XY sex determination

The XY sex determination system is the determination of sex via sex chromosomes in most mammals (including humans), some insects and some plants. In this system, males are the heterogametic sex, XY, while females are the homogametic sex, XX.

In fertilisation, an ovum can only carry an X chromosome. It is therefore the sex chromosome found in the sperm - either X or Y - that determines whether a zygote will be female or male, respectively.

The Y chromosome carries either a gene or mutliple genes that signal the embryo towards a male developmental pathway. In humans, a single gene (SRC) signals the development of maleness; however, not all genes for maleness are located on the Y chromosome.

Occasionally, a human may be genotypically one sex, and phenotypically the other. For instance, in XX male syndrome a male has an XX sexual genotype but a phenotype of 'maleness'. This is due to abnormal crossing over of genetic material between the X and Y chromosomes during the formation of gametes in meiosis, causing some Y-chromosomal genes to be carried on the X chromosome.

Other types of abnormal sex inheritance include the presence of additional, or fewer, sex chromosomes than is expected. For instance, in Kleinfelter's Syndrome human males have the genotype XXY. The additional X chromosome may cause symptoms such as smaller testicle size and reduced fertility; it affects 1 in 1000 males making it the most common sex-chromosomal disorder, and the second most common general chromosomal disorder. In Turner Syndrome, a condition affecting 1 in 2500 human females, the sexual genotype is XO, with O denoting the absence of a second sex chromosome (it is sometimes called monosomy X). Symptoms include infertility, reduced growth, a broad chest, a webbed neck and often a range of concurrent problems such as learning difficulties, heart disease, hyper- or hypothyroidism, diabetes and other endocrinological problems.