Question 1

Which of the following best describes the difference between incomplete penetrance and variable expressivity?

Accepted Answer

Incomplete penetrance means that not all individuals with a genotype show the expected phenotype, while variable expressivity means that individuals with the same genotype show the phenotype to different degrees.

Question 2

A pedigree shows several individuals with an autosomal dominant trait, but some individuals with the genotype do not express the phenotype. What is the most likely explanation?

Accepted Answer

Incomplete penetrance

Question 3

Which of the following is an example of pleiotropy?

Accepted Answer

A single gene mutation causing both anemia and resistance to malaria in humans

Question 4

In a dihybrid cross involving two genes with recessive epistasis, what phenotypic ratio would you expect in the F2 generation?

Accepted Answer

$$9:3:4$$

Question 5

A cross between two pure-breeding white-flowered sweet peas produces all purple F1 offspring. When the F1 are self-crossed, the F2 generation shows a $$9:7$$ ratio of purple to white flowers. What type of gene interaction is this?

Accepted Answer

Complementary gene interaction

Question 6

In Labrador retrievers, the B gene controls eumelanin production (black or brown pigment), and the E gene controls deposition of pigment in the hair. What is the genotype of a yellow lab?

Accepted Answer

$$B\_ee or$$ $$bb\_ee$$

Question 7

A geneticist crosses two pure-breeding mutants with the same phenotype. All F1 offspring have the wild-type phenotype. What does this indicate?

Accepted Answer

The mutations are in different genes

Question 8

Which of the following best describes the concept of epistasis?

Accepted Answer

One gene masking or modifying the effect of another gene

Question 9

A patient with phenylketonuria (PKU) can avoid symptoms by following a strict diet low in phenylalanine. What does this illustrate about the PKU phenotype?

Accepted Answer

It is influenced by environmental factors

Question 10

In a cross between $$PpRr 	imes PpRr$$ bean plants, where P and R are duplicate genes for purple color, what proportion of the F2 generation will have white flowers?

Accepted Answer

$$1/16$$

Question 11

A pedigree shows individuals with Waardenburg syndrome, but each affected individual displays a different combination of symptoms. What genetic concept does this illustrate?

Accepted Answer

Variable expressivity

Question 12

Which of the following is true about the one-gene–one-enzyme hypothesis proposed by Beadle and Tatum?

Accepted Answer

Each gene encodes a single enzyme that affects a single step in a metabolic pathway.

Question 13

A dihybrid cross produces a $$9:6:1$$ phenotypic ratio in the F2 generation. What type of gene interaction does this indicate?

Accepted Answer

Dominant gene interaction

Question 14

Which of the following best explains why genetic counseling for traits with incomplete penetrance and variable expressivity can be challenging?

Accepted Answer

The same genotype can result in different phenotypes or degrees of expression.

Question 15

In the arginine biosynthetic pathway, a mutant grows on minimal medium supplemented with citrulline but not with ornithine. Which enzyme is most likely defective?

Accepted Answer

Ornithine transcarbamylase

Question 16

A cross between two dihybrid budgerigars ($$BbYy 	imes BbYy) $$produces four feather color phenotypes in the F2 generation. Which ratio is expected if there is no gene interaction?

Accepted Answer

$$9:3:3:1$$

Question 17

Which of the following is an example of a sex-limited trait?

Accepted Answer

Horn development in male moose

Question 18

If two mutations fail to complement each other in a complementation analysis, what does this indicate?

Accepted Answer

The mutations are in the same gene

Question 19

Which of the following best describes duplicate gene action?

Accepted Answer

Two genes encode products with the same effect, so a dominant allele at either locus produces the wild-type phenotype.

Question 20

A genetic disorder is caused by a mutation in a single gene, but the symptoms include anemia, pain crises, and resistance to malaria. What genetic concept does this illustrate?

Accepted Answer

Pleiotropy

Genetic Interactions: Penetrance, Expressivity, Pleiotropy, and Epistasis

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