Question 1

Which of the following best defines penetrance in genetics?

Accepted Answer

The proportion of individuals with a specific genotype who exhibit any signs or symptoms of the associated disease.

Question 2

A family has an autosomal dominant disorder with 80% penetrance. If an affected parent has a child, what is the probability that the child will develop the disease?

Accepted Answer

$$0.5 	imes 0.8 = 0.4 (40$$%)

Question 3

Which of the following is an example of reduced (incomplete) penetrance?

Accepted Answer

BRCA1/2-associated hereditary breast and ovarian cancer

Question 4

What is the main difference between penetrance and expressivity?

Accepted Answer

Penetrance refers to the presence or absence of a phenotype, while expressivity refers to the severity or extent of the phenotype.

Question 5

A patient with Huntington disease has 38 CAG repeats in the HTT gene. What is the most likely explanation for their clinical presentation?

Accepted Answer

They have a reduced penetrance allele and may or may not develop symptoms.

Question 6

Which of the following best describes genetic heterogeneity?

Accepted Answer

A genetic disorder can be caused by mutations in different genes (locus heterogeneity) or different mutations in the same gene (allelic heterogeneity).

Question 7

Which of the following is an example of locus heterogeneity?

Accepted Answer

Retinitis pigmentosa caused by mutations in multiple different genes

Question 8

A genetic disorder is caused by more than 1600 different mutations in the same gene. This is an example of:

Accepted Answer

Allelic heterogeneity

Question 9

Which of the following statements about anticipation is correct?

Accepted Answer

It refers to the earlier onset or increased severity of a genetic disorder in successive generations, often due to expansion of unstable repeat sequences.

Question 10

A child presents with multiple café-au-lait spots, neurofibromas, and Lisch nodules. The parent also has the same mutation but only mild skin findings. This is an example of:

Accepted Answer

Variable expressivity

Question 11

Which of the following is NOT a criterion for classifying a DNA change as a pathogenic mutation rather than a benign polymorphism?

Accepted Answer

The mutation must be found at high frequency in the general population.

Question 12

Which of the following best describes genotype-phenotype correlation?

Accepted Answer

The relationship between specific mutations and the severity or features of a condition.

Question 13

In Huntington disease, what is the relationship between the number of CAG repeats and the age of onset?

Accepted Answer

There is a significant inverse correlation: more repeats are associated with earlier onset.

Question 14

A patient with 25 CAG repeats in the HTT gene is tested for Huntington disease. What is the interpretation?

Accepted Answer

No risk for Huntington disease and no known risk to children.

Question 15

Which of the following is a real-world example of allelic heterogeneity?

Accepted Answer

Cystic fibrosis caused by many different mutations in the CFTR gene

Question 16

Which of the following statements about neurofibromatosis type 1 (NF1) is correct?

Accepted Answer

It is fully penetrant after childhood but shows extreme clinical variability (variable expressivity).

Question 17

Which of the following best describes the concept of age-dependent penetrance?

Accepted Answer

The likelihood of expressing a phenotype increases with age among individuals with a disease-causing genotype.

Question 18

A patient with hemophilia A has a mutation in the F8 gene, while another with hemophilia B has a mutation in the F9 gene. Both have indistinguishable symptoms. This is an example of:

Accepted Answer

Locus heterogeneity

Question 19

Which of the following is most likely to show anticipation?

Accepted Answer

A triplet repeat disorder such as Huntington disease

Penetrance, Expressivity, and Genetic Heterogeneity in Human Genetics

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