X chromosome

Eva Rudolf-Müller is a freelance writer in the medical team. She studied human medicine and newspaper sciences and has repeatedly worked in both areas - as a doctor in the clinic, as a reviewer, and as a medical journalist for various specialist journals. She is currently working in online journalism, where a wide range of medicine is offered to everyone.

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The X chromosome is one of two sex chromosomes (the other is the Y chromosome). Women have two X chromosomes, while men have one X and one Y chromosome. Read everything you need to know about the X chromosome here!

What is the X Chromosome?

The X chromosome is one of the two sex chromosomes (gonosomes). The woman has two X chromosomes (XX), the man one X and one Y chromosome (XY).

The X chromosome is larger than the Y chromosome. Like all chromosomes, it consists of deoxyribonucleic acid (DNA, English abbreviation: DNA) and special proteins. DNA is the carrier of genetic information (in the form of genes). There are around 2000 genes on the X chromosome.

What is the function of the X chromosome?

The sex chromosomes (X and Y chromosomes) determine the genetic sex of a person. To understand this, one has to go back a little further:

The cell nuclei of almost all cells in the body have a double (diploid) set of chromosomes: 46 chromosomes, consisting of 22 pairs of autosomes and 2 sex chromosomes. The abbreviation for the female chromosome set is therefore 46XX and for the male chromosome set 46XY.

Egg and sperm cells

Only the germ cells - i.e. the woman's egg cells and the man's sperm cells - have a simple (haploid) set of chromosomes: the progenitor cells of egg and semen cells are still diploid. They then go through the so-called reduction division (meiosis), in the course of which the double set of chromosomes is halved. Haploid daughter cells emerge from meiosis - the mature germ cells. They each have 22 autosomes and 1 sex chromosome (gonosome).

Since women only have X chromosomes as sex chromosomes, each of their egg cells also contains only one X chromosome. It is different with men who have an X and a Y chromosome as gonosomes: The sperm cells either have an X chromosome or a Y chromosome - depending on which of the two gonosomes they receive from their precursor cells during meiosis to have.

fertilization

During fertilization, an egg cell fuses with a sperm cell (sperm). With regard to the sex chromosomes, this means: The sex chromosome from the egg cell (an X chromosome) and that from the sperm cell (either X or Y chromosome) combine in a common cell, from which the embryo then develops.

Son or daughter?

The genetic sex of the child is determined by the sperm cell (and thus by the father):

• When a sperm with an X chromosome fertilizes the egg cell (with an X chromosome), the resulting embryo has two X chromosomes in each of its cells - it becomes a girl.
• If a sperm with the Y chromosome fuses with the egg cell, the cells of the resulting embryo each have an X and an X chromosome - a boy develops.

Sex-linked genes

All genes that are located on the sex chromosomes are called sex-linked genes. Contrary to the assumption, however, they not only contain the genetic information for sexual characteristics (formation of the ovaries, male chest hair, etc.). The X chromosome (as well as the Y chromosome) also contains genes that code for characteristics that have nothing to do with gender, for example the blueprints for the blood clotting factors.

Where is the X chromosome located?

It is found in all cell nuclei of female and male body cells.

What problems can the X chromosome cause?

Sometimes there is also genetic information on the X chromosome that - if incorrect - leads to a health disorder or hereditary disease. Duchenne muscular dystrophy is the leg game for such a sex-linked hereditary disease. In this disease, the gene that codes for a muscle protein is missing on the X chromosome. As a result, patients develop muscle weakness and incoordination.

Hemophilia is also inherited as an x-linked trait. The gene that codes for a specific coagulation factor is defective. The body can therefore not produce enough functional copies of this coagulation factor. The result is an increased tendency to bleed and a prolonged bleeding time (e.g. in the case of injuries).

However, not every person who carries the “hemophilia gene” will also get it. The hereditary disease is inherited recessively: In women, the disease only occurs if both of their X chromosomes have the hemophilia gene (which happens very rarely). More often, the hemophilia gene is only found on one X chromosome, while the second X chromosome has a healthy gene. The woman in question then does not have the hemophilia. She can pass this predisposition on to her child (via the defective X chromosome). If it is a son, he will definitely get the hemophilia (he does not have a second and possibly healthy X chromosome that could compensate for the defect).

Color blindness (red-green poor eyesight) is also an x-linked recessive inherited disorder.

Another genetic condition associated with the X chromosome is Klinefelter syndrome, which can only occur in men. Those affected have an extra X chromosome. So they have a total of three sex chromosomes (47XXY) in their cells. As a result, the testicles are underdeveloped, the male breasts are often enlarged (gynecomastia) and the affected men are sterile.

Women can also have an additional (third) X chromosome (47XXX). The chromosome excess has no health effects here.

However, it has serious consequences if one of the two X chromosomes in women is missing or structurally defective and therefore not functional (45X0): This is Ull-rich-Turner syndrome (monosomy X). Typical signs are short stature, shortened metacarpal bones, a deep hairline and malformations of internal organs. An underfunction of the ovaries and infertility can also be traced back to the missing X chromosome.

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