What is the X chromosome?
The X chromosome is one of the two sex chromosomes (gonosomes). The female has two X chromosomes (XX), the male has 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) and special proteins. DNA is the carrier of hereditary information (in the form of genes). There are about 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, you have to go a little further:
The nuclei of almost all cells in the body have a double (diploid) set of chromosomes: 46 chromosomes, consisting of 22 pairs of autosomes and two sex chromosomes. The abbreviation for the female chromosome set is therefore 46XX and for the male chromosome set 46XY.
Egg and sperm cells
Since women have only X chromosomes as sex chromosomes, each of their eggs also contains only one X chromosome. This is different for men, who have one X and one Y chromosome as gonosomes: Sperm cells carry either an X chromosome or a Y chromosome – depending on which of the two gonosomes they received from their precursor cells during meiosis.
Fertilization
Fertilization is when an egg fuses with a sperm cell (sperm). In terms of sex chromosomes, this means that the sex chromosome from the egg cell (an X chromosome) and that from the sperm cell (either X or Y chromosome) unite in a common cell, which then develops into the embryo.
Son or daughter?
The genetic sex of the child is therefore determined by the sperm cell (and therefore by the father):
- If a sperm with one X chromosome fertilizes the egg (with X chromosome), the resulting embryo carries two X chromosomes in each of its cells – it becomes a girl.
- If a sperm with a Y chromosome fuses with the egg, the cells of the resulting embryo will have one X and one Y chromosome each – it will develop into a boy.
Sex-linked genes
Where is the X chromosome located?
It is located in all cell nuclei of female and male body cells.
What problems can the X chromosome cause?
Sometimes hereditary information is found on the X chromosome, which – if faulty – leads to a health disorder or hereditary disease. In leg play for such a sex-linked hereditary disease is Duchenne’s muscular dystrophy. In this disease, the gene encoding a muscle protein is missing on the X chromosome. As a result, patients develop muscle weakness and coordination problems.
Hemophilia is also inherited on the x-chromosome. The gene that codes for a certain clotting factor is defective. The body is therefore unable to produce enough functional copies of this clotting factor. The result is an increased tendency to bleed and a prolonged bleeding time (for example in the case of injuries).
Color blindness (red-green vision deficiency) is also an x-linked recessive inherited disorder.
Another genetic disorder related to the X chromosome is Klinefelter syndrome, which can only occur in males. Affected individuals have an extra X chromosome. Thus, they have a total of three sex chromosomes (47XXY) in their cells. As a result, the testes are underdeveloped, the male breasts are often enlarged (gynecomastia) and the affected men are infertile.
Females may also have an extra (third) X chromosome (47XXX). In this case, however, the chromosome excess has no health consequences.
However, it has serious consequences if one of the two X chromosomes of women is missing or structurally defective and thus non-functional (45X0): This is when Ullrich-Turner syndrome (monosomy X) is present. Typical signs include short stature, shortened metacarpal bones, a low hairline and malformations of internal organs. Hypofunction of the ovaries and infertility can also be due to the missing X chromosome.