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The chromosomal mechanism of determination of sex in human beings is of the genotype XX-XY. The nucleus of each cell consists of 23 pairs of chromosomes or 46 chromosomes out of which 22 pairs are autosomes and the last pair, the 23rd pair is the sex chromosome. Females are homomorphic, i.e., they possess two same sex chromosomes XX whereas males are heteromorphic, they possess two different sex chromosomes XY. Females are homogametic, producing only one type of egg (22+X) whereas males are heterogametic producing two types of sperms – (22+X) & (22+Y).
In the process of fertilization, if the sperm containing X chromosome fertilizes the egg, then the developing offspring would be a female (XX) and if the sperm containing Y chromosome fertilizes the egg, then the developing offspring would be a male (XY). Therefore, the sex ratio that is produced in the progeny is 1:1. Heterogamesis is the chromosomal mechanism of sex determination which can be male heterogamety or female heterogamety.