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What Is a Sex Chromosome?

By A.M. Boyle
Updated May 21, 2024
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A sex chromosome is a stringlike structure within the nucleus of a cell that determines the sex of an organism. In humans, each cell normally has one pair of sex chromosomes designated by the letters X and Y. The presence of the Y chromosome distinguishes male from female traits. Most people have only one pair of these chromosomes in each cell, but some individuals have more or less, resulting in certain genetic disorders.

Composed of twisted strands of DNA, chromosomes are the building blocks of an organism’s genetic makeup. In the human body, the nucleus of each cell contains 22 chromosomes called autosomes and one pair of sex chromosomes. A single human sex chromosome is labeled either X or Y, and these chromosomes pair up as either XX or XY. An XX designation signifies that the person will have female traits, while an XY pair indicates that the individual will carry male characteristics. The particular biology of these chromosomes and the way they link together means it is possible to have a pairing of two Xs but not two Ys.

For females, during the embryonic stage of development, only the egg cell is permitted to carry two active X chromosomes. For all other cells that contain two X chromosomes, one will be deactivated. During conception, when the egg cell containing two X chromosomes meets the sperm cell that contains one X and one Y chromosome, a random pair is made, including either two X or an XY combination.

Not only does a sex chromosome determine a person’s reproductive organs, but it is also responsible for other characteristics that are normally associated with either men or women. For instance, a person who carries an XY sex chromosome generally develops thick facial hair, a prominent Adam’s apple, and a comparatively deep voice. In the opposite regard, individuals who have an XX chromosome normally develop distinctive female characteristics, such as mammary glands, enlarged breasts, and finer, thinner facial hair.

There are situations when a person has an abnormal sex chromosome pair. For instance, sometimes an extra X attaches to the pair, or occasionally, there is only one chromosome rather than two. In those instances, the person will likely develop certain disorders, such as Turner’s syndrome.

In other cases, the X chromosome can carry an abnormality that manifests as a particular disease, such as hemophilia or Duchene’s muscular dystrophy. These types of conditions are considered recessive in that they are nearly always exclusive to men. This is because, for a woman to display symptoms, both X chromosomes would have to be defective. On the other hand, because a man only has one X chromosome, if it is defective, he will likely demonstrate symptoms of the disorder.

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