Question 1

Which of the following best describes the molecular basis of dominance in a gene where the wild-type allele (R+) produces 50 units of enzyme and the mutant allele (r) produces 0 units, with the phenotype requiring at least 40 units for normal function?

Accepted Answer

Haplosufficiency, because one copy of the wild-type allele is sufficient for normal function.

Question 2

A rabbit with genotype $$c^{ch}c$ displays a chinchilla phenotype. Which type of mutation does the c$$^{ch}$$ allele represent, and what is its effect on tyrosinase activity?

Accepted Answer

Hypomorphic (leaky) mutation; reduced but not absent tyrosinase activity.

Question 3

In the ABO blood group system, which genotype(s) will result in the expression of both A and B antigens on red blood cells?

Accepted Answer

$$I^A I^B$$

Question 4

A genetic disorder shows a phenotype in only 70% of individuals with the mutant genotype. What is the term for this phenomenon?

Accepted Answer

Incomplete penetrance

Question 5

Which of the following best explains why Himalayan rabbits ($$c^{h}c$) have dark extremities but a white body?

Accepted Answer

The $$c^{h}$$ allele is a temperature-sensitive hypomorphic mutation, so tyrosinase is active only in cooler body regions.

Question 6

A patient with Waardenburg syndrome displays premature graying, a white forelock, hearing loss, and different-colored eyes. What genetic concept does this illustrate?

Accepted Answer

Pleiotropy

Question 7

Which of the following is an example of a gene–environment interaction?

Accepted Answer

Phenylketonuria (PKU) symptoms prevented by dietary restriction of phenylalanine.

Question 8

In a dihybrid cross, the observed phenotypic ratio is 9:7 instead of the expected 9:3:3:1. Which genetic phenomenon most likely explains this result?

Accepted Answer

Epistasis

Question 9

A goat displays a beard only if it is male, even though both sexes carry the gene for beard growth. What is this an example of?

Accepted Answer

Sex-limited trait

Question 10

Which of the following best describes a dominant negative mutation?

Accepted Answer

A mutant allele whose product interferes with the function of the wild-type protein, especially in multimeric proteins.

Question 11

If a gene has an allelic series with the following dominance hierarchy: $$C \u003e c$$^{ch}$$ \u003e c$$^{h}$$ \u003e c$$, which genotype would produce a Himalayan phenotype?

Accepted Answer

$$c^{h}c$

Question 12

Which of the following scenarios best illustrates incomplete dominance?

Accepted Answer

A heterozygous flower with genotype $$Rr$$ displays a color intermediate between red ($$RR) $$and white ($$rr) $$flowers.

Question 13

Which of the following best describes the genotype of a person with blood type O?

Accepted Answer

$$ii$$

Question 14

A mutation in a gene results in a protein with a novel function not seen in the wild-type. What type of mutation is this?

Accepted Answer

Neomorphic

Question 15

Which of the following is an example of variable expressivity?

Accepted Answer

Individuals with the same genotype for Waardenburg syndrome show different combinations of symptoms.

Question 16

A cross between two heterozygous pea plants for flower color ($$Rr x$$ $$Rr) $$produces offspring with a 1:2:1 ratio of red:pink:white flowers. What inheritance pattern does this illustrate?

Accepted Answer

Incomplete dominance

Question 17

Which of the following best describes the effect of a leaky (hypomorphic) mutation?

Accepted Answer

Partial loss of gene function, resulting in a less severe phenotype than a null mutation.

Question 18

Which of the following is true about codominance?

Accepted Answer

Both alleles in a heterozygote are fully expressed, resulting in a phenotype distinct from either homozygote.

Question 19

A genetic trait is expressed more strongly in males than in females, even though both sexes have the same genotype. What is this an example of?

Accepted Answer

Sex-influenced trait

Question 20

Which of the following best describes pleiotropy?

Accepted Answer

A single gene affects multiple distinct traits.

Gene Interaction and Extensions to Mendelian Inheritance: Study Notes

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