SEX CHROMOSOMES
Sex chromosomes are chromosomes in cells of heteroecious organisms, which are different from autosomes. They determine sex of the organism. The sex with two similar sex chromosomes, which are usually denoted as X-chromosomes, is called homogametic. Heterogametic sex in various species of animals and plants has either one X-chromosome, or a pair of differing sex chromosomes, X and Y. As a rule, male sex is heterogametic.
Sex chromosomes contain genes, which determine not only sexual characters, but also other kinds of characters in an organism. These characters are called sex-linked. Y-chromosome is more meagre in respect of gene composition in comparison with X-chromosome. It contains much constitutive heterochromatin and, as a rule, it is less in size.