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76 Cards in this Set

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What is genetics?
It is the study of how traits are passed on from one generation to the next
What is the basic unit of heredity?
The gene
What are genes?
They are composed of DNA and are located on chromosomes
What are the alternative forms of genes called when they exist in more than one form?
Alleles
What is the genotype?
It is the genetic makeup of an individual
What is the physical manifestation of the genetic makeup called?
It is the phenotype
What did Gregor Mendel do in the 1860s?
He developed the basic principles of genetics through his experiments with the garden pea
What did he study?
He studied the inheritance of individual pea traits by performing genetic crosses
What are true breeding individuals?
They are individuals that if self-crossed produce progeny only with the parental phenotype
He crossed true breeding individuals how?
He crossed true breeding individuals with different traits, mated them, and statistically analyzed the inheritance of the traits in the progeny
What are the four principles of inheritance Mendel postulated?
He said genes exist in alternative forms. An organism has two alleles for each inherited trait, one inherited from each parent. The two alleles segregate during meiosis, resulting in gametes that carry only one allele for any given inherited trait. If two alleles in an individual organism are different, only one will be fully expressed and the other will be silent
What is the expressed allele called?
It is the dominant allele
What is the silent allele?
It is recessive
What is a monohybrid cross?
It is how Mendelian inheritance can be illustrated. It is a cross between two true-breeding pea plants, one with purple flowers and the other with white flowers
What is it called since only one trait is being studied in this particular mating?
A monohybrid cross
What are the individuals called that are being crossed?
They are the parental or P generation
What are the progeny generations called?
They are the filial or F generations
What does the purple flower plant have a genotype of?
PP
What does the white flower parent have?
It has a genotype pp
What is a testcross?
It is used to determine the unknown genotype of an organism
How come?
Only recessive phenotypes can be predicted with 100 percent accuracy
What happens if the dominant phenotype is expressed?
The genotype can be either homozygous dominant or heterozygous
What does this imply?
It means homozygous recessive organisms always breed true
What is a testcross or backcross?
It is when an organism with a dominant phenotype of unknown genotype is cross with a phenotypically recessive organism
What is Mendel’s law of independent assortment?
The alleles of unlinked genes assort independently during meiosis
What are diploid species?
They have chromosomes pairs called homologues
What can recombination frequencies be used to construct?
A genetic map
What is one map unit?
It is defined as a 1 percent recombinant frequency
How does it work?
Recombination frequencies are roughly additive
What does this mean?
If genes are found on a map in the order XYZ, then the recombination frequency between X and Y plus the recombination frequency between Y and Z will be roughly equal to the recombination frequency between X and Z
What if you are given the recombinant frequencies for X and Y, X and Z, and Y and Z?
You can determine the relative positions of these genes on the chromosome
How?
If X and Y have a recombination frequency of 8 percent, they are 8 map units apart. If X and Z recombine 12 percent of the time, then they are 12 map units apart. Depending on where you draw Z in relation to X on your map, Y and Z are either 20 map units apart or 4 map units apart. Since we are also given that Y and Z recombine with a frequency of 4 percent, the genes are in the order of XYZ (or ZYX) on the chromosome.
What is incomplete dominance?
It is when some progeny phenotypes are a blend of the parental phenotypes
What is an example?
An example is in snapdragons. If a homozygous dominant red snapdragon is crossed with homozygous recessive white snapdragons, they produce 100 percent pink progeny
What happens when the pink are self crossed?
They produce red, pink, and white progeny in the ratio of 1:2:1
What is codominance?
It occurs when multiple alleles exist for a given gene and more than one of them is dominant
What is an example of codominance?
It deals with multiple alleles in the inheritance of ABO blood groups in humans
What is blood type determined by?
It is determined by three different alleles, IA, IB, and i
With is penetrance?
The penetrance of a genotype is the percentage of individuals within the population carrying the allele who actually express the phenotype associated with it
What is expressivity?
It is the degree of a genotype to which the phenotype associated with the genotype is expressed in individuals who carry the allele
Can they be influenced by environment?
Yes
What is an example?
In the fruitfly Drosophila melanogaster, the dominant gene curly (Cy) gives rise to adult flies with abnormal wings that curl up if the pupae are kept at 25C, but remain uncurled if the pupae are kept at 19C.
So what is the expressivity?
It is the degree of curliness of the wings
Most lethal genes are what?
They are early acting, ire they program an early death for homozygotes, typically during embryonic development
Can lethal genes ever be positive?
Yes, for example in heterozygous individuals as in the case of sickle-cell anemia. Heterozygous individuals are resistant to malaria, while homozygous individuals have abnormal hemoglobin
Can lethal alleles be dominant?
Yes, these are late-acting genes
What is an example of this?
The gene for Huntington’s disease in humans. It is 100% percent penetrant and fully dominant
What is the sad part of this?
Since the Huntington’s disease gene isn’t expressed until middle age, most of its victims have already had children by the time of diagnosis
Will the recessive phenotype be more or less frequently found in males?
It will be more frequent
What are examples of sex-linked recessives?
The genes for hemophilia and for color-blindness
Since the gene is carried on the X chromosome, and males pass the X chromosome only to their daughters, what does this imply?
Affected males cannot pass the trait to their male offspring
What does it mean to their daughters?
Males will pass the gene to all of their daughters
What does this mean for the daughter concerning the mother?
Unless the daughter also receives the gene from her mother, she will be a phenotypically normal carrier of the trait
So, what does this amount to ultimately?
Since all of the daughter’s male children will receive their only X chromosomes from her, half of her sons will receive the recessive sex-linked allele
So?
Sex-linked recessives generally affect only male
What is a pedigree?
It is a family tree depicting the inheritance of a particular genetic trait over several generations
In analyzing pedigrees, what should you look for?
Individuals with the recessive phenotype
Why?
Matings between them and the dominant phenotype behave as test crosses
What will the ratio of the phenotypes be among their offspring show?
It allows the deduction of the dominant phenotype
What is nondisjunction?
It is either the failure of homologous chromosomes to separate properly during meiosis I, or the failure of sister chromatids to separate properly during meiosis II
What happens if during meiosis, the failure occurs in the secondary spermatocytes?
It results in one gamete with two copies of the chromosome (polyploidy), two normal haploid gametes, and one gamete with no copies of the chromosome, some (aneuploid)
What can the resulting zygote have?
It might have either three copies of that chromosome, which is called trisomy, or might have a single copy of that chromosome, called monosomy
What is an example of this?
The birth defect Down syndrome, which is caused by trisomy of chromosome 21.
What is wrong with victims of down syndrome?
They are of short stature, have characteristic facial features, and are mentally retarded
What else can nondisjunction do to sex chromosomes?
It can result in individuals with extra or missing copies of the X and or Y chromosomes
XXY chromosomes are afflicted with what?
They have Klinefelter’s syndrome
What is that?
They are sterile males with abnormally small testes
What about XO females?
They have only one sex chromosome and suffer from Turner’s syndrome. It means they fail to develop secondary sexual characteristics, and are sterile and of short stature
What about XXX females?
They are referred to as metafemales
What’s wrong with them?
They are usually mentally retarded and sometimes infertile
What about XYY males?
They are normal, though they tend to be taller than average, and according to some studied, may be more violent
What is a chromosome that loses a fragment said to be called?
They are said to have a deficiency
What occurs when the fragment may join with its homologous chromosome?
It results in duplication
What happens if it joins with a nonhomologous chromosome?
It is an event termed translocation
What else can the fragment do?
It may rejoin its original chromosome, but in the reverse position
What is this called?
It is called an inversion