Genes and Sex Determination 1905 – Sex Chromosomes

For centuries, doctors wondered how a baby's sex was determined in the womb. In 355 BC, Aristotle put forward the mistaken idea that “hot sperm produced males and cold sperm produced females”. Today, we know that sexual differentiation depends on genes in our chromosomes. 

Usually, a woman has two X chromosomes (symbolically written as XX), while a man has one X and a much smaller Y chromosome (XY). A boy inherits his Y chromosome from his father. This “XY sex-determination system” was first described in 1905 by American geneticists Nettie Stevens and Edmund Wilson. Chromosomes are not always arranged as XX or XY:

• X: Women with Turner syndrome have only a single X chromosome. This suggests that genes on the Y chromosome are not necessary for survival but are needed to trigger male development in the embryo. 
• XXY: Men with Klinefelter syndrome have two X chromosomes and one Y chromosome. They usually have reduced facial hair and small testes. 
• XXX: Female individuals with triple X syndrome have normal intelligence, but females with more X chromosomes have intellectual disabilities. 
• XYY: These males are taller than average. In general, a single copy of the Y chromosome contains the male-determining gene. Even in the presence of several X chromosomes, it produces an individual who appears male. 
• XY individuals with androgen insensitivity syndrome resemble women, lack a uterus and have a blind-ending structure.

In the 1990s, a team of scientists discovered the SRY gene on the Y chromosome, which plays an important role in the chain of events leading to masculinisation of the embryo. 

XX males can also occur: they have no Y chromosome, but they have a copy of the SRY region on one of their other chromosomes. XY females with a mutation that inactivates the SRY gene can also occur.

Developing human embryos can potentially be male or female. Depending on which genes are subsequently activated, testes or ovaries can develop. The testes secrete testosterone, the hormone responsible for male characteristics.

Description of the article image: Male (XY) chromosomes are shown on the right and female (XX) chromosomes on the left.

Historical figures mentioned in the article: Edmund Beecher Wilson (1856–1939), Nettie Maria Stevens (1861–1912)

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