Question

A family has 9 girls, probability of son at 10${}^{th}$ birth would be _________.

• A. 50%
• B. 100%
• C. 25%
• D. 75%

Answer 1

The correct option is A 50%

In humans, both males and females have 23 pairs of chromosomes. Of these 1 pair is called sex chromosomes.

Females have a perfect pair of sex chromosomes, both called X chromosomes. But males have a mismatched pair in which one is a normal-sized X while the other is a short one called Y. So women are XX, while men are XY.

During meiosis, all the gametes produced by females contain only the X chromosome, whereas males produce two kinds of gametes, 1/2 gametes with X-chromosome and 1/2 with Y-chromosome.

Fertilization of the egg with sperm carrying X chromosomes results in a female child (XX).

Fertilization of the egg with sperm carrying Y chromosomes results in a male child (XY).

Thus, each time there is a 50% chance for a child to be a son and 50% to be a daughter; thus, the probability of 10${}^{th}$ child to be a son is 50%.

So, the correct answer is option A.