Chromosomal close ups
in many species, the sex of an individual is determined by the chromosomal constitution of the cells in that individual. For example, in humans, both males and females have 23 pairs of chromosomes; 22 of these are the same and are called autosomes. In females, the 23rd pair consists of two identical chromosomes, denoted xx , whereas in males this pair has two different chromosomes, denoted xy . In short, males have a chromosome (the y ) that females lack.
The x and the y are called sex chromosomes. Both these chromosomes are believed to have a common ancestry, with the y getting depleted of most of its genes in the course of evolution. As a result of this