Mastering Genetics (2) Exam 1

Which of the following symbols indicates a wild-type allele? • D • R • e • +

+

Two forms of hemophilia are determined by genes on the X chromosome in humans. Assume that a phenotypically normal woman whose father had hemophilia is married to a normal man. What is the probability that their first daughter will have hemophilia?

0 Father of woman: XªY Woman: XªX (because one X is from the dad) Man: XY ____|___X____|__Y__ _Xª_|__XªX___|_XªY_ _X__|__XX____|_XY_ Look at the girl section, and notice that none will have hemophilia, but their is a 1/2 chance that their first daughter might be a carrier.

Two forms of hemophilia are determined by genes on the X chromosome in humans. Assume that a phenotypically normal woman whose father had hemophilia is married to a normal man. What is the probability that their first son will have hemophilia?

1/2 Father of woman: XªY Woman: XªX (because one X is from the dad) Man: XY ____|___X____|__Y__ _Xª_|__XªX___|_XªY_ _X__|__XX____|_XY_ Look at the boy section, and notice that 1/2 of the boys could have hemophilia

Assume that a dihybrid F2 ratio, resulting from epistasis, was 9:3:4. If a double heterozygote is crossed with the fully recessive type, what phenotypic ratio is expected among the offspring?

1:1:2 For a 9:3:4, the parents genotype are: AABB and aabb AaBb (double heterozygote) is crosse with the fully recessive type (aabb) http://www.yourarticlelibrary.com/biology/6-most-important-kinds-of-epistasis-biology/6436/

With incomplete dominance, a likely ratio of phenotypes in the F2 generation resulting from a monohybrid cross between two true-breeding parents would be ___. • 3:1 • 1:2:1 • 3:3 • 9:3:3:1 • 1:2:2:4

1:2:1

Assume that a dihybrid cross (AaBb X AaBb) is made in which the gene loci are autosomal, independently assorting, and incompletely dominant. What phenotypic ratio would you expect from such a cross? Just provide the ratio, not the phenotypes.

1:2:1:2:4:2:1:2:1

Ratio in the F2 generation: 12/16 solid white 3/16 black-and-white spotted 1/16 solid black How many gene pairs are involved in the inheritance of cattle coat color?

2 1/4ⁿ, where n=2 because 4² = 16

An F2 generation is produced in this ratio: 1 dark-red 4 medium-dark-red 6 medium-red 4 light-red 1 white Based on the ratios in the F2 population, how many genes are involved in the production of color?

2 genes There are 5 phenotypes # of phenotypes = 2n +1 5 = 2n +1 4 = 2n n = 2 (gene pairs)

Assume that in the F2 of a series of crosses, 1/64 of the offspring resemble one of the parents (P). How many gene pairs are involved in producing these results?

3 Remember # of individuals with extreme phenotype is: 1/4ⁿ , where n is the gene pairs. 1/4³ = 1/64

Assume that you are studying a trait determined by a number of polygenes. If there are seven phenotypic categories, how many genes are probably involved?

3 Remember: Equation: 2n + 1 = # of phenotypes where n = genes pairs involved 2n + 1 = 7 2n = 6 n = 3

An inbred stain of plants has a mean height of 24 cm. A second strain of the same species from a different geographical region also has a mean height of 24 cm. When plants from the two strains are crossed together, the F1 plants are the same height as the parent plants. However, the F2 generation shows a wide range of heights; the majority are like the P1 and F1 plants, but approximately 4/1000 are only 12 cm high, and about 4 of 1000 are 36 cm high. How much does each allele contribute to plant height?

3 cm Since now we are talking about "additive alleles," not just gene pairs, multiple the number of gene pairs by 2 to get the total amount of additive alleles. 4 gene pairs x 2 = 8 additive alleles Mean height = 24 cm 24 cm / 8 = 3 cm per (additive) allele

Assume that a dihybrid F2 ratio, resulting from epistasis, was 15:1. If a double heterozygote is crossed with the fully recessive type, what phenotypic ratio is expected among the offspring?

3:1 http://www.yourarticlelibrary.com/biology/6-most-important-kinds-of-epistasis-biology/6436/

An inbred stain of plants has a mean height of 24 cm. A second strain of the same species from a different geographical region also has a mean height of 24 cm. When plants from the two strains are crossed together, the F1 plants are the same height as the parent plants. However, the F2 generation shows a wide range of heights; the majority are like the P1 and F1 plants, but approximately 4/1000 are only 12 cm high, and about 4 of 1000 are 36 cm high. How many gene pairs are involved?

4 1/4ⁿ = 1/individuals 4/1000 = 1/250 = 1/4⁴ n = 4 gene pairs

In a cross between a strain of large guinea pigs and a strain of small guinea pigs, the F1 are phenotypically uniform, with an average size about intermediate between that of the two parental strains. Among 1017 F2 individuals, 1 is about the same size as the small parental strain and 1 is about the same size as the large parental strain. How many gene pairs are involved in the inheritance of size in these strains of guinea pigs?

5 1/4ⁿ = 1/4⁵ = 1/1024 ≈ 1/1017 If it was asking for "additive alleles" then you would multiply the gene pairs by 2 and get the amount 10.

In a mating between individuals with the genotypes IaI0 x IoIo, what percentage of the offspring are expected to have the O blood type?

50% ___|__Ia__|_Io_ _Io_|_IaIo_|_IoIo _Io_|_IaIo_|_IoIo

Assume that four polygenic gene pairs are involved in determining the phenotypes of F2. How many phenotypic classes are expected?

9 Remember # of phenotypes = 2n + 1 # of phenotypes = 2(4) + 1 # of phenotypes = 9

Typical ratios resulting from epistatic interactions in dihybrid crosses would be _____. • 3:1, 1:1 • 9:3:4, 9:7 • 1:1:1:1, 1:4:6:4:1 • 9:3:3:1, 1:2:1 • 1:2:2:4:1:2:1:2:1, 1:2:1

9:3:4, 9:7

Which of the following statements is always true when mutations occur in genes whose products are essential to an organism's survival? • A homozygote for a recessive lethal allele will not survive. • Reversion of the mutation must occur for a heterozygous individual to survive. • One normal copy of the gene will allow survival. • The heterozygote always has a normal phenotype.

A homozygote for a recessive lethal allele will not survive.

The distribution of continuous traits can best be described as ____. • Hyperbolic • Logarithmic • Bell-shaped • Linear

Bell-shaped Remember: Most continuously-varying traits follow a bell-shaped distribution, also known as a normal distribution.

Quantitative inheritance involves the interaction of a number of gene loci. The pattern of genetic transmission typical of quantitative inheritance is ___. • continuous variation of phenotypic expression • a 9:3:3:1 ratio • typical of Mendelian inheritance • usually a patron that clearly reflects dominance and recessiveness • discontinuous distributions such as 3:1

Continuous variation of phenotypic expression

All EXCEPT which of the following statements are major points in the multiple-gene hypothesis? • Each additive allele contributes roughly equally to the phenotype • Together, the genes controlling the trait under study produces a wide range of phenotypic variations. • Analysis of multiple-gene traits requires the study of large numbers of progeny. • Each gene locus involved in the phenotype may be occupied by either an additive allele, which contributes to the phenotype, or a nonadditive allele, which offsets the additive alleles.

Each gene locus involved in the phenotype may be occupied by either an additive allele, which contributes to the phenotype, or a nonadditive allele, which offsets the additive alleles. Remember: In the multiple-gene hypothesis, nonadditive alleles do not contribute quantitatively to the phenotype. [so in other words, they don't offset anything]

Traits such as height, general body structure, skin color, and some behavioral traits are probably caused primarily by genes that behave codominantly or epistatically. True or False

False

The term expressivity defines the percentage of individuals who show at least some degree of expression of a mutant genotype. True or False

False (pp) Expressivity is the range of expression of a gene. (online) The relative capacity of a gene to affect phenotypes of the organisms of which it is a a part

Penetrance specifically refers to the expression of lethal genes in heterozygotes. True or False

False (pp) Penetrance is the fraction of individuals which carry a gene that express it. (online) The proportion of individuals of a particular genotype that express its phenotypic effect in a given environment

Concordane refers to the frequency with which members of a twin pair express a different trait. True or False

False (pp) When a pair of twins display the same phenotype they are "concordant" (pp) When they have a different phenotype, they are "discordant" (online) In genetics, the occurrence of a given trait in both members of a twin pair. Ex. In a trait such as schizophrenia, a pair of identical twins is concordant if both are affected or both are unaffected; it is discordant if only one is affected.

A 9:7 ratio indicates incomplete dominance True or False

False If you look at the ratio, its close to 1:1

Pattern baldness and hen/cock feathering in fowl are examples of X-linked inheritance. True or False

False Remember that this is sex-influenced.

Which of the following statements about twins and twin studies is FALSE? • If the concordance values for monozygotic twins are much higher than for dizygotic twins, the trait probably has a strong genetic basis. • Twin studies have been used to try to determine the role of heredity versus environment. • Fraternal twins are as close genetically to each other as are monozygotic twins. • Dizygotic twins are an important control for any study using monozygotic twins.

Fraternal twins are as close genetically to each other as are monozygotic twins. Remember: This statement is false. Fraternal twins arise from two sperm fertilizing two eggs and are no more genetically similar than any two other siblings. Monozygotic twins have identical genetic compositions.

Assume that a cross is made between tall and dwarf tobacco plants. The F1 generation showed intermediate height, whereas the F2 generation showed a distribution of height raining from tall to dwarf, like the original parents, and many heights between the extremes. These data are consistent with which one of the following modes of inheritance? • multiple-factor inheritance • alternation of generations • codominance • incomplete dominance • hemizygosity

Multiple-factor inheritance

Two mothers give birth to sons at the same time at a busy urban hospital. The son of mother 1 is afflicted with hemophilia, a disease caused by an X-linked recessive allele. Neither parent has the disease. Mother 2 has a normal son, despite the fact that the father has hemophilia. Several years later, couple 1 sues the hospital, claiming that these two newborns were swapped in the nursery following their birth. As a genetic counselor, you are called to testify. What information can you provide the jury concerning the allegation?

Passage of X-linked genes typically occurs from carrier mother to affected son. The fact that the father in couple 2 has hemophilia would not predispose his son to hemophilia. The first couple has no valid claim. Parents 1: Neither parent has the disease (does not mean they are not carriers) Mother 1: XXª Father 1: XY Son 1: XªY Parents 2: Father has hemophilia, but doesn't matter because father gives Y not X. Mother 2: XX Father 2: XªY Son 2: XY

An inbred stain of plants has a mean height of 24 cm. A second strain of the same species from a different geographical region also has a mean height of 24 cm. When plants from the two strains are crossed together, the F1 plants are the same height as the parent plants. However, the F2 generation shows a wide range of heights; the majority are like the P1 and F1 plants, but approximately 4/1000 are only 12 cm high, and about 4 of 1000 are 36 cm high. What mode of inheritance is occurring here? • autosomal recessive • autosomal dominant • mitochondrial • polygenic

Polygenic

"Gain of function" mutations are generally dominant because one copy in a diploid organism is sufficient to alter the normal phenotype. True or False

True

Assume that a mutation occurs in the gene responsible for the production of hexosaminidase A, such that only about 50% of the enzyme activity is found in the heterozygote compared with a homozygous normal individual. If heterozygotes are phenotypically normal, we would say that the mutant allele is recessive to its normal allele.

True

The multiple factor hypothesis suggests that many factors or genes contribute to the phenotype in a cumulative or quantitative manner. True or False

True

With multiple alleles, there can be more than two genetic alternatives for a given locus. True or False

True

Know the following Parental Phenotypes: IªIᶹ IᵇIᶹ IªIᵇ

Type A IªIᶹ = IaI = IaIa = Can receive blood from A & O Type B IᵇIᶹ = IbI = IbIb = Can receive blood from B & O IªIᵇ = Can receive blood from A, B, AB, and O I I = Can receive blood from O

Which term describes the phenomenon that occurs when the expression of one gene pair masks the expression of another gene pair? • epistasis • hypostatic • incomplete dominance • pleiotropy

epistasis

A gene in which the heterozygous condition is displayed differently in males and females is referred to as ___. • X-linked • sex-limited • imprinting • sex-influenced

sex-influenced Remember that pattern baldness is expressed in heterozygous males, but not in heterozygous females.

Of the human traits listed below, which are likely to be under the control of a polygenic mode of inheritance? Select all that apply. • skin color • sickle cell anemia • intelligence • ABO blood type • height • color blindness

skin color, intelligence, height

In a mating between individuals with the genotypes IaIb x IoIo, what percentage of the offspring are expected to have the O blood type?

zero

What is the difference between incomplete dominance and codominance? • The phenotype reveals the genotype only in codominance. • In codominance, both phenotypes are expressed in heterozygotes. In incomplete dominance, the heterozygote shows a phenotype that is intermediate between the two homozygotes. • In incomplete dominance, the dominant phenotype is expressed in heterozygotes. In codominance, both phenotypes are expressed in heterozygotes. • In both types of inheritance, the heterozygotes express the phenotypes dictated by both alleles.

• In codominance, both phenotypes are expressed in heterozygotes. In incomplete dominance, the heterozygote shows a phenotype that is intermediate between the two homozygotes.

A mutation in a gene often results in a reduction of the product of that gene. The term for this type of mutation is ____ . • codominance • gain of function • multiple allelism • loss of function or null (in the case of complete loss) • incomplete dominance

• loss of function or null (in the case of complete loss)

The presence of more than two alternative forms of a given gene would be called ____. • codominance • multiple alleles • incomplete dominance • hemizygosity • alternation of generations

• multiple alleles

Environmental factors typically influence inheritance of ____. • Polygenic traits • Codominance • Trihybrid crosses • Dominantly inherited traits • Multiple alleles

Polygenic Traits

A condition in which one gene pair masks the expression of a non allelic gene pair is called ___. • recessiveness • additive alleles • epistasis • codominance • dominance

epistasis