Question 1

Which of the following best explains why female mammals are considered genetic mosaics?

Accepted Answer

Because each cell randomly inactivates either the maternal or paternal X chromosome, leading to populations of cells expressing different X-linked alleles.

Question 2

A female heterozygous for an X-linked gene (alleles Xᴹ and Xᴾ) displays patches of two different phenotypes in her tissues. What is the molecular basis for this observation?

Accepted Answer

Random X-chromosome inactivation leads to some cells expressing the maternal allele and others the paternal allele.

Question 3

Which RNA molecule is directly involved in X-chromosome inactivation in placental mammals?

Accepted Answer

XIST (X-inactivation-specific transcript)

Question 4

A wild-type allele (R⁺) produces 50 units of enzyme, and a mutant allele (r) produces 0 units. If 40 or more units are required for a wild-type phenotype, what is the phenotype of an R⁺r heterozygote?

Accepted Answer

Wild-type, because $50 \geq 40$ units are produced.

Question 5

Which term describes a mutation that results in a gene product with a new function not found in the wild type?

Accepted Answer

Neomorphic mutation

Question 6

A patient has a mutation in a gene encoding a multimeric protein. The mutant protein interferes with the function of the normal protein, resulting in a disease phenotype even in heterozygotes. What type of mutation is this?

Accepted Answer

Dominant negative mutation

Question 7

Which of the following best describes a hypomorphic mutation?

Accepted Answer

A mutation that results in partial loss of gene function, producing less gene product than wild type.

Question 8

In a population, a gene has three alleles: A, B, and C. Which statement is correct regarding allelic interactions?

Accepted Answer

There may be more than two alleles for a given gene within a population, but each individual can only have two alleles.

Question 9

Which of the following is an example of incomplete dominance?

Accepted Answer

A heterozygote displays a phenotype intermediate between the two homozygotes.

Question 10

A plant with genotype $T_1T_2$ for flowering time flowers at day 3.7, while $T_1T_1$ flowers at day 0 and $T_2T_2$ at day 5.2. What genetic phenomenon does this illustrate?

Accepted Answer

Incomplete dominance

Question 11

Which of the following best describes a null (amorphic) mutation?

Accepted Answer

A mutation that produces no functional gene product.

Question 12

A wild-type allele is described as haplosufficient. What does this mean?

Accepted Answer

One copy of the wild-type allele is sufficient for normal function in heterozygotes.

Question 13

Which of the following statements about gain-of-function mutations is correct?

Accepted Answer

They can result in either increased gene activity (hypermorphic) or novel gene activity (neomorphic).

Question 14

A gene product is only functional when two or more polypeptides assemble together. A mutation in one allele causes the resulting polypeptide to disrupt the function of the entire protein complex. What is this type of mutation called?

Accepted Answer

Dominant negative mutation

Question 15

If a wild-type allele is haploinsufficient, what is the expected phenotype of a heterozygote carrying one wild-type and one mutant allele?

Accepted Answer

Mutant, because one wild-type allele is not enough for normal function.

Question 16

Which of the following best describes dosage compensation in mammals?

Accepted Answer

Equalization of gene expression between males and females by inactivating one X chromosome in females.

Question 17

A mutation causes a gene to be expressed at much higher levels than normal, resulting in a more severe phenotype in homozygotes than heterozygotes. What type of mutation is this?

Accepted Answer

Hypermorphic mutation

Question 18

Which of the following scenarios best illustrates the concept of gene interaction?

Accepted Answer

Two different genes both influence the color of a flower, resulting in a phenotype not seen when either gene is mutated alone.

Question 19

Which of the following is true regarding the molecular basis of dominance?

Accepted Answer

Dominance is determined by the activity of the protein products of the alleles.

Question 20

A mutation in a gene results in a partial loss of function, and the severity of the phenotype depends on the level of activity of the mutant allele. What is this type of mutation called?

Accepted Answer

Leaky (hypomorphic) mutation

Genetic Mechanisms: X-Chromosome Inactivation, Gene Interaction, and Mutation Effects

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