All the genetic information is contained in the cell nucleus of each individual somatic cell, packed into chromosomes. In each cell of our body, there are 50,000 to 100,000 genes which are distributed over 46 chromosomes. These 46 chromosomes occur as 23 pairs. They differ in size and structure. The last in the row are the X and the Y chromosome. They decide the sex of a person and are, therefore, also called gonosomes (sex chromosomes).
Women have two X chromosomes and men have an X chromosome and a Y chromosome in their cell nuclei. The karyotype (Greek: karyon - cell nucleus) is the appearance of a set of chromosomes and represents the number and structure of the chromosomes of a person.
If the structure and/or number of chromosomes changes without the loss or gain of genetic material, this is a balanced chromosomal change. Such changes have no effect on the somatic cells of an organism and, therefore, do no cause disease in the affected person. During the maturation of gametes (development of mature egg cells or sperm), unbalanced egg cells or sperm (gametes) may be created.
In this case, the blastoderm created after fertilisation or the first cell of the nascent human (zygote) carries an unbalanced set of chromosomes, which results in an undesirable development of the embryo. This leads either to the absence of a pregnancy, as the nascent embryo does not have the ability to be implanted in the uterine lining or to miscarriages or births of children with deformities and mental retardation.
In men, balanced chromosomal changes can also lead to azoospermia or oligozoospermia and, therefore, to infertility.
If someone carries an unbalanced set of chromosomes, it generally develops in accordance with its own genetic programme which is associated with a particular external appearance, organ malformations and mental retardation.
If the number of sex chromosomes is affected, this is a matter of numerically unbalanced sets of chromosomes which are only associated with mild external changes. The carriers do not usually stand out. This chromosomal peculiarity is only discovered through a chromosome analysis which is performed because of an unfulfilled desire to have children.
Boys who have an additional X chromosome in their cell nuclei do not generally stand out after birth and lead a normal adult life. Due to a testicular atrophy, they are infertile. In a few cases, men with the karyotype 47,XXY can have children with the help of the methods of reproductive medicine.
Women with a karyotype 45,X are small and have a few special external characteristics. Sometimes, organ malformations occur, such as heart defects or hypothyroidism. Their ovaries rearrange themselves into cords of connective tissue during childhood. These women are infertile.
In some women, only a few cell nuclei have a set of chromosomes with a missing X chromosome. The rest of the cell nuclei have normal chromosomes. Fertility is more or less restricted in these women.
Women with this set of chromosomes are healthy and fertile, but may pass on an additional X chromosome to their children. Sons will then have the karyotype 47,XXY (see above) and daughters the same set of chromosomes as their mothers.