Sex chromosomes

There have been many weird and wonderful ideas about sex determination in humans. Some Ancient Greeks thought that the sex of a baby was determined by which testicle the sperm came from. Apparently, this belief was adopted by some European kings who tied off or removed their left testicle to ensure a male heir to the throne. Other people believed that the sex of a baby could be controlled by conceiving when the Moon was in a particular phase, when the wind was blowing in a certain direction, or whilst speaking certain words. We now know that human sex is determined by a pair of sex chromosomes called X and Y. Because these chromosomes do not look alike, they are sometimes called heterosomes. All other chromosomes are called autosomes.Females have two X chromosomes (XX). Males have one X and one Y chromosome (XY). Although the sex chromosomes determine the sex of an individual, it is important to realise that they do not carry all the genes responsible for the development of sexual characteristics.

During meiosis, the sex chromosomes pair up and segregate into the daughter cells. Males are called the heterogametic sex because they produce different sperm: approximately 50% contain an X chromosome and 50% have a Y chromosome. Females are called the homogametic sexbecause (usually) all of their eggs contain an X chromosome. This arrangement applies to all mammals and some insects (including Drosophila, the fruit fly commonly used in genetic experiments). However, in birds, moths, and butterflies, females are the heterogametic sex with the XY genotype (or XO, meaning the second sex chromosome may be absent). In some species, sex determination depends on a complex interaction between sex chromosomes and autosomes, or between inherited factors and environmental ones. The sex of some turtles, for example, depends on the temperature of the sand in which eggs are laid: those laid in sand warmed by the Sun develop into females; those laid in cool sand in the shade develop into males.

In humans, the father's sperm determines the sex of the baby: if a baby inherits a Y chromosome from its father it will be a boy; if it inherits an X chromosome from its father it will be a girl. So the sex of a baby depends on which sperm fertilises the egg cell: a sperm with an X chromosome or one with a Y chromosome. However, there are cases where having a Y chromosome does not necessarily mean that an embryo will become a boy.