Question

4. Note: the genotype of females is XX; for males is XY. Use a Punnett square to determine the genotypes and phenotypes of the offspring. a. If a couple has 3 girls, what are possibilities of the 4th child to be another girl?

Answer 1

Answer:
Possibility of the fourth child to be a girl:
The genotype of male is XY and that of female is XX. During normal gamete formation, males produce two types of gametes viz. one containing the X chromosome and the other containing the Y chromosome. But, females produce only one type of gamete i.e., the one containing an X chromosome.
Each of the gametes produced by the male has equal chance for fertilizing each of the gamete produced by the female. Thus, the possible genotypes of the offspring are:
X from male + X from female = XX child = Female child.
Y from male + X from female = XY child = male child.
The chances of occurrence of the above two conditions is equal for all fertilization events.
Hence, the probability of the child from any pregnancy to be a male child = number of possible events producing male child ÷ total number of possible events = 1 ÷ (1 + 1) = 0.5
Similarly, the probability of any child in any pregnancy to be a female = 0.5
It must be noted here that the possibility of occurrence of either male child or female child is not related to any of the previous pregnancies. That Is, the probability for a male child is always equal to the probability of a female child and is equal to 0.5 or 50%.
Hence, the probability of the fourth child being girl is also 0.5 or 50%.