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The X and Y chromosomes determine whether a person, or any other mammal, is male or female. Together they are known as sex chromosomes.

Chromosomes are grouped in pairs and humans have 23 of them, with 22 autosome pairs and one pair being the sex chromosomes. Women have two X chromosomes, while men have one X and one Y chromosome.

As the mother always gives an X chromosome, a baby’s sex is determined by whether the sperm of the father carries either an X or Y chromosome. If a sperm with an X chromosome fertilizes the egg, the baby will be female (XX). If a sperm with a Y chromosome fertilizes the egg, the baby will be male (XY).

> X-linked inheritance patterns

Because women have two X chromosomes and men only one, the chance of inheriting an X-linked disease depends on the baby’s gender. However, it is not as simple as to only distinguish between affected or healthy, as some women may be carriers of the disease (they do not experience any symptoms but can transmit the disease to their male children).

Examples of X-linked dominant disorders:

Examples of X-linked recessive disorders:

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