Question

A man and a women who both appear normal have a child together who has sickle cell anaemia. Sickle cell anaemia is an autosomal recessive trait. The women becomes pregnant again and is told that she is carrying fraternal twins. What is the probability that both of the couple's twins will develop sickle cell anaemia?

• A. $1/16$
• B. $1/4$
• C. $1/2$
• D. $9/16$

Answer 1

The correct option is A $1/16$

Fraternal twins are produced when two different eggs are fertilized forming two different zygotes.
Let the recessive gene of sickle cell anaemia be $a$. As the first child is affected by sickle cell anaemia of the couple, the genotype of the first child is homozygous $aa$. Hence, normal parents are of heterozygous genetype i.e, Aa.
The probability of having an infected progeny is $1/4$ according to the cross between parents. Hence, for two children (fraternal twins), the probability will be $\left(1/4\right)\times \left(1/4\right)=1/16$.