Among mammals, the gender of an organism is determined by the sex chromosomes. Humans have two sex chromosomes, X and Y. Every human diploid cell has 22 pairs of autosomes and one pair of sex chromosomes. A human female has two X chromosomes, while a male has one X chromosome and one Y chromosome.

The germline cells such as egg and sperm cells carry only half the number of chromosomes, i.e., 22 autosomes and one sex chromosome. All eggs have an X chromosome, while sperm cells can carry an X or Y chromosome. If a sperm carrying an X chromosome fuses with an egg, it produces a female embryo, whereas fusion of a sperm carrying a Y chromosome with an egg produces a male embryo.

The human genome sequencing project estimates that sex chromosomes have evolved from autosomes within the last 300 million years. Over time, the Y chromosome lost most autosomal sequences and currently has only around 55 genes. On the other hand, the X chromosome has over 1000 genes, most of which are unrelated to sex determination but include immune-related genes and several housekeeping genes. The mutations in the X- chromosome gene are often associated with X-linked disorders. These disorders are more prominent and lethal among males because they do not have another copy of the X chromosome to compensate for the genetic defects.

Additionally, an incorrect number of sex chromosomes can lead to genetic disorders. For example, females with Turner's syndrome have only one X chromosome instead of two. Such females are infertile with a broad chest and a wide, webbed neck. In contrast, Klinefelter syndrome (XXY) affects only males. Primary features include infertility, poor motor abilities, weaker muscle, among several other symptoms.