Q: A couple, both heterozygous for a recessive allele ($$Dd$$), have three children. What is the probability that exactly one child will be affected by the recessive disorder?

$$3 \times \left(\frac{1}{4}\right) \times \left(\frac{3}{4}\$$right)^2$$ = \frac{27}{64}$$

Question 1

Which of the following is a hallmark of an autosomal recessive trait in a pedigree?

Accepted Answer

The trait often skips generations and can appear in siblings with unaffected parents.

Question 2

A couple has a child with galactosemia, an autosomal recessive disorder. Neither parent is affected. What is the probability that their next child will also have galactosemia?

Accepted Answer

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

Question 3

In a pedigree, a shaded square represents:

Accepted Answer

An affected male

Question 4

If both parents are heterozygous for an autosomal recessive disorder, what is the expected genotypic ratio among their children?

Accepted Answer

$$1:2:1 ($$homozygous dominant : heterozygous : homozygous recessive)

Question 5

A pedigree shows a trait that is present in every generation and affected individuals have at least one affected parent. What is the most likely mode of inheritance?

Accepted Answer

Autosomal dominant

Question 6

A woman is a carrier for an X-linked recessive disorder. Her partner is unaffected. What is the probability that their son will be affected?

Accepted Answer

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

Question 7

Which of the following scenarios would most likely prompt a referral to a genetic counselor?

Accepted Answer

A family history of a genetic disorder

Question 8

A pedigree shows that all daughters of an affected father are also affected, but none of his sons are. What is the most likely mode of inheritance?

Accepted Answer

X-linked dominant

Question 9

If a couple has one child with an autosomal recessive disorder and one unaffected child, what is the probability that their next child will be a carrier?

Accepted Answer

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

Question 10

A pedigree shows a trait that is more common in males and is never passed from father to son. What is the most likely mode of inheritance?

Accepted Answer

X-linked recessive

Question 11

A couple, both with no family history of galactosemia, have a child diagnosed with galactosemia. What is the most appropriate next step for their future pregnancies?

Accepted Answer

Carrier screening for both parents

Question 12

Which of the following best describes the purpose of newborn genetic screening?

Accepted Answer

To identify treatable, rare genetic diseases early in life

Question 13

In a pedigree, what does a half-shaded circle represent?

Accepted Answer

A female carrier of a recessive trait

Question 14

If two heterozygous parents (Dd) have a child, what is the probability that the child will be homozygous dominant?

Accepted Answer

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

Question 15

A pedigree shows a trait that appears only in males and is passed from father to all sons. What is the most likely mode of inheritance?

Accepted Answer

Y-linked

Question 16

A woman with genotype $$Dd ($$where $$d is a $$recessive disease allele) marries a man with genotype $$Dd. $$What is the probability that their first two children will both be affected by the disease?

Accepted Answer

$$\frac{1}{16}$$

Question 17

Which of the following is NOT a goal of genetic counseling?

Accepted Answer

To guarantee a healthy child

Question 18

A pedigree shows a trait that is present in both males and females, skips generations, and affected children can be born to unaffected parents. What is the most likely mode of inheritance?

Accepted Answer

Autosomal recessive

Question 19

If a pedigree shows that all children of two affected parents are also affected, what is the most likely mode of inheritance?

Accepted Answer

Autosomal recessive

Question 20

A couple, both heterozygous for a recessive allele ($$Dd$$), have three children. What is the probability that exactly one child will be affected by the recessive disorder?

Accepted Answer

$$3 	imes \left(\frac{1}{4}ight) 	imes \left(\frac{3}{4}\$$right)^2$$ = \frac{27}{64}$$

Pedigrees: Analysis, Construction, and Genetic Counseling in Human Genetics

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