X Chromosome: Definition & Disorders

Instructor: Angela Lynn Swafford

Lynn has a BS and MS in biology and has taught many college biology courses.

In humans, the X chromosome is one example of a sex chromosome. If you are a girl, you have two X chromosomes. If you are a boy, you have only one X chromosome. Learn more about the X chromosome and disorders associated with it in this lesson.

Definition and Function

All living things have chromosomes in their cells. Chromosomes are structures that carry genetic information in the form of DNA. You, as well as other human individuals, have 46 chromosomes in each of your body cells. Two of these 46 chromosomes are called sex chromosomes because they are important for determining whether you are a boy or a girl.

There are two different sex chromosomes: the X chromosome and the Y chromosome. A girl will have two X chromosomes (written as XX), but a boy will have one X and one Y chromosome (written as XY). This means that no matter your gender, you will always have at least one X chromosome. The Y chromosome is very small and has only a few genes (50 to 60). These genes are mostly involved with male characteristics because only males have a Y chromosome.

Picture of all 46 chromosomes in a human male. X and Y are the two human sex chromosomes. (A female would have two X chromosomes and no Y chromosome.)
Human Chromosomes

On the other hand, everybody has an X chromosome. The X chromosome is larger and has 800 to 900 genes. These genes are called X-linked genes and make proteins associated with many non-gender related traits and processes, including:

  • Pigmentation of skin and eyes
  • Vision
  • Blood clotting
  • Digestion of materials using lysosomes
  • Making collagen
  • Making tooth enamel

In females, one X chromosome is turned off in a process called X-inactivation. This occurs so that the same amount of proteins are made from X-linked genes in both females and males. Remember that males only have one X chromosome, while females have two. So if both X chromosomes were active in females, they would be making twice as many proteins as males.


There are many different types of disorders that are associated with the X chromosome. Some of them have to do with numbers of X chromosomes, such as having extra copies or missing a copy. Three examples are shown in the table below.

Disorder Cause Symptoms
Turner Syndrome One X chromosome missing in females (written as XO instead of XX) Do not mature sexually, not fertile, short stature
Triple X Syndrome Extra X chromosome in females (written as XXX) Usually healthy and fertile females
Klinefelter Syndrome Extra X chromosome in males (written as XXY instead of XY) Enhanced feminine features, reduced genitals, unable to reproduce

Other X-chromosome-associated disorders are due to mutations in X-linked genes. Remember, X-linked genes are just those that are on the X chromosome. For many X-linked genetic disorders, a female needs to have two copies of the mutation, one on each X chromosome. Males only have one X chromosome, so they only need one copy of the mutation to have the disorder. This means that most X-linked disorders are more common in males than in females because it is much easier to inherit one copy of a mutant gene than two copies of the same mutant gene. Three examples of X-linked genetic disorders are listed in the table below.

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