What is the X chromosome? Learn about the X chromosome definition, how many X chromosomes males and females have, and some of the disorders associated with the X chromosome.

## What Is the X Chromosome?

Inherited genetic material in the form of deoxyribonucleic acid (DNA) is present in the nucleus of eukaryotic cells. DNA is wrapped around [protein structures](/content/academy/lesson/factors-affecting-protein-structure.html) named histones in the form of chromatin. During cell division, chromatin is condensed into distinct bodies called **chromosomes**. Chromosomes contain genes, which code for proteins that are vital for cell function and life. Humans have 46 chromosomes in their somatic cells, 23 from each parent. 22 pairs of these chromosomes, namely chromosomes 1-22, are called [autosomal chromosomes](/content/academy/lesson/autosomes-definition-lesson-quiz.html). The last pair of chromosomes are labeled as sex chromosomes.

There are two types of sex chromosomes. X and Y. If one has the genotype XX, then they are genetically female. However, if their genotype is XY, then they are genetically male. The X and Y chromosomes differ in structure, where the Y chromosome is significantly smaller than the X chromosome, and they were given their names due to their resemblance to the letters X and Y when their DNA is duplicated. The X chromosome carries approximately 900 genes, while the Y chromosome carries 55.

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An Illustration of the Human X Chromosome

## Sex Chromosomes Definition: X Chromosome Gender

The terms [gender and sex](/content/academy/lesson/sex-and-gender-in-society-differences-preferences-characteristics.html) are widely misused in science and in our everyday lives. Gender is often used in place of sex, which is a biological term. Biological sex refers to the individual's genetic makeup in terms of the presence of either the XX genotype or the XY genotype. On the other hand, gender is the cultural and social expression of roles that refer to men and women.

All humans must have at least one X chromosome, which is inherited from the mother. The chromosome inherited from the father is what determines biological sex, and it can be either the X chromosome or the Y chromosome. The inheritance of the Y chromosome accounts for the acquisition of the XY genotype in the offspring, yielding a genetically male offspring. The absence of the Y chromosome and the inheritance of the X chromosome from the father yields the XX genotype in the offspring, which is followed by the development of the female reproductive tract. The X chromosome encodes sex-linked characteristics, as well as some immune functions.

### How Many X Chromosomes Do Males Have?

While the male sex possesses one X and one Y chromosome in their cells (XY genotype), some disorders such as Klinefelter syndrome account for the presence of an extra X chromosome (XXY genotype). The presence of the Y chromosome in Klinefelter individuals is responsible for the development of the [male reproductive system](/content/academy/lesson/male-reproductive-system-functions-organs-anatomy.html) due to the expression of the SRY gene. However, the extra X chromosome is responsible for some setbacks in terms of bodily functions. Klinefelter patients present with symptoms such as:

- Reduced testicular size leading to reduced testosterone
- [Gynecomastia](/content/academy/lesson/what-is-gynecomastia-causes-symptoms-treatment.html) or enlarged breast tissue
- Reduced facial and body hair growth
- Low or absent sperm counts.

### How Many X Chromosomes Do Females Have?

In most cases, the female sex is characterized by the presence of 2 X chromosomes. However, a process that occurs in somatic cells called X-inactivation turns one of the X chromosomes into a structure called a [Barr body](/content/academy/lesson/barr-body-definition-formation.html), thus partially inactivating it. Disorders stemming from either the presence of an extra X chromosome or the absence of one can affect females in different ways including:

- Turner syndrome: largely brought about by the presence of only one X chromosome, thus yielding a genotype of XO. In some cases, segments of the X chromosome carrying important genes are missing due to errors in cell division, thus bringing about symptoms of Turner syndrome. Turner syndrome can have serious medical implications, such as heart failure, and poor development of the female reproductive system.
- Triple X syndrome: brought about by the presence of an extra X chromosome, yielding a genotype of XXX. This disorder usually goes undiagnosed due to its lack of serious implications in most cases. However, in rare cases, it can cause developmental delays and cognitive impairment.
- Swyer syndrome: brought about by the female having an XY genotype, with the Y chromosome containing impaired genes for male sexual development. Therefore, affected individuals possess external female genitalia with a normal uterus and Fallopian tubes but lack functional gonads (ovaries). Patients diagnosed with Swyer syndrome are typically treated with hormone replacement therapy at a young age.

## X Chromosome Disorders

Disorders in cell division stemming from errors in chromosome segregation account for the phenomenon of having the wrong number of X chromosomes. Mutations in genes carried on the X chromosome could be responsible for many other recessively inherited disorders such as color blindness and hemophilia. Since the male sex usually only has one copy of the X chromosomes, the presence of only one disease-causing allele results in the said disease's manifestation. However, two copies of the faulty allele are required for disease manifestation in females. These disorders are known as sex-linked [genetic disorders](/content/academy/lesson/what-are-genetic-disorders-definitions-descriptions.html).

### Red-Green Color Blindness

Red-green color blindness, or sometimes referred to as deuteranopia, is an X-linked sex disorder that affects an individual's ability to perceive red, green, and sometimes yellow colors. This disorder is caused by mutations in the genes OPN1LW or OPN1MW present on the X chromosome. These genes code for pigmentation required by retina cells in the eye to perceive colors. Red-green color blindness frequently goes undiagnosed until it is identified by a doctor. There are many online picture charts available for individuals to self-check.

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Color Blindness Test

### Hemophilia

Hemophilia is another example of an X-linked sex disorder, which is caused by mutations in the F8 and F9 genes on the X chromosome. The F8 and F9 genes code for factor 8 and factor 9 clotting proteins, and therefore mutations to them can cause blood clotting disorders. Mutations in the F8 genes cause abnormal concentrations of Factor 8 proteins in the blood, thus causing hemophilia A, while F9 mutations cause abnormal concentrations of Factor 9 proteins and subsequently hemophilia B. Hemophilia B is normally only found in males.

Even though it is a recessively inherited disorder, the presence of only one mutated allele could still cause symptoms of the disorder in carrier females. Hemophilia can result in a myriad of medical complications such as bleeding into the joints, brain, skin, urine, and frequent nosebleeds. Hemophilia is treated by supplementing the patient with doses of the missing clotting factors caused by this disorder.

## Lesson Summary

- The X chromosome is a sex chromosome that also codes for some immune system functions.
- The inheritance of 2 copies of chromosome X results in the female sex phenotype, while the inheritance of one X chromosome and one Y chromosome results in the male sex phenotype.
- Sex is not to be confused with gender. Sex is the biological term used to differentiate between XX and XY genotypes, while gender is associated with the cultural and societal implications brought about by biological sex.
- Disturbances in the number of inherited X chromosomes result in disorders including Klinefelter syndrome, Swyer syndrome, Turner syndrome, and triple X syndrome.
- Mutations in genes carried on the X chromosome can result in sex-linked disorders including hemophilia and red-green color blindness.