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Higher Biology – Sex Linkage

This week we have been learning a little about sex linkage. A person’s sex is determined by the sex chromosomes they inherit from their parents. Either, XX (female) or XY (male). Any other chromosomes the person inherits from their parents are known as autosomes. In a large population the sex ratio is roughly 50% male and 50% female.

During meiosis, the X and Y chromosomes behave as a homologous pair. However, they do differ in a number of ways. For example, the X chromosome is larger than the Y chromosome. The genes which are carried by the X chromosome but not by Y are said to be sex-linked. The arrows below show the region where these sex linked genes can be found.

Characteristics which are known to be sex linked include, colour blindness and haemophilia in humans. In Drosophila, eye colour is a sex linked characteristic. Here is an example of a cross;


A white-eyed female drosophila is crossed with a red-eyed male. This is a sex-linked trait in which red eye is dominant to white eye.

Always write out “XX” and “XY” when you read that the condition is sex linked.  Then follow the steps below;


A female who is heterozygous for such a condition is known as a carrier.


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