map based on the frequencies of recombination between markers during crossing over of homologous chromosomes
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Linkage map
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Genes located close enough together on a chromosome to be usually inherited together.
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Linked genes
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A unit of measurement of the distance between genes. One map unit is equivalent to a 1% recombination frequency.
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Map unit
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An offspring with a phenotype that matches one of the parental phenotypes.
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Parental type
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An offspring whose phenotype differs from that of the parents; also called recombinant type.
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Recombinant
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A human genetic disease caused by a sex-linked recessive allele; characterized by progressive weakening and a loss of muscle tissue.
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Duchenne muscular dystrophy
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A human genetic disease caused by a sex-linked recessive allele; characterized by excessive bleeding following injury.
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Hemophilia
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A gene located on a sex chromosome.
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Sex-linked gene
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A chromosomal aberration in which one or more chromosomes are present in extra copies or are deficient in number.
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Aneuploidy
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(1) A deficiency in a chromosome resulting from the loss of a fragment through breakage. (2) A mutational loss of one or more nucleotide pairs from a gene.
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Deletion
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A human genetic disease caused by presence of an extra chromosome 21; characterized by mental retardation and heart and respiratory defects.
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Down syndrome
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An aberration in chromosome structure due to fusion with a fragment from a homologous chromosome, such that a portion of a chromosome is duplicated.
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Duplication
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An aberration in chromosome structure resulting from reattachment in a reverse orientation of a chromosomal fragment to the chromosome from which the fragment originated.
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Inversion
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Referring to a cell that has only one copy of a particular chromosome, instead of the normal two.
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Monosomic
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An error in meiosis or mitosis, in which both members of a pair of homologous chromosomes or both sister chromatids fail to move apart properly.
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Nondisjunction
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A chromosomal alteration in which the organism possesses more than two complete chromosome sets.
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Polyploidy
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(1) An aberration in chromosome structure resulting from attachment of a chromosomal fragment to a nonhomologous chromosome. (2) During protein synthesis, the third stage in the elongation cycle when the RNA carrying the growing polypeptide moves from the A site to the P site on the ribosome. (3) The transport of organic nutrients in the phloem of vascular plants
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Translocation
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Referring to a cell that has three copies of a particular chromosome, instead of the normal two.
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Trisomic
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A virus that infects bacteria; also called a phage.
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Bacteriophage
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The form of native DNA, referring to its two adjacent polynucleotide strands wound into a spiral shape.
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Double helix
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A virus that infects bacteria; also called a bacteriophage.
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Phage
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A linking enzyme essential for DNA replication; catalyzes the covalent bonding of the 3’ end of a new DNA fragment to the 5’ end of a growing chain.
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DNA ligase
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An enzyme that catalyzes the elongation of new DNA at a replication fork by the addition of nucleotides to the existing chain.
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DNA polymerase
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An enzyme that untwists the double helix of DNA at the replication forks.
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Helicase
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A discontinuously synthesized DNA strand that elongates in a direction away from the replication fork.
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Lagging strand
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The new continuous complementary DNA strand synthesized along the template strand in the mandatory 5’ ( 3’ direction.
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Leading strand
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The cellular process that uses special enzymes to fix incorrectly paired nucleotides.
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Mismatch repair
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An enzyme that hydrolyzes DNA and RNA into their component nucleotides.
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Nuclease
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The process of removing and then correctly replacing a damaged segment of DNA using the undamaged strand as a guide.
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Nucleotide excision repair
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A short segment of DNA synthesized on a template strand during DNA replication. Many Okazaki fragments make up the lagging strand of newly synthesized DNA.
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Okazaki fragment
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Site where the replication of a DNA molecule begins.
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Origin of replication
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An enzyme that joins RNA nucleotides to make the primer.
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Primase
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A polynucleotide with a free 3´ end, bound by complementary base pairing to the template strand, that is elongated during DNA replication.
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Primer
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A Y-shaped region on a replicating DNA molecule where new strands are growing.
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Replication fork
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Type of DNA replication in which the replicated double helix consists of one old strand, derived from the old molecule, and one newly made strand.
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Semiconservative model
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During DNA replication, molecules that line up along the unpaired DNA strands, holding them apart while the DNA strands serve as templates for the synthesis of complementary strands of DNA.
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Single-strand binding protein
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An enzyme that catalyzes the lengthening of telomeres. The enzyme includes a molecule of RNA that serves as a template for new telomere segments.
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Telomerase
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The protective structure at each end of a eukaryotic chromosome. Specifically, the tandemly repetitive DNA at the end of the chromosome’s DNA molecule. See also repetitive DNA.
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Telomere
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A protein that functions in DNA replication, helping to relieve strain in the double helix ahead of the replication fork.
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Topoisomerase
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Define independent assortment, crossing over, and random fertilization with regards to meiosis and sexual life cycles. Describe how independent assortment, crossing over and random fertilization effect genetic variation in offspring of humans.
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Independent assortment means that each pair of homologous chromosomes in an individual is aligned on the metaphase plate independently of the other pairs of chromosomes. Since homologous chromosomes are not completely alike, this means there are many different combinations of the paired chromosomes that can be produced when the homologous pairs separate during meiosis I. Crossing over occurs during prophase of meiosis I and during the process pieces of chromatids are exchanged between paired chromosomes. This produces chromosomes that are potentially unlike those contributed by the parent gametes. Random fertilization means that any of a fathers gametes is equally likely to fertilize any of the mothers gametes. Humans possess 46 chromosomes and independent assortment of chromosome pairs means that there are potentially 223 or about 8 million different possible combinations of paired chromosomes. Crossing over produces unique chromosome pairs so that there could be significantly more different gametes possible. Given two parents with 8 million different gametes, random fertilization could potentially produce 70 trillion genetically different offspring.
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Charles Darwin published “Origin of Species” in 1859. What are the four observations and hypotheses that he put forward to explain the variation he observed among species? Specifically, how is the term natural selection related to these observations and hypotheses?
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Darwin's observations and hypotheses were 1) Populations produce more offspring than the environment can support, 2) Individuals with characteristics best fit to an environment will survive over less-fit individuals, 3) Favorable, heritable characteristics accumulate in a population over time 4) This accumulation produces adaptation of populations to their environment. In no. 2, natural selection leads to differential survival. In other word, the strongest individuals survive and strength has a significant genetic component.
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There was no accepted scientific explanation for how characteristics could be inherited when Darwin published Origin of the Species in 1859. Gregor Mendel was discovering how characteristics were inherited. Who was Gregor Mendel and when did he publish his treatise on inheritance in plants? What organism did Mendel use for his genetic studies? What three advantages did Mendel have in his studies over other investigators?
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Gregor Mendel was an Austrian monk who published "Experiments in Plant Hybridization" in 1865. Mendel used common garden peas for experiments. Mendel was able to control all of his crosses; he knew the identity of the "parents" in each cross. Mendel studied traits with qualitative differences, in other words the physical characteristics of his traits could only be of two different types. Finally, Mendel started his studies with true-breeding individuals meaning crosses of such individuals would always give the same results.
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What are stamens and carpels in flowers? Peas can self-fertilize, what does that mean? Describe how the structure of the pea flower allowed Mendel to control his experimental crosses.
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Stamens are the male portion of flowers and they produce pollen which contains the male gametes. Carpels are the female parts of flowers and they contain ovules which contain female gametes. Pollen from a pea stamen can pollinate the carpel of the same flower in peas. Mendel removed the stamens before they matured on one flower and then introduced the pollen from another flower to the carpel of first flower. Using this approach and keeping the flower covered except when he introduced pollen guaranteed he knew the parents of his cross.
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Mendel deduced an explanation for the results of his studies of garden pea crosses. What are the features of Mendel's explanation? Using his logic, explain his results for crosses of plants with purple flowers with plants possessing white flowers.
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Mendel's concluded from his results that traits were governed by two alleles and one allele was completely dominant over the other. Further, the alleles for a trait segregated (separated) during gamete formation. Therefore each gamete contained only one allele for a given trait. Based on his conclusions, the phenotypes, genotypes, and gametes for his monohybrid cross are illustrated as follows:
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What are alleles? How many alleles did Mendel's pea plants possess per trait? Define the terms phenotype and genotype. Differentiate between homozygous and heterozygous genotypes. How do homozygous and heterozygous genotypes relate to phenotype? Use Mendel's pea flower color as an example to illustrate the difference.
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Alleles are alternative versions of genes that code for a given trait. In Mendel's pea plants there were two alleles for each trait. Phenotype is the physical appearance of an organism based on its genetic makeup. Genotype is the genetic makeup of an organism referring to the specific alleles possessed by the organism. An organism is homozygous when it possesses either two dominant or 2 recessive alleles for a trait while an organism is heterozygous when it possesses one dominant and one recessive allele for a trait. For pea flower color where the dominant allele is P and the recessive allele is p, the homozygous genotypes would be PP or pp. The heterozygous genotype would be Pp.
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When Mendel crossed a true-breeding pea plant that produced yellow seeds with a plant that produce green seeds the result was all plants that produced yellow seeds. What were the genotypes of the parental (P), true-breeding plants? What kinds of gametes would each of these parents produce? What was the phenotype and genotype of the F1 offspring? What kind of gametes would each of these F1 offspring produce? If two F1 individuals are crossed what are the possible phenotypes and genotypes of the resulting F2 progeny and in what proportion would they be expected to be produced?
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The P genotypes would be YY (yellow phenotype) and yy (green phenotype). Each parent (P generation) would produce one kind of gamete, Y from yellow seed individuals and y from green see individuals. All of the F1 offspring produce yellow seeds but they are heterozygous: their genotype is Yy. F1 individuals would produce two kinds of gametes, Y and y. Crossing two F1 plants would produce both yellow seed and green seed individuals in a 3:1 ratio. The genotypes would be YY, Yy and yy in a 1:2:1 ratio.
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What is a Test Cross? What is the purpose of a test cross? Illustrate the Test Cross by explaining how you would determine the genotype of the pea plant that produces purple flowers.
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A test cross is the breeding of recessive individual (thus having a known genotype, two recessive alleles) with an individual with a dominant phenotype but unknown genotype. By examining the offspring produced by the cross, the genotype of the dominant individual can usually be determined. Crossing a purple flowered plant (which could have either a homozygous dominant or a heterozygous genotype) with a white flowered plant (homozygous recessive) would produce two possible outcomes depending on the genotype of the purple-flowered plant. If the purple flowered plant was homozygous, all the offspring would produce purple flowers. If the purple flowered plant was heterozygous, the offspring should be a mix of plants producing purple and white flowers in a 1:1 ratio.
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Tall pea plants are dominant to dwarf pea plants and green pod color is dominant to yellow pod color in pea plants (note this is the opposite of seed color, Table 14.1). True-breeding tall pea plants producing green pods are crossed with dwarf pea plants producing yellow pods. Two resulting F1 individuals are crossed to produce an F2 generation. What are the possible phenotypes of the F1 and F2 generations and in what ratios? Determine the ratio of phenotypes in the F2 using a Punnett square.
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P generation: Tall, green pods (TTGG) x dwarf, yellow pods (ttgg). Each parent produces one kind of gamete with an allele for each trait. The gametes are TG and tg. The F1 generation would all be the same, tall and green with a TtGg genotype. The F1 individuals can produce 4 different types of gametes: TG, Tg, tG, tg. The possible phenotypes would be tall and green, tall and yellow, dwarf and green and dwarf and yellow in a 9:3:3:1 ratio.
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You are given a tall pea plant that produces green pods but you do not know if it is true-breeding or not. How would you determine the genotype of the plant (see question 4)? Explain your answer.
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Make a test cross for two traits. The genotype of the tall and green individual could be TTGG, TTGg, TtGG, or TtGg. If the tall and green parent was TTGG and crossed with a dwarf and yellow parent, all of the offspring would be tall and green (TtGg). If the parent was TTGg, half of the offspring would be tall and green and half would be tall and yellow. If the parent was TtGG, half of the offspring would be tall green and half would be dwarf green. If the parent was TtGg, 1/4 o the offspring would tall and green, 1/4 would be tall and yellow , 1/4 would dwarf green and 1/4 would be dwarf yellow.
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Referring to the crosses in question 4, describe how you would determine the ratio of tall, yellow-pod offspring to all offspring in the F2 using probabilities.
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We need to determine the expected number of offspring in the F2 that have a dominant phenotype for both traits in question 4. Determine the probability for offspring with a dominant phenotype for each trait and then multiply the probabilities. For example, the F1 cross for the first trait would be Tt X Tt. We would expect 3/4 of the offspring from this cross to have a dominant phenotype as we would for the second trait. Therefore when we consider both traits the probability would be 3/4 X 3/4 or 9/16.
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Lecture 24 Questions:
(Also, see "Study Questions" from Chapt. 14 (p. 272-273): 1-5, 7,10, 12-13) |
See paper for answers.
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How do incomplete dominance and codominance differ from complete dominance in Mendel's traits.
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In complete dominance, the phenotype for the dominant allele completely masks the recessive allele. In incomplete dominance the heterozygous genotype produces a phenotype that is intermediate between the phenotypes produced by the homozygous genotypes foe each of the two alleles. In codominance, the phenotype of each allele is expressed in the heterozygous genotypes.
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A cross between snap dragon plants with red flowers and white flowers (P generation) produces pink flowers (F1 generation). Give the genotypes of the plants from these two generations. Two F1 individuals are then crossed. What gametes will be formed by the F1 individuals? What will be the phenotypes and genotypes of F2 individuals?
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In the P generation, each parent is homozygous for one of the two alleles that produce flower color Symbolically, one parent would be CrCr and the other parent would be CwCw where C is the gene for color and r is one allele and w is the other allele. The F1 offspring would all have the genotype - CrCw . The F1 offspring would produce two different types of gametes, Cr , and Cw. The F2 offspring would have three phenotypes, red, pink and white. The genotypes for these phenotypes would be CrCr , CrCw , and CwCw respectively.
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What are the alleles that control A-B blood groups in humans? Which alleles are dominant and which are recessive? What are the different possible phenotypes that can be found for A-B blood groups? What genotypes produce these phenotypes? What controls compatibility for transfusions among different blood groups? How does the control of A-B blood groups relate to the genetic term multiple alleles?
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The three alleles controlling A-B blood types are Ia ,Ib (each are dominant) and i (which is recessive). The possible phenotypes and their genotypes are: type A (IaIa, or Iai), type B ( IbIb, or Iai), type AB ( IaIb) or type O (ii). Compatibility is controlled by antibodies produced against the A and B carbohydrates produced by the Ia and Ib alleles. Individuals with the and Ia allele and no Ib allele produce an antibody against the B carbohydrate, individuals with the Ib allele and no Ia allele produce an antibody against the A carbohydrate, individuals with ii genotype produce antibodies against both the A and B carbohydrates and individuals with the IaIb genotype do not produce antibodies against either A or B carbohydrates. A trait that is controlled by three or more alleles is defined as control by multiple alleles.
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A man with group A blood marries a woman with group B blood and they produce a child with type O blood. What are the genotypes of these three individuals? What other genotypes and in what frequencies would you expect in offspring from this marriage of the original man and woman?
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To be type O, the child must have the genotype ii. Therefore the child received one i allele from each parent. That means the genotype of the man must be Iai and the genotype of the mother must be Ibi. Each parent could produce two different gametes and the four possible combinations from these gametes would be IaIb, Iai, Ibi, and ii. The ratio for all of the genotypes would be 1:1:1:1. Each genotype has a separate phenotype.
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What is pleiotropy? Give an example of a pleiotropic trait. What is epistasis? Give an example of epistasis.
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Pleiotropy is a case where a single has multiple phenotypic effects. This is not uncommon and a specific example is sickle-cell disease where the alleles cause many different health (phenotypic) problems. Epistasis is where a single phenotype is the product of more than one gene. An example is coat color in certain mice (or dogs). One gene regulates whether or not hair will have pigment and therefore color while a second gene regulates the color in individuals that have pigment.
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What is polygenic inheritance? How are the phenotypes of polygenic traits thought to be determined? How is a Bell-Curve related to phenotype in polygenic inheritance?
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Polygenic inheritance is where phenotypes are varied and controlled by the additive effect of two or more genes. The phenotypes are determined by the number of dominant alleles for two or more genes. In polygenic inheritance, the distribution of phenotypes approximates a bell curve. Individuals with the extreme phenotypes are relatively rare while the number of individuals with the intermediate phenotype are relatively common.
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LECTURE 25 QUESTIONS \
Also, see "Study Questions" from Chapt. 14 (p. 272-273): question 14) |
See paper for answers.
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Explain how Cystic Fibrosis, Tay-Sachs Disease and Sickle-Cell Disease are inherited. Briefly describe the symptoms of each of these genetic disorders. While rare, each of these disorders are more common in certain groups of individuals. In what groups of people are each of these rare disorders more common than normal?
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All three of these genetic disorders are inherited as a Mendelian trait where the disease is expressed in individuals that are homozygous recessive. Cystic fibrosis, is caused by defective chloride channels which produce the build up of mucous in the pancreas, lungs and other organs leading to recurrent infections and early death. The white, US population has an abnormally high number of carriers for cystic fibrosis and thus a relatively high number of individuals are born with the disease. Tay-Sachs disease is caused by a defective enzyme needed to breakdown lipids in the brain. The accumulation of the lipids causes neural degeneration and early death. There are abnormally high individuals born with Tay-Sachs disease in Jews of central European descent, French Canadians and the Cajun population in Louisiana. Defective structure of the hemoglobin protein causes formation of abnormal red blood cells which clog arteries and often leads to early death. Sickle-Cell is relatively common in African-Americans and its stability in the population is due to the fact that heterozygous individuals are more resistant to malaria than non carriers of the disease.
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What are the symptoms of Huntington’s disease? How does Huntington’s disease differ from Cystic Fibrosis, Tay-Sachs and Sickle-Cell diseases in the way that it is inherited? Even though Huntington's disease is fatal it is still maintained in human populations. Why?
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Huntington's disease is characterized by progressive degeneration of the nervous system which leads to dementia and loss of motor control and eventual death. Huntington's disease, unlike cystic fibrosis, Tay-Sachs disease and sickle-cell disease is inherited as a Mendelian dominant trait. The disease is not expressed in individuals until they are 35 years old or older. Therefore individuals with the disease will appear normal and can produce children before they know they have the disease.
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What is a pedigree? You are given a family pedigree for a trait that is obviously inherited. What clues would you look for in the pedigree to decide if the trait was inherited as either a Mendelian dominant or a Mendelian recessive?
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A pedigree is a diagram of a family tree showing the occurrence of heritable characters in parents and offspring over multiple generations. For a recessive trait, look to see if the trait skips generations in its inheritance. Skipping generations is typical if an individual must have the homozygous recessive condition for the trait to be expressed. Heterozygous individuals would be carriers and would not have the trait. If the trait is inherited as a Mendelian dominant it would be present in each generation.
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Thomas Hunt Morgan was a geneticist who used fruit flies to study inheritance. Why are fruit flies a useful model system for studying genetics? What trait allowed Morgan to first show that genes were on chromosomes?
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Flies are useful because 1) they have a short (two week) life cycle and they produce 100's of offspring with each cross, 2) flies are simple genetically, they are 2n=8 (diploid with 3 pairs of autosomes and 1 pair of sex chromosomes), and 3) flies are easily mutagenized to produce genetic variants. Morgan's group discovered an eye color mutation in which individuals had white eyes instead of the normal red eye color. Morgan discovered that inheritance of this trait was different depending on whether an individual was male or female showing it had to be inherited on the X chromosome. This proved traits were on chromosomes.
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What trait allowed Thomas Hunt Morgan to first show that genes were on chromosomes? How did Morgan explain inheritance of this trait?
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The rare mutation producing white eye was the first mutation that Morgan discovered and understanding the genetics of inheritance of this trait allowed him to confirm that genes were linked with chromosomes. Morgan started with a normal (red-eye, wild type) female and a white-eye male. All of the F1 offspring had red-eyes consistent with this normal condition being dominant over white eye. Morgan found when two F1 individuals were crossed that the phenotype of the resulting F2 offspring were 3/4 red eye and 1/4 white eye again consistent with the allele for red-eye being dominant over the allele for white eye. However, Morgan also found that in the F2 all of the females had normal, red eyes but the males were 1/2 red eye and 1/2 white eye. Morgan concluded that the only way to produce these results was that the trait was inherited on the X chromosome. Therefore, females have two alleles for eye color because they have two X chromosomes and males have only one allele for eye color because they have only one X chromosome. In the F2 generation, females have one X chromosome from their father which had to have the dominant allele because he had red eye. Therefore F2 females were either homozygous or heterozygous giving them the dominant phenotype. F2 males, however, could receive either a dominant allele or a recessive allele from their mother which would determine their phenotype since they received a Y chromosome from their father which had no allele. Overall, the results showed that the trait was on the X chromosome and therefore genes were transmitted on chromosome, not by some other type of molecule.
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In flies, vestigial wing condition is controlled by two alleles, vg+ and vg. For fly terminology, which allele is dominant and which is recessive? What is the logic behind this symbolism and how does this differ from the symbols Mendel used? What would be the phenotype of a fly with the genotype vg vg, a fly with the genotype vg+vg?
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In fly terminology vg+ is the dominant allele and vg is the recessive allele. Morgan's lab named the trait after the mutation or recessive condition. Thus this trait is named vg after vestigial, the condition of the wings and the dominant or normal allele is designated with a superscript + while the recessive or mutant allele is just designated as vg. Mendel named his traits with a letter symbolizing the dominant condition and the dominant allele was capitalized and the recessive allele was not. A fly with the genotype vg vg has two recessive alleles and would have the shrunken or vestigial wings. A fly with the genotype vg+vg would have one dominant allele and therefore would have normal or wild-type wings.
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In studying the linked genes described in question 3, Morgan crossed an F1 individuals with individuals recessive for both traits (test cross). The resulting phenotypes and numbers were: wild type (both traits) 965, recessive both traits 944, grey-body and vestigial wings 206, black-body and regular wings 185. Which of these are considered parental offspring? Why? Which are considered recombinant offspring? Why? How do you determine Recombinant Frequency from these data?
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The parental offspring are grey body, straight wings and black body, vestigial wings. These are the same phenotypes found in the original P parents and they are linked on the same chromosome (see diagram for question 3). The recombinant offspring are grey body, vestigial wings, and black body, normal or straight wings. These are called recombinant because they are unlike either of the original P parents and they arise from crossing over in Meiosis I where adjacent sister chromatids exchange parts. Recombinant frequency is calculated from the formula ((# recombinants/total number of offspring)*100. In this case, the total number of offspring equals 2300 and the number of recombinant offspring is 391 (206+185). The recombinant frequency is 17%.
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In flies, gray body color and straight wings are wild type to black body color and vestigial wings. These traits are linked on autosomal chromosomes. A true breeding gray bodied, straight winged male is crossed with a true breeding black bodied, vestigial winged female. What are the genotypes of each parent and what are the possible gametes produced by each parent? What would be the phenotype and genotype of the resulting offspring (F1 generation)? Assuming no cross over, what types of gametes would individuals of the F1 generation produce?
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One parent would be homozygous dominant and the second parent would be homozygous recessive. Each parent would produce one type of gamete, dominant for each trait on a single chromosome for one parent and recessive for each trait on a single chromosome for the second parent. The F1 generation would have a dominant or wild type phenotype for each character ((body color and wing shape). Each individual would have a heterozygous genotype for each trait. Without cross over, the F1 individuals would produce two types of gametes. One gamete would have dominant alleles for each trait and the other type of gamete would have recessive alleles for each trait. The following figure summarizes genotypes and gametes for the P and F1 generations.
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Why is the frequency of crossing over between two genes on a chromosome considered proportional to the distance between them?
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Crossing over was thought to be a random event and likely to occur at any point on a chromosome. Therefore the greater the distance between two genes, the greater the chances of crossing over and the greater the recombinant frequency. Therefore 1% recombinant frequency in a cross involving two traits was arbitrarily set to equal 1 map unit on a relative map of genes on a chromosome.
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Define the terms linked genes and sex-linked genes. Be able to distinguish between the two terms.
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Linked genes are two or more genes on the same chromosome. Sex-linked genes are genes that occur on a sex chromosome. Linked genes refer to two or more genes. To be sex linked, however, a gene just needs to reside on a sex chromosome. In humans, sex-linked genes are primarily found on the X chromosome.
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In humans, genes can be sex-linked on either the X chromosome or more rarely on the Y chromosome. How would inheritance be different for rare, recessive, sex-linked traits on the Y chromosome as compared to on the X chromosome?
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For sex-linked genes on the X chromosome, males would have the recessive phenotype much more frequently that females since they possess only one X chromosome. Sons would always get the recessive allele and therefore the condition from their mother (the source of the son's X chromosome). For inheritance of characteristics on the Y chromosome, females would never have the recessive condition and every son of a father with the recessive condition would have the condition.
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Hemophilia is a sex-linked trait on the X chromosome. Individuals with this genetic disorder fail to produce a clotting factor and may die when they receive minor cuts or even bruises. A normal male marries a normal female and they produce a son with hemophilia. What are the genotypes of these three individuals? What other genotypes and in what frequencies would you expect in all offspring from the original parents? What would be the phenotypes and in what proportions?
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The son would have an X chromosome with the defective allele producing hemophilia and of course his second sex chromosome would by a Y. Let XH represent the normal allele and Xh the allele that produces hemophilia. The genotype for the son would be Xh Y. Since he was normal, the father would have to be XH Y. The mother would have to be a carrier since she is the source of her son's X chromosome and her genotype would have to be XH Xh . Expected genotypes of offspring from these two parents would be XH XH , XH Xh ,XH Y, and Xh Y in a 1:1:1:1 ratio. Overall, the proportion of normal to hemophiliacs would be 3:1. All female offspring would be normal and males would be expected to be both normal and hemophiliac in a 1:1 ratio.
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What is aneuploidy? Give and example from humans.
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Aneuploidy - abnormal number of chromosomes in an individual due to non-disjunction of paired chromosomes during gamete formation (meiosis) and the formation of a zygote with the incorrect number of chromosomes. An example is trisomy 21 or Down's syndrome. The extra chromosome 21 apparently affects a number of structural and physiological traits. Other examples include unusual numbers of sex chromosomes: XXY, XYY, and X0. The last situation is the only known viable case of a missing chromosome in humans.
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What is polyploidy? What are examples from humans and plants?
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Polyploidy - cases where individuals have 3 or more sets of chromosomes (3n, etc.). There are no known viable examples of polyploidy in humans. Polyploidy is more common in plants. For example, commercial bananas are triploid and sterile. Non-disjunction is fairly common in ferns where species may have up to a 1000 chromosomes.
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Reciprocal crosses between two primrose varieties, A and B, produced the following results: A female X B male --> offspring with all green (non-variegated) leaves; B female X A male --> offspring with spotted (variegated) leaves. Explain how variegated leaves are inherited in these primrose varieties.
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There are two clues here. First, in variegated leaves there are areas of green and areas of white color indicating this has something to due with chloroplasts. Second, variegation in offspring depended on which variety served as the mother in the cross. The best explanation of the results is that variegation is related to something coded for by chloroplast DNA. In the B female X A male cross, the chloroplasts come from the cytoplasm of the mother ovum which contains the unusual chloroplasts. Therefore all of the cells developing in the embryo have the potential to have the unusual chloroplasts.
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What is mitochondrial myopathy and how is it inherited?
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In mitochondrial myopathy, there is incorrect development of proteins in the mitochondria leading to deficiencies in the Citric acid cycle and/or the electron transport chain. As a result individuals are weak and intolerant of exercise. Mitochondrial myopathy is inherited with the mitochondria in the ovum.
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Lecture 27 ANSWERS
(Answers to questions 1, 3-5, 8, and 9 starting on page 291 are in the textbook index) |
(Answers to questions 1, 3-5, 8, and 9 starting on page 291 are in the textbook index)
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Before 1940, many scientists believed the information for “genes” that resided on chromosomes was not found in the DNA of eukaryotic cells. In what type of molecule did they think the information was stored and why?
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Many scientists thought that the information was stored in proteins. Proteins could be very complex and they were built with 20 different amino acids giving a complex way to store a lot of information. The other possibility was DNA but it was composed of only four different bases so this seemed to not be complex enough to hold the information for proteins containing 20 different amino acids.
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What type of structure is a bacteriophage and how does it get its name? What kind of molecules are found in the type of bacteriophage studied by Hershey and Chase and how is the bacteriophage reproduced?
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A bacteriophage is a virus and the name bacteriophage comes from the fact that the virus is a parasite on bacterial cells. The bacteriophage studied by Hershey and Chase was made up of two kinds of molecules, protein and DNA. When the bacteriophage infects a bacterial cell, it redirects the bacterial cell to make new virus protein and DNA. After a great deal of protein and DNA is produced these molecules self assemble into new virus particles, the bacterial cell lyses and the the virus particles are distributed into the environment.
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Describe how Hershey and Chase demonstrated that DNA contained the genetic information in their bacteriophage.
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Hershey and Chase produced two different types of labeled virus by allowing virus to be reproduced in the presence of 35S which would only be incorporated into protein or in the presence of 32P which would only be incorporated into DNA. They then infected bacterial cells with virus particles containing radioactive protein or radioactive DNA and after a time removed the virus particle from the infected bacteria. They found that only the DNA had been incorporated into the infected bacteria and not the protein, indicating that it was the DNA that was directing the bacteria to make new virus protein and DNA.
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What were two key features of DNA structure that were known in 1952 before the structure of the complete molecule was described? How are nucleotides bonded together to make a DNA strand? Explain what is meant by the description 5’ to 3’ (or 3’ to 5’) for a nucleic acid.
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First, it was known that the nucleic acid in DNA (not the total molecule) was a polymer made from four different nucleotides. The four nucleotides differed in the type of nitrogenous base they possessed and the four bases were adenine, guanine, cytosine and thymine. Second, it was shown by E. Chargaff when he analyzed the nucleic acids that they were composed of almost equal amounts of adenine and thymine bases as well as guanine and cytosine bases. Human DNA contains approximately 30% adenine, 30% thymine, 20% guanine, 20% cytosine.
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In what year did the first publication that correctly described the structure of a DNA molecule appear? Who were the authors? Who shared the Nobel prize in 1962 for determining the structure of DNA?
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James Watson and Francis Crick published a description of the DNA molecule in 1953 in the journal Nature. Watson, Crick and Maurice Wilkins shared the Nobel Prize.
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What is the general structural difference between a pyrimidine and a purine? What pyrimidines and purines are found in DNA? What type of bonds form between pyrimidines and purines found in the nucleotides making up two DNA nucleic acids? What is the specific way that bases are paired when nucleic acid strands are bonded together?
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Pyrmidines are single-ringed structures and purines are double-ringed structures. DNA contains the purines adenine and guanine and the pyrimidines thymine and cytosine. Hydrogen bonding holds the bases together. Adenine always pairs with thymine because each of these bases are capable of forming two hydrogen bonds while cytosine always pairs with guanine because these bases form three hydrogen bonds.
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A DNA molecule can be described as a ladder. How many strands of DNA make up a single molecule? What makes up the sides of the ladder and what makes up the rungs of the ladder? Why is the molecule termed antiparallel? Describe the three dimensional structure of the DNA molecule.
How is genetic information stored in a DNA molecule? |
The DNA molecule is made up of 2 strands of DNA. The molecule resembles a ladder with the backbones of the nucleic acid strands forming the sides of the ladder and the rungs are formed from hydrogen bonding of two nitrogenous bases, one from each strand. Each strand of DNA has a 3' and a 5' end and the strands are arranged in opposite directions (one strand is 3' to 5' and the other strand is 5' to 3'). The ladder form of the two strands can be thought of as a ribbon and the ribbon forms an alpha helix in space. In other words the ribbon forms a spiral around an imaginary cylinder in space.
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How is genetic information stored in a DNA molecule?
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Genetic information in DNA is stored in the sequence of bases.
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Why is DNA replication termed semi-conservative? In DNA replication, what is a bubble, a point of origin, a replication fork? What is the general functions of the following enzymes: helicase, DNA pol III, primase, DNA pol I, and ligase? What does the abbreviation "pol" stand for?
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Semi-conservative refers to the fact that when a DNA molecule is replicated to form two new molecules, each new molecule contains one parental strand and one newly synthesized strand. A bubble is a portion of the DNA molecule that is untwisted and the hydrogen bonds between bases are broken separating a portion of the two strands. The point of origin is the center of the bubble and the replication fork is the point where strands are separating. The helicase enzyme separates the two DNA strands. DNA pol III, a DNA polymerase, reads a template strand and orders the correct complementary nucleotides to form the complementary DNA strand. Primase aligns RNA nucleotides to form a primer on a DNA template strand where DNA pol III will then bind and begin synthesizing the new daughter strand (adding nucleotides to the 3' end). DNA pol I replaces the RNA primer nucleotides with DNA nucleotides after the DNA pol III has finished its work. Ligase bonds pieces of the new DNA strand together. For example, after the DNA pol I has replaced the RNA primer nucleotides, the replacement nucleotides must be bonded to the growing daughter strand.
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What are "leading" and "trailing" DNA strands? Why are separate mechanisms required for replication of these two types of strands? What do Okazaki fragments have to do with DNA replication?
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DNA replication is from the point of origin to the replication fork. One of the daughter strands being formed will be orientated 5' to 3', from the point of origin to the replication fork. This is a leading strand because the DNA pol III reads directly from the primer at the point of origin to the replication fork. Another daughter strand will read 3' to 5' from the point of origin to the replication fork. Since the DNA pol III can only add nucleotides to the 3' end of a strand it has to work in small stretches of DNA in a back word fashion. That is a primase establishes a primer several nucleotides toward the replication fork and the DNA pol II then synthesizes a segment of DNA from the primer back towards the point of origin. This is the lagging strand. The small stretches of DNA produced in this fashion on the lagging strand are termed the Okazaki fragments. Eventually the fragments are joined to make a continuous strand that overall grows from the point of origin to the replication fork.
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What are two ways that cells correct errors in DNA sequences? How many base pairs are there in a human somatic cell and approximately how long does it take to replicate this DNA?
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Errors in the DNA sequence are corrected by DNA pol III which checks its own synthesis of daughter strands. There are also other enzymes that can detect and repair damaged DNA. In a human somatic cell there are approximately 6 billion base pairs in the DNA and this DNA can be duplicated in a few hours.
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Where do you find telomeres and what do they consist of? In what kind of cells do you find telomerases, what do they do and why are they important for organisms?
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Telomeres are stretches of DNA at the end of eukaryotic chromosomes. To form a daughter strand that is oriented with its 5' prime end at the end of the DNA molecule, a primer must be formed at the end of the strand. DNA pol I can only work with DNA adjacent to the primer and thus the primer at the end of the daughter strand is not replaced with DNA nucleotides. This means the end of the strand is shortened. The telomere region at the end of the strand does not contain bases that are part of a gene so at least for a few replications of the chromosome there is no damage to genes on the chromosome. After several divisions, the shortening of the DNA strand will damage genes on the chromosome. Reproductive cells contain telomerases which are enzymes that replace the lost parts of telomeres after DNA duplication. Telomerases guarantee that gametes will contain complete chromosomes.
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In 1981, a stray black cat with unusual rounded, curled-back ears was adopted by a family in California. Hundreds of descendants of the cat have since been born, and cat fanciers hope to develop the curl cat into a show breed. Suppose you owned the first curl cat and wanted to develop a true-breeding variety. How would you determine whether the curl allele is dominant or recessive? How would you obtain true-breeding curl cats? How could you be sure they are true-breeding?
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Matings of the original mutant cat with true-breeding noncurl cats will produce both curl and noncurl F1 offspring if the curl allele is dominant, but only noncurl offspring if the curl allele is recessive. You would obtain some true-breeding offspring homozygous for the curl allele from matings between F1 cats resulting from the original curlxnoncurl crosses whether the curl trait is dominant or recessive. You know that cats are true-breeding when curlxcurl matings produce only curl offpsring. As it turns out, that allele that causes curled ears is dominant.
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In tigers, a recessive allele causes an absence of fur pigmentation (a white tiger) and a cross-eyed condition. If two phenotypically normal tigers that are heterozygous at this locus are mated, what percentage of their offspring will be cross-eyed? What percentage will be white?
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25% will be cross-eyed; all of the cross-eyed offspring will also be white.
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In corn plants, a dominant allele I inhibits kernel color, while the recessive allele i permits color when homozygous . At a different focus, the dominant allele P causes purple kernel color, while the homozygous recessive genotype pp causes red kernels. If plants heterozygous at both Ioci are crossed, what will be the phenotypic ratio of the offspring?
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The dominant allele I is epistatic to the P/p locus, and thus the genotypic ratio for the F1 generation will be 9 I_P_ (colorless): 3 I_pp (colorless) : 3 iiP_ (purple) : 1 iipp (red). Overall, the phenotypic ratio is 12 colorless: 3 purple: 1 red.
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