Question

Mr. Stevan is suffering from haemophilia and cystic fibrosis. His father is hetrozygous for cystic fibrosis. The probability of Stevan's sperm having recessive X-linked as well as autosomal allele is

• A. $\frac{1}{4}$
• B. $\frac{1}{16}$
• C. $\frac{1}{2}$
• D. $\frac{1}{8}$

Answer 1

The correct option is A $\frac{1}{4}$

Mutations in the CFTR gene cause cystic fibrosis(CF). It is an example of an autosomal recessive disease. Implying, a homozygous recessive condition is needed for the expression of cystic fibrosis. People heterozygous for CFTR gene mutation act as carriers. Haemophilia is an X-linked disease. It is expressed in males as they carry one copy of X chromosome. Females act as a carrier only. (Homozygous and affected females die at an embryonic stage.)

Mr. Steven is suffering from hemophilia so his sperms can have either an affected X chromosome or normal Y chromosome. Both with a probability of 1/2

Also, since his father was heterozygous for CF, there's 50% or 1/2 probability of Mr. Steven carrying the affected allele.

So, by the probability law of product, there's 1/2 x1/2= 1/4 probability of Stevan's sperm carrying recessive X-linked as well as an autosomal allele.

So, the correct option is '1/4'.