Q: A healthy sibling of a person with cystic fibrosis (incidence $$1/2500) $$marries an unrelated partner. What is the probability that their first child will be affected?

$$\frac{2}{3} \times \frac{1}{25} \times \frac{1}{4} = \frac{1}{150}$$

Q: A woman with a prior probability of $$\frac{2}{3}$$ for being a carrier of cystic fibrosis tests negative on a mutation screen that detects 90% of all mutations. What is her new carrier probability?

$$\frac{2}{3} \times 0.1 / (\frac{2}{3} \times 0.1 + \frac{1}{3}) \approx 0.067$$

Question 1

A couple, both healthy, have a child affected with an autosomal recessive disorder. What is the probability that their next child will also be affected, assuming Mendelian inheritance?

Accepted Answer

$$\frac{1}{4}$$

Question 2

In a family with an autosomal dominant disorder showing reduced penetrance $$P$$, what is the probability that the unaffected child of an affected parent is a carrier of the mutant allele?

Accepted Answer

$$\frac{1-P}{2-P}$$

Question 3

A woman has a brother and a maternal uncle affected with Duchenne muscular dystrophy. She has three unaffected sons. Using Bayes’ theorem, what is her posterior probability of being a carrier?

Accepted Answer

$$\frac{1}{9}$$

Question 4

If the probability of having a boy is $$0.5$$ and the probability of having a girl is $$0.5$$, what is the probability that a couple’s two children will both be boys?

Accepted Answer

$$0.25$$

Question 5

A healthy sibling of a person with cystic fibrosis (incidence $$1/2500) $$marries an unrelated partner. What is the probability that their first child will be affected?

Accepted Answer

$$\frac{2}{3} 	imes \frac{1}{25} 	imes \frac{1}{4} = \frac{1}{150}$$

Question 6

In a binomial distribution, what is the probability of observing exactly $$r$$ events of one type in $$n$$ trials, where the probability of the event is $$q$$?

Accepted Answer

$$P = \frac{n!}{(n-r)!r!} p$$^{n-r}$$ q^r$$

Question 7

A woman is found to be a carrier of a BRCA1 mutation. What is her approximate cumulative risk of developing breast cancer by age 70, based on multiple-case family studies?

Accepted Answer

$$70\% - 85\%$$

Question 8

If a disorder is inherited in an X-linked recessive manner and the mutation rate in males is much higher than in females, what happens to the probability that the mother of an isolated case is a carrier?

Accepted Answer

It increases above $$\frac{2}{3}$$

Question 9

A consanguineous couple (first cousins) with no family history of a recessive disorder want to know the risk that their first child will have a serious autosomal recessive disorder, assuming each common ancestor carries one deleterious gene. What is the risk?

Accepted Answer

$$\frac{1}{32}$$

Question 10

A woman with a family history of Huntington disease is 60 years old and unaffected. If 75% of heterozygotes are affected by age 60, what is her posterior probability of being a carrier?

Accepted Answer

$$0.2$$

Question 11

A couple, both carriers for different autosomal dominant disorders, want to know the probability that their child will inherit both disorders. What is the probability?

Accepted Answer

$$\frac{1}{4}$$

Question 12

In a population in Hardy-Weinberg equilibrium, if the incidence of an autosomal recessive disorder is $$1/10,000$$, what is the approximate carrier frequency?

Accepted Answer

$$\frac{1}{50}$$

Question 13

A woman is a known carrier of an X-linked recessive disorder. What is the probability that her son will be affected?

Accepted Answer

$$\frac{1}{2}$$

Question 14

A healthy sibling of a person with an autosomal recessive disorder marries a first cousin. What is the probability that their first child will be affected?

Accepted Answer

$$\frac{1}{24}$$

Question 15

A woman with a prior probability of $$\frac{2}{3}$$ for being a carrier of cystic fibrosis tests negative on a mutation screen that detects 90% of all mutations. What is her new carrier probability?

Accepted Answer

$$\frac{2}{3} 	imes 0.1 / (\frac{2}{3} 	imes 0.1 + \frac{1}{3}) \approx 0.067$$

Risk Calculation in Genetic Counseling: Mini-Textbook Study Notes

Study Guide - Practice Questions

Study Guide - Flashcards