Genetics Chapter 3: Mendelian Genetics

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transmission genetics

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The field of genetics concerned with heredity and the mechanisms by which genes are transferred from parent to offspring.

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trait

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Any detectable phenotypic variation of a particular inherited character.

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monohybrid cross

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A genetic cross involving only one character (e.g., AA × aa).

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selfing

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In plant genetics, the fertilization of a plant’s ovules by pollen produced by the same plant. Reproduction by self-fertilization.

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F1 generation

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First filial generation; the progeny resulting from the first cross in a series.

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F2 generation

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Second filial generation; the progeny resulting from a cross of the F1 generation.

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reciprocal cross

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A pair of crosses in which the genotype of the female in one is present as the genotype of the male in the other, and vice versa.

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T/F: Mendel's monohybrid crosses were sex-dependent.

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False. In each cross, the F1 and F2 patterns of inheritance were similar regardless of which P1 plant served as the source of pollen (sperm) and which served as the source of the ovum (egg). The crosses could be made either way—pollination of dwarf plants by tall plants, or vice versa. Crosses made in both these ways are called reciprocal crosses. Therefore, the results of Mendel’s monohybrid crosses were not sex-dependent.

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Describe Mendel's First Postulate

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UNIT FACTORS IN PAIRS
Genetic characters are controlled by unit factors existing in pairs in individual organisms.
In the monohybrid cross involving tall and dwarf stems, a specific unit factor exists for each trait. Each diploid individual receives one factor from each parent. Because the factors occur in pairs, three combinations are possible: two factors for tall stems, two factors for dwarf stems, or one of each factor. Every individual possesses one of these three combinations, which determines stem height.

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Describe Mendel's Second Postulate

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DOMINANCE/RECESSIVENESS
When two unlike unit factors responsible for a single character are present in a single individual, one unit factor is dominant to the other, which is said to be recessive.
In each monohybrid cross, the trait expressed in the F1 generation is controlled by the dominant unit factor. The trait not expressed is controlled by the recessive unit factor. The terms dominant and recessive are also used to designate traits. In this case, tall stems are said to be dominant over recessive dwarf stems.

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Describe Mendel's Third Postulate

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SEGREGATION
During the formation of gametes, the paired unit factors separate, or segregate, randomly so that each gamete receives one or the other with equal likelihood.
If an individual contains a pair of like unit factors (e.g., both specific for tall), then all its gametes receive one of that same kind of unit factor (in this case, tall). If an individual contains unlike unit factors (e.g., one for tall and one for dwarf), then each gamete has a 50 percent probability of receiving either kind of unit factor (either the tall or the dwarf).

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segregation

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The separation of maternal and paternal homologs of each homologous chromosome pair into gametes during meiosis.

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phenotype

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The overt appearance of a genetically controlled trait.

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gene

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The fundamental physical unit of heredity, whose existence can be confirmed by allelic variants and which occupies a specific chromosomal locus. A DNA sequence coding for a single polypeptide.

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allele

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One of the possible mutational forms of a gene, often distinguished from other alleles by phenotypic effects. For example, the unit factors representing tall and dwarf are alleles determining the height of the pea plant.

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genotype

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The allelic or genetic constitution of an organism; often, the allelic composition of one or a limited number of genes underinvestigation.

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homozygote

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An individual with identical alleles for a gene or genes of interest. These individuals will produce identical gametes (with respect to the gene or genes in question) and will therefore breed true. Example (DD or dd)

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heterozygote

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An individual with different alleles at one or more loci. Such individuals will produce unlike gametes and therefore will not breed true. Example Dd

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testcross

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A cross between an individual whose genotype at one or more loci may be unknown and an individual who is homozygous recessive for the gene or genes in question.

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dihybrid cross

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A genetic cross involving two characters in which the parents possess different forms of each character (e.g., yellow, round × green, wrinkled peas).

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product law

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In statistics, the law holding that the probability of two independent events occurring simultaneously is equal to the product of their independent probabilities.

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Describe Mendel's Fourth Postulate

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INDEPENDENT ASSORTMENT
During gamete formation, segregating pairs of unit factors assort independently of each other.
This postulate stipulates that segregation of any pair of unit factors occurs independently of all others. As a result of random segregation, each gamete receives one member of every pair of unit factors. For one pair, whichever unit factor is received does not influence the outcome of segregation of any other pair. Thus, according to the postulate of independent assortment, all possible combinations of gametes should be formed in equal frequency.

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independent assortment

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The independent behavior of each pair of homologous chromosomes during their segregation in meiosis I. The random distribution of maternal and paternal homologs into gametes.

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Describe Mendel’s 9:3:3:1 dihybrid ratio.

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In every F1 × F1 fertilization event, each zygote has an equal probability of receiving one of the four combinations from each parent.If many offspring are produced, 9/16 have yellow, round seeds, 3/16 have yellow, wrinkled seeds, 3/16 have green, round seeds, and 1/16 have green, wrinkled seeds. This is an ideal ratio based on probability events involving segregation, independent assortment, and random fertilization.

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trihybrid cross

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three pairs of contrasting traits, or three-factor cross.

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continuous variation

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Phenotype variation in which quantitative traits range from one phenotypic extreme to another in an overlapping or continuous fashion. held that offspring were a blend of their parents’ phenotypes.

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discontinuous variation

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Pattern of variation for a trait whose phenotypes fall into two or more distinct classes. Mendel theorized that variation was due to a dominance–recessive relationship between discrete or particulate units

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chromosome theory of inheritance

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The idea put forward independently by Walter Sutton and Theodore Boveri that chromosomes are the carriers of genes and the basis for the Mendelian mechanisms of segregation and independent assortment.

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Mendel’s Principle of Uniformity in F1

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F1 offspring of a monohybrid cross of true-breeding strains resemble only one of the parents.

Why? Smooth seeds (allele S) are completely dominant to wrinkled seeds (allele s).

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Mendel’s Principle of Segregation Summary

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Recessive characters masked in the F1 progeny of two true-breeding strains, reappear in a specific proportion of the F2 progeny.

Two members of a gene pair segregate (separate) from each other during the formation of gametes. Inheritance is particulate, not blending as previously believed.

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Mendel’s Principle of Independent Assortment Summary

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Alleles for different traits assort independently of one another.

Genes on different chromosomes behave independently in gamete production.

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First Filial Generation F1 =

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P1 X P2

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Second Filial Generation F2 =

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F1 X F1

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Backcross one, B1 =

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F1 X P1

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Backcross two, B2 =

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F1 X P2

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reciprocal cross

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switching the phenotype of the male and female parents

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dominant

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phenotype visible in the F1 generation

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recessive

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trait which reappears in the F2 generation after self cross

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gene

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hereditary element that gets segregated

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allele

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particular form of a gene

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homozygous

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organism carrying two copies of the same allele

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heterozygous

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organism carrying different alleles of the same gene

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diploid (2n)

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A condition in which each chromosome exists in pairs; having two of each chromosome.

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The independent behavior of Mendel’s pairs of unit factors is due to what?

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their presence on separate pairs of homologous chromosomes.

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locus (pl., loci)

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The site or place on a chromosome where a particular gene is located.

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criteria necessary to classify two chromosomes as a homologous pair?

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1. During mitosis and meiosis, when chromosomes are visible in their characteristic shapes, both members of a homologous pair are the same size and exhibit identical centromere locations. The sex chromosomes (e.g., the X and the Y chromosomes in mammals) are an exception.

2. During early stages of meiosis, homologous chromosomes form pairs, or synapse.

3. Although it is not generally visible under the microscope, homologs contain the identical linear order of gene loci.

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Genetic variation results because?

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the two members of any homologous pair of chromosomes are rarely, if ever, genetically identical. As the maternal and paternal members of all pairs are distributed to gametes through independent assortment, all possible chromosome combinations are produced, leading to extensive genetic diversity.

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the number of possible gametes, each with different chromosome compositions?

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2n, where n equals the haploid number. Thus, if a species has a haploid number of 4, then 2 4, or 16, different gamete combinations can be formed as a result of independent assortment.

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product law

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says that the probability of two or more events occurring simultaneously is equal to the product of their individual probabilities

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When are two or more events independent of one another?

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if the outcome of each one does not affect the outcome of any of the others under consideration.

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sum law

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The law that holds that the probability of one of two mutually exclusive events occurring is the sum of their individual probabilities.

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conditional probability

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what is the probability that one outcome will occur, given the specific condition upon which this outcome is dependent.What is the probability that a tall plant is heterozygous?

Pc = Pa/Pb
Pa : p of heterozygous
Pb: p of a plant being tall
Pc= (1/2)/(3/4)=2/3

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binomial theorem

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(a + b)^ n = 1
where a and b are the respective probabilities of the two alternative outcomes and n equals the number of trials.

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null hypothesis (H0)

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Used in statistical tests, the hypothesis that there is no real difference between the observed and expected datasets. Statistical methods such as chi-square analysis are used to test the probability associated with this hypothesis.

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chi-square (χ2) analysis

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Statistical test to determine whether or not an observed set of data is equivalent to a theoretical expectation.his test takes into account the observed deviation in each component of a ratio (from what was expected) as well as the sample size and reduces them to a single numerical value. The value for χ2 is then used to estimate how frequently the observed deviation can be expected to occur strictly as a result of chance.

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degrees of freedom (df)

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equal to n − 1, where n is the number of different categories into which the data are divided, in other words, the number of possible outcomes. For the 3:1 ratio, n = 2, so df = 1. For the 9:3:3:1 ratio, n = 4 and df = 3. Degrees of freedom must be taken into account because the greater the number of categories, the more deviation is expected as a result of chance.

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probability

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Ratio of the frequency of a given outcome to the frequency of all possible outcomes.

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pedigree

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In human genetics, a diagram showing the ancestral relationships and transmission of genetic traits over several generations in a family.

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consanguineous

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Related by a common ancestor within the previous few generations.

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sibling species

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Species that are morphologically almost identical but that are reproductively isolated from one another.

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monozygotic twins

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Twins produced from a single fertilization event; the first division of the zygote produces two cells, each of which develops into an embryo. Also known as identical twins.

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dizygotic twins

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Twins produced from separate fertilization events; two ova fertilized independently. Also known as fraternal twins.

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proband

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An individual who is the focus of, or who called attention to, the genetic study leading to the production of a pedigree tracking the inheritance of a genetically determined trait of interest. Formerly known as a propositus.

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albinism

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A condition caused by the lack of melanin production in the iris, hair, and skin. In humans, it is most often inherited as an autosomal recessive trait.

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Huntington disease type

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caused by an autosomal dominant allele. The key to identifying a pedigree that reflects a dominant trait is that all affected offspring will have a parent that also expresses the trait. It is also possible, by chance, that none of the offspring will inherit the dominant allele. If so, the trait will cease to exist in future generations. Like recessive traits, provided that the gene is autosomal, both males and females are equally affected.

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familial hypercholesterolemia

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Heterozygotes display a defect in their receptors for low-density lipoproteins, the so-called LDLs (known popularly as “bad cholesterol”). As a result, too little cholesterol is taken up by cells from the blood, and elevated plasma levels of LDLs result. Such heterozygous individuals almost always have heart attacks during the fourth decade of their life, or before. While heterozygotes have LDL levels about double that of a normal individual, rare homozygotes have been detected. They lack LDL receptors altogether, and their LDL levels are nearly ten times above the normal range. They are likely to have a heart attack very early in life, even before age 5, and almost inevitably before they reach the age of 20.

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Tay–Sachs disease (TSD)

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an inherited disorder that causes unalterable destruction of the central nervous system. This condition is particularly tragic because infants with TSD are unaffected at birth and appear to develop normally until they are about six months old.

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Pedigrees give information on:

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Dominance or recessive nature of allelesRisks (probabilities) of having affected offspring

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Chief method used in human genetics is...?

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pedigree analysis.
Pedigree analysis is used to determine the patterns of inheritance for a trait. A pedigree shows a family tree with respect to a given trait.

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Autosomal Dominant Example

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Achondroplasia  (dwarfism)
Fibroblast growth factor receptor 3

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Autosomal recessive

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Affected persons must be homozygous for the disease allele
Both parents are normal, but may see multiple affected individuals in the sibship, even though the disease is very rare in the population.
Usually “skipped” generations: Because most mating are with homozygous normal individuals and no offspring are affected.

Expect increased consanguinity between the parents,
That is, the parents are more likely to be relatives

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T/F: A horizontal pattern of inheritance indicates a rare recessive trait

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True.

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T/F: A true-breeding plant is one that is homozygous for the trait under study.

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True. True-breeding plants always give rise to offspring whose phenotypes are just like the parents.
True-breeding plants are homozygous for the trait under study, and their offspring always have phenotypes identical to those of the parents.

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Which two genotypes in an individual would be expressed as the same phenotype in a diploid organism?

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HINT: How do genotypes correlate with phenotypes?

Homozygous dominant and heterozygous
Both of these genotypes would express the dominant phenotype. The homozygous dominant individual would have both copies of the dominant allele, and the heterozygous individual would have one copy of the dominant allele and one copy of the recessive allele, but only the dominant phenotype would be observed.

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T/F: The term phenotype refers to the genetic constitution of an individual.

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False.
HINT: The phenotype is the physical result of the genotype’s message.
The term phenotype refers to the appearance of a characteristic or trait that results from an individual’s genetic constitution, or genotype

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A test cross is designed to determine the genotype of a parent with the dominant tall phenotype. A test cross produces progeny that are 1⁄2 tall and 1⁄2 dwarf. What is the genotype of the parent?

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Dd

HINT: A test cross involves a heterozygous or homozygous dominant parent and a homozygous recessive parent.
The tall parent must be heterozygous because we know the dwarf parent is homozygous recessive, by definition of the test cross, and the only way to produce 1/2 recessive offspring is for the tall parent to be heterozygous.

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How many different types of gametes can be formed by individuals with genotype AaBbcc?

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ABc, Abc, aBc, abc
These are the four possible gamete combinations. HINT: Match each A or a with each B or b and each c in combinations of three to get the different types of gametes.

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How did Mendel's work support the chromosomal theory of inheritance?

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The behavior of chromosomes during meiosis (as observed under a microscope) correlated with Mendel’s principles of inheritance.
The observation that chromosomes segregated independently and randomly during meiosis fit very nicely with Mendel’s postulates. Mendel’s work supported the idea that the genetic information was carried on the chromosomes.
HINT: Mendel observed the results of chromosome assortment during sexual reproduction.

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On the basis of segregation and independent assortment, how many different types of gametes can be formed from an organism that has a diploid number of 12?

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64
If the number of complementary chromosomes in the genome (two because it is a diploid) is raised to the nth power (where n = haploid number = 1/2 of the diploid number 12, or 6), the answer is 64.

HINT: The number of different types of gametes formed is based on the number of complementary chromosomes found in the genome.

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A conditional probability is one for which...?

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A specific condition must occur before the event of interest in can occur
If the initial condition does not occur, the event of interest cannot occur.

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The chi-square analysis can help us to decide whether...?

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Our observations of an event differ from our expectations

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What information can be obtained from a human pedigree?

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How a gene is inherited
Human pedigrees are useful for determining the mode of inheritance of a gene; for example, whether it is passed as a dominant, recessive, or sex-linked trait.
HINT: Human pedigrees show the transmission of genetic traits through several generations in a family.

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Mendel's unit factors in pairs are most accurately known to be?

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Two alleles on paternal and maternal homologs
Mendel observed patterns of inheritance in offspring that stemmed from traits found in the parents. Mendel explained this phenomenon as the transmission of "unit factors" from the parents to the offspring.

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Normal parents have a child with Tay-Sachs disease, a recessive condition. What is the probability that their next child will be normal?

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0.75 (75%)
If two normal individuals have one child with Tay-Sachs disease, they are both heterozygous carriers for the recessive gene. The probability of producing a child that is either homozygous dominant or heterozygous for the Tay-Sachs gene is 3:1 for every meiotic event. Thus, the chances for the next child to be normal are 75%.

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If a home garden were planted with all hybrid round peas, what phenotypic proportions would be observed in the progeny? (The wrinkled trait is recessive.)

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3/4 round and 1/4 wrinkled
The hybrid round peas are heterozygous, and therefore the expected ratio of round to wrinkled peas (if the round trait is dominant) will be 3:1.

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If a family has five children, what is the probability that four of them are girls?

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5 (1/2)^4 (1/2)^1
Check the use of the binomial expansion or Pascal's triangle

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Round (R) seed shape is dominant to wrinkled (r) seed shape in pea plants. If an RR plant is crossed with an rr plant, what is the frequency of phenotypes in the F2 generation?

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3 round seeds, 1 wrinkled seed
The F1 generation would have the genotype Rr, so crossing two heterozygotes would result in 3 plants with round seeds and 1 plant with wrinkled seeds.

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T/F:The law of independent assortment states that one gene in a pair is always dominant to the other.

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False. The law of independent assortment states that during gamete formation, segregating pairs of unit factors assort independently of each other; the law says nothing about dominance.

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If a yellow pea plant with round seeds that has the genotype GgWw is crossed to itself, what proportion of the offspring will be green with round seeds?

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HINT: Predict all possible gamete combinations and then use the forked-line method to predict the probability of all possible phenotypes.
a. 3/16
This is the probability of offspring that are either green with round seeds or yellow with wrinkled seeds.

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What is the probability of a family with six children having three boys and three girls?

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HINT: Use the binomial theorem and Pascal’s triangle.
20/64
The term of interest in the expansion (a + b)^6 is the middle term: (a^3)(b^3). Using Pascal’s triangle, the coefficient of this term is 20, so the probability of three boys and three girls is 20/64.

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Which term describes the individual in a pedigree whose phenotype was first brought to the attention of a medical researcher?

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Proband
The proband is the affected individual who is first brought to the attention of a medical researcher; usually the pedigree is constructed around this individual.