gonosomes

Gonosomes are the chromosomes that determine the genetic development of an organism’s sexes. In many species, the gonosomes are the sex chromosomes, such as the X and Y in humans and other mammals. In other lineages, different systems exist, for example Z and W in birds, where females are ZW and males are ZZ. Some organisms have multiple sex chromosome pairs or switch between sex-determining mechanisms during evolution.

In humans and most mammals, females carry two X chromosomes (XX) and males carry one X and one Y chromosome (XY). The Y chromosome contains the SRY gene, which initiates a pathway leading to testes development and male differentiation. The X chromosome is larger and gene-rich, while the Y chromosome has fewer genes and many repetitive sequences. During male meiosis, recombination occurs between the X and Y only in the small pseudoautosomal regions, leaving most of the two chromosomes non-recombining. To balance gene dosage between XX females and XY males, females inactivate one X chromosome in somatic cells, a process known as X-inactivation.

Gonosomes have evolved from autosomes through the suppression of recombination and subsequent divergence. This can lead to degeneration of the Y (or W) chromosome in many lineages, though some species retain functional, gene-rich sex chromosomes. Across the tree of life, sex chromosome systems vary widely, and some groups exhibit rapid turnover or environmental/epigenetic sex-determination mechanisms rather than distinct chromosomal ones.

In genetics, gonosome terminology is used to denote sex chromosomes and their role in inheritance and development, in contrast to autosomes which carry the bulk of non-sex-specific genes.