Question

(a) In humans, males are heterogametic and females are homogametic. Explain. Are there any examples where males are homogametic and females heterogametic?
(b) Also, describe as to, who determines the sex of an unborn child? Mention whether temperature has a role in sex determination.

Answer 1

(a) In humans, the ${23}^{rd}$ pair of chromosome contains and $X$ chromosome and a $Y$chromosome. Hence, males are called heterogametic. Females, on the other hand, have XX-chromosomes in the ${23}^{\text{rd}}$ pair. Hence, females are called homogametic. But in bird's females have ZW chromosome and males have ZZ chromosome. So, in some cases, males can be homogametic and females can be heterogametic.
(b) In the case of humans, sex is determined by $X$ and $Y$ -chromosomes. Out of the 23 pairs of chromosomes in human beings, the ${23}^{rd}$ pair is called sex chromosome while the remaining 22-pairs are called autosomes. All males have X and Y-chromosomes in the 23 ${}^{\text{rd}}$ pair while females have XX-chromosomes in the ${23}^{rd}$ pair. Thus, a sperm can have either X or Y chromosome, while all the eggs will have an X-chromosome. When a sperm with an X-chromosome fertilizes the ovum; the zygote will result in the development of a girl child. If a sperm with Y-chromosome fertilizes the ovum; the zygote will result in the development of a male child. Temperature-dependent sex determination is found in many animals, e.g. In crocodiles. When eggs are incubated at a higher temperature, it results in the birth of male crocodiles.