The XY sex-determination system is the sex-determination system found in humans, most other mammals, some insects (Drosophila) and some plants (Ginkgo). In this system, females have two of the same kind of sex chromosome (XX), and are called the homogametic sex. Males have two distinct sex chromosomes (XY), and are called the heterogametic sex. However, an opposite scheme is found in birds.
The XY sex determination system was first described independently by Nettie Stevens and Edmund Beecher Wilson in 1905.
Ancient ideas on gender determination
Since ancient times, people believed that the gender of an infant is determined by how much heat a man's sperm had during insemination. Aristotle wrote that:
Aristotle claimed that the male principle was the driver behind gender determination, such that if the male principle was insufficiently expressed during reproduction, the foetus would develop as a female. In contrast, modern genetics has developed a view on sex determination in which no one single factor is responsible with determining gender, a number of pro-male, anti-male and pro-female genes being responsible.
Beginnings of genetics of sex
Edmund Beecher Wilson and Nettie Stevens are credited with discovering in 1905 the chromosomal XY sex-determination system, the idea that males have XY sex chromosomes and females have XX sex chromosomes.
The first clues to the existence of a factor which determines the development of testis in mammalians came from experiments carried out by Alfred Jost, who castrated embryonic rabbits in utero and noticed that they all developed as female. In the wake of Jost's experiments, C. E. Ford and his team discovered that the Y gene was needed for a foetus to develop as male when they examined patients with Turner's syndrome, who grew up as phenotypic females, and found them to be X0 (hemizygous for X and no Y). All these observations lead to a consensus that a dominant gene which determines testis development (TDF) must exist on the mammalian chromosome Y.
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