Explain the chromosomal basis of sex-determination in animals.

Explanation

Chromosomal Theory of Sex-Determination
The five main methods of sex-determination by chromosomes are as follows
(i) XX-XY method In this mechanism, half of the male gametes carry X and the other half carry Y-chromosomes. The females have a pair of X-chromosomes, e.g. mammals (humans), Drosophila melanogaster, etc.
(ii) XX-XO method In this method, there is differenc in number of chromosomes in two sexes with .O. indicating the absence of X-chromosome. The female has a pair X-chromosomes and produces homogametic eggs while male has only one X-chromosome (XO) and produces two types of sperms, with X and without X.
(iii) ZZ-ZW method This type of sex-determination is seen commonly in birds, fowls and fishes. In this type, females have one Z and one W-chromosome along with autosomes (heterogametic), whereas males have a pair of Z-chromosomes (homogametic).
(iv) ZO-ZZ method In this type, the female is heterogametic having only one Z-chromosome, while the male is homogametic having a pair of Z-chromosomes, e.g. moths and certain butterflies.
(v) Haploid-diploid method In this method, the unfertilised egg develops into female. This type of sex-determination is the characteristic feature of insects like honeybees, ants, etc.