$(a)$ 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.

$(a)$ The term homogametic and heterogametic refer to the organism depending upon whether all the gametes contain one or same type of sex chromosome (Homo = same) or two different types of sex chromosomes (Hetero = different).

$(i)$ Humans show $XX/XY$ type of sex determination, i.e. Females contain two copies of $\mathrm{X}$ chromosome and males contain one $\mathrm{X}$ and one $Y$ chromosome. $(ii)$ Therefore, ova produced by females contain the same sex chromosome, i.e. $X$. $(iii)$ On the other hand the sperms contain two different types of chromosomes, i. e. $50 \%$ sperms have $\mathrm{X}$ and $50 \%$ have $Y$ chromosome open from half the autosomes (Meiosis). Therefore, the sperms are different with respect to the composition of sex chromosome. In case of humans, females are considered as homogametic $(X X)$. Males are considered as heterogametic $(XY)$ . Yes, there are examples where males are homogametic and females are heterogametic. In some birds the mode of sex determination is denoted by $ZZ$ (males) and $ZW$ (females),

$(b)$ As a rule the heterogametic organism determines the sex of the unborn child. In case of humans, since males are heterogametic it is the father and not the mother who decides the sex of the child. In some animals like crocodiles,it is the temperature which decides the sex of the organism. Lower temperature favour hatching of female offsprings. Higher temperatures lead to hatching of male off springs.