The establishment of sex through differential development in an individual at the time of zygote formation is termed as sex-determination. The genes are the units which control hereditary characters of an organism. The genes are located on the chromosomes. These are transmitted from the parents to the offspring through gametes during fertilisation. The chromosomes are responsible for the sex-determination in many organisms. The chromosomes controlling the sex are called sex chromosomes and the remaining chromosomes controlling the normal metabolism are known as autosomes. The organisms may either be male or female depending upon the sex organs. There are various mechanisms of sex-determination in different groups of organisms. Sex-Determination in Insects XX-XO Mechanism In this mechanism, only the X-chromosome is present, the Y being absent. The female is homogametic (XX) and the male is heterogametic (XO). Maleness is determined by a single X-chromosome. O denotes the absence of a Y-chromosome (e.g. grasshopper and bug).
XX-XO mechanism of sex-determination in grasshopper Sex-Determination in Butterflies ZZ-ZO Mechanism In Lepidoptera (e.g. moths and butterflies), the male is homogametic with two Z-chromosomes (ZZ), while the female is heterogametic with one each of Z and W (ZW). However, some females have ZO complement indicating that W is not essential for femaleness.
