Question

1a. A boy has Duchenne muscular dystrophy, an X-linked recessive trait. His maternal uncle had died of muscular dystrophy at the age of 21 years. What is the probability that this child's siblings will be affected?

i. It depends whether his mother is a carrier or not

ii. 50%

iii. 25%

iv. 75%

v. 0%

1b. 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?

i. If their first child is affected then their child will not be affected

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

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

iv. The probability that their third child will be affected if their first two children are not affected is 1 in 2.

Answer 1

1a) Option (iii) is the correct answer.

There is a 25% probability of the boy's siblings having Duchenne muscular dystrophy. It is given in the question that boy's maternal uncle suffered from tthe disease, which means that the mother of the boy is most likely a carrier for the disease. A cross between a carrier mother and a normal father is depicted in the punnet square below:

Gametes	X	X
X	XX = carrier daughter	XX = normal daughter
Y	XY = diseased son	XY = normal son

Thus, as can be seen from the punnet square, there is a 25% probability of their child suffering from disease. Hence, there is 25% probability of the boy's siblings having muscular dystrophy.

1b) Option (iv) is the correct answer.

Neurofibromatosis type 1 is an autosomal dominant disorder which means that the presence of even one allele, i.e., in heterozygous condition is sufficient to cause the disorder. A cross between  woman with neurofibromatosis type 1 and an unaffected partner is depicted in the punnet square below:

Gametes	A	a
a	Aa = diseased	aa = normal
a	Aa = diseased	aa = normal

Thus, there is a probability of 1 in 2 children suffering from the disorder or 50% probability, hence Option (iv) is the correct answer.