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The X and Y chromosomes. The sex chromosomes determine the biological sex of an individual: XX in women and XY in men. The non-sex chromosomes (numbered 1 to 22) are known as the autosomes.

Use in clinical context

The X chromosome has many more genes compared to the Y chromosome, which means there are only a handful of genetic conditions linked to the Y chromosomes compared to the X. The X chromosome is present in different numbers in males and females, which results in different inheritance patterns of X-linked conditions compared to autosomal conditions. For instance, recessive variants on the X chromosome are always expressed in males as they only have one copy.  In females, however, both copies need to be the same to be expressed because there are two X chromosomes. This is why conditions such as colour blindness are more common in males than females. It is possible, although rare, for individuals to have any number of sex chromosomes from one to five. Aneuploid individuals display different symptoms with a varying degree of associated health problems depending on the number and combination of X and Y chromosomes that they have.

Last updated on 31st May, 2019