Question 1

Which of the following best explains why most loss-of-function (LOF) mutations produce recessive phenotypes?

Accepted Answer

Half the amount of wild-type gene product is sufficient to produce a wild-type phenotype.

Question 2

A heterozygous individual for a gene with alleles $$A^1 ($$wild-type) and $$A^2 ($$mutant) shows a mutant phenotype. Which genetic concept does this illustrate?

Accepted Answer

Haploinsufficiency

Question 3

In the case of a dominant-negative mutation, what is the effect of the mutant protein subunit in a heterozygote?

Accepted Answer

It interferes with the normal functioning of the wild-type protein, resulting in a mutant phenotype.

Question 4

Which genotype would most likely result in an intermediate phenotype due to incomplete dominance in snapdragon flower color, given the alleles $$C^R ($$red) and $$C^I ($$ivory)?

Accepted Answer

$$C^R C^I$$

Question 5

A null (amorphic) mutation in a gene encoding an enzyme results in:

Accepted Answer

No functional enzyme produced

Question 6

If a gene is haplosufficient, what is the expected phenotype of a heterozygote ($$WT/mut) $$for a loss-of-function mutation?

Accepted Answer

Wild-type phenotype

Question 7

Which of the following scenarios best describes a case of haploinsufficiency?

Accepted Answer

A heterozygote has half the normal amount of protein, but this is not enough for a wild-type phenotype.

Question 8

Which equation best represents the total amount of functional protein in a heterozygote for a gene with a wild-type allele ($$A) $$and a null allele ($$a$$)?

Accepted Answer

$$P_{total} = P_A + P_a$$

Question 9

In a gene with a threshold effect, what happens if the amount of functional protein in a heterozygote falls below the threshold?

Accepted Answer

The phenotype will be mutant.

Question 10

Which of the following is TRUE about recessive alleles?

Accepted Answer

Their phenotype is only observed in the homozygous state.

Question 11

A heterozygous individual for a gene with alleles $$A^1$$ and $$A^2 is $$used to determine the relationship between the two alleles. Why is the heterozygous state considered informative?

Accepted Answer

It allows comparison of the expression patterns of both alleles in the same genetic background.

Question 12

Which of the following is an example of a dominant-negative mutation?

Accepted Answer

A mutant protein subunit interferes with the function of the wild-type subunit in a multimeric enzyme.

Question 13

In the case of incomplete dominance, what is the expected phenotype of a heterozygote compared to the two homozygotes?

Accepted Answer

The heterozygote shows a phenotype intermediate between the two homozygotes.

Question 14

Which of the following best describes a hypomorphic mutation?

Accepted Answer

A mutation that results in reduced but not absent gene function

Question 15

A gene encoding a pigment enzyme in snapdragons has no threshold for activity. What is the expected phenotype of a $$C^R C^I$$ heterozygote?

Accepted Answer

Pink flowers due to incomplete dominance

Question 16

Which of the following is NOT an exception to the rule that most LOF mutations have recessive phenotypes?

Accepted Answer

Codominance

Question 17

In a gene with a threshold effect, the wild-type phenotype is observed when:

Accepted Answer

The amount of functional protein is above the threshold

Question 18

Which of the following equations represents the concept of haploinsufficiency for a gene product threshold?

Accepted Answer

$$P_{het} \u003c P_{threshold}$$

Question 19

A patient has a heterozygous mutation in a gene encoding a multimeric enzyme, and the mutant subunit disrupts the function of the entire enzyme complex. What type of mutation is this?

Accepted Answer

Dominant-negative mutation

Question 20

Which statement is TRUE regarding the transcription of alleles in a heterozygote?

Accepted Answer

Both alleles are transcribed unless a mutation prevents transcription.

Genetic Diversity: Effects of Mutations – Loss-of-Function Mutations and Their Phenotypic Consequences

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