Which of the following is a characteristic of an x-linked recessive disorder pedigree?

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Question 1

Neurofibromatosis type 1 is one of the most common autosomal dominant disorders. A woman with neurofibromatosis type 1 has an unaffected partner. Which of the following is correct regarding their children?

a] The probability that each of their children will be affected is 1 in 4.

b] The probability that their second child will be affected if their first child is affected is 1 in 4.

c] The probability that their third child will be affected if their first two children are affected is 1 in 2.

d] If their first child is affected then their second child will not be affected.

Question 2

Retinoblastoma usually occurs as a sporadic event but in around 10% of cases there is a positive family history due to autosomal dominant inheritance. In these families the penetrance is approximately 80% [= 0.8]. A woman with retinoblastoma, whose father and grandfather were also affected, has an unaffected partner with no relevant family history. Which of the following is correct regarding their children?

a] The probability that their first child will be affected is 0.25.

b] The probability that their first child will be affected is 0.4.

c] The probability that each of their children will be affected is 0.5.

d] The probability that each of their children will be affected is 0.4 X 0.5.

Question 3

Achondroplasia shows autosomal dominant inheritance with complete penetrance. A man with achondroplasia has an unaffected partner of normal height. Their first two children are not affected. Which of the following is correct?

a] The probability that their next child will be affected is 1 in 2.

b] The probability that each of their children will be affected is less than 1 in 2 because the parents of the man with achondroplasia are not affected.

c] The mother of these children has a sister with achondroplasia. This increases the risk that the children could be carriers.

d] If their third child is unaffected, then the probability that their fourth child will be affected is 1 in 8.

Question 4

Tay-Sachs disease shows autosomal recessive inheritance. Parents of a newly diagnosed affected child are referred for genetic counseling. It would be correct to tell them that:

a] the probability that their next child will be affected is 1 in 2.

b] the probability that the older unaffected sister of the affected child is a carrier is 1 in 2.

c] the fact that their last child was affected means that their next three children will not be affected.

d] the probability that each parent is a carrier is 1.

Question 5

Phenylketonuria shows autosomal recessive inheritance with an incidence of 1 in 10 000. Assume that the population is in Hardy-Weinberg equilibrium. Which of the following is correct?

a] The frequency of the mutant allele is 1 in 1000.

b] The frequency of the normal [wild-type] allele is 49/50.

c] The carrier frequency is 1 in 100.

d] The carrier frequency is approximately 1 in 50.

Question 6

A young man with phenylketonuria, who was successfully treated following diagnosis on newborn screening, is planning to start a family with his healthy, unaffected, and unrelated partner, who has no family history of phenylketonuria . Phenylketonuria shows autosomal recessive inheritance with an incidence of 1 in 10 000. Assume that the population is in Hardy-Weinberg equilibrium. Which of the following is correct?

a] The probability that his partner is a carrier is 1 in 100.

b] The probability that the man will transmit a mutant allele to his first child is 1 in 2.

c] The probability that his partner will transmit a mutant allele to their first child is 1 in 50.

d] The probability that their first child will be affected is 1 in 100.

Question 7

Cystic fibrosis shows autosomal recessive inheritance with an incidence of 1 in 2500. The unaffected brother of an affected girl is referred for genetic counseling. Assume that the population is in Hardy-Weinberg equilibrium. Which of the following is correct?

a] The probability that the brother is a carrier is 1/2.

b] If the brother has a child with a healthy unrelated partner the probability that the child will be affected is 1 in 150.

c] If the brother has a child with his first cousin the probability that the child will be affected is 1 in 12.

d] If the affected sister has a child with her first cousin the probability that the child will be affected is 1 in 4.

Question 8

Hunter syndrome is a rare form of mucopolysacchararidosis that differs from all other forms in that it shows X-linked recessive inheritance. A woman with two affected brothers is referred for genetic counseling. Which of the following is correct?

a] The probability that she is a carrier is 2/3.

b] If she is a carrier then the probability that each of her sons will be affected is 1 in 4.

c] If she is a carrier then the probability that each of her daughters will be a carrier is 1 in 4.

d] Before any carrier testing is carried out the woman can be correctly informed that the probability that her first son will be affected is 1 in 4.

Question 9

Haemophilia A is a severe coagulation disorder that shows X-linked recessive inheritance. Red-green colour blindness also shows X-linked recessive inheritance. A man with both haemophilia A and colour blindness is referred for genetic counseling. Assume that his partner is not a carrier of either of these conditions. Which of the following is correct?

a] The probability that each of his daughters will be a carrier of haemophilia A is 1 in 2.

b] The probability that each of his daughters will be a carrier of colour blindness is 1 in 2.

c] The probability that each of his daughters will be a carrier of both conditions is 1.

d] The probability that each of his sons will be affected with haemophilia A is 1 in 2.

Question 10

A woman who has two brothers and a maternal uncle [her mother's brother] with non-specific X-linked mental retardation is referred for genetic counseling. There are no diagnostic tests available to help determine whether or not she is a carrier. Which of the following is correct?

a] The prior probability that she is a carrier is 1 in 4.

b] The woman already has two unaffected sons. Therefore the probability that she is a carrier can be calculated using Bayes' theorem to be 1 in 5.

c] This means that the probability that her next son will be affected or that her next daughter will be a carrier is 1 in 5.

d] The probability that the maternal grandmother of this woman is a carrier is 1 in 2.

Question 11

In this pedigree A and B represent alleles at a marker locus closely linked to the disease locus. Affected individuals are shown as shaded. The disease status in III 1 is unknown. Which of the following is correct?

a] The probable pattern of inheritance shown by the disease in this family is autosomal recessive.

b] If recombination does not occur the probability that III 1 will be affected if she has an AB marker genotype is 1.

c] If recombination does not occur, the probability that III 1 will be affected if she has a BB marker genotype is 1.

d] If the recombination fraction between the disease and marker loci equals 0.04, the probability that III 1 will be affected if she inherits an AB marker genotype equals 0.96.

Question 12

In this pedigree II 1 is affected with an autosomal recessive disorder. The disease status for II 2 and II 3 is unknown. A and B represent alleles at a locus which is tightly linked to the disease locus with recombination fraction of 0. On the basis of the linked marker genotypes II 2 can be told that:

a] he is affected.

b] he is a carrier.

c] he is homozygous unaffected.