Question

In humans, the disease galactosemia is inherited as a rare autosomal recessive trait in a simple Mendelian manner. A normal woman whose father had galactosemia intends to marry a normal man whose grandfather had galactosemia. What is the probability that their first child will have galactosemia?

A. 1/2

B. 1/8

C. 1/16

D. 1/4

E. 0

Correct Answer: B. 1/8

Can you explain why?

Answer 1

Correct answer : 1/8

Galactosemia is an autosomal recessive disorder. These disorders are usually passed on by two carriers. Their health is rarely affected, but they have one mutated gene (recessive gene) and one normal gene (dominant gene) for the condition. Therefore, the probability of inheriting the disease from an affected parent is 50%, i.e., 1/2. Hence, in the question given above, the probability of the woman inheriting the affected allele from her father is 1/2 and the probability of the man inheriting the affected allele from his grandfather is 1/4

Hence, the probability of the child carrying the affected alleles = 1/2 x 1/4 = 1/8, i.e., there is a 1/8 probability that their first child will have galactosemia