How is gender determined

Inheritance of gender

Sex determination

In some living beings, gender is determined by environmental influences. In the case of Mississippi alligators, for example, the temperature when the eggs are hatching is crucial. Only females hatch at breeding temperatures of around 30 degrees Celsius, and mostly males at temperatures of around 34 degrees Celsius. The same applies to various species of turtles.

During the human embryonic development in the womb, neither temperature nor other external factors play a gender-determining role. In all mammals, and thus also in humans, the sex is determined genetically. Like all genetic information, that of sex is also on chromosomes.

The human organism has two types of sex chromosomes: the X chromosome and the Y chromosome. In addition to the 44 chromosomes (autosomes) that are identical in both sexes, every cell in the woman's body contains two X chromosomes as sex chromosomes (gonosomes). The man's body cells, on the other hand, contain an X chromosome and a Y chromosome.

In addition to the body cells, men and women have cells that specialize in reproduction: egg cells and sperm. Egg cells and sperm contain only half of the chromosomes and therefore only one sex chromosome each. Every egg cell in a woman therefore contains an X chromosome.

Men, on the other hand, produce two types of sperm: one half contains an X chromosome, the other half a Y chromosome. That is why the sex of a person is determined at the moment of conception.

If a sperm with one X chromosome penetrates the egg cell, the embryo carries two X chromosomes and develops into a female organism. If, on the other hand, a sperm with a Y chromosome fertilizes the egg cell, the result is XY and a male organism develops.

Ultimately, the father is the one who determines the sex of his offspring, albeit without any possibility of influence. The sex determination remains random. A 50:50 chance for female or male.

The Y chromosome

The man's Y chromosome is much smaller than the X chromosome and, with a few dozen genes, also carries much less genetic information than the X chromosome, which has more than 1000 genes. Some genes on the Y chromosome and the X chromosome are identical, they are homologous.

There is no homologous equivalent on the X chromosome to other genes on the Y chromosome. This also includes the so-called SRY gene. A British research team identified this gene in the 1990s, which directs the development of the embryo in the direction of the male organism. They named the gene the "sex-determining region of the Y chromosome" (SRY).

At the beginning of pregnancy, embryos do not yet show any gender-specific differences. Male and female embryos initially develop identical-looking genital bumps and folds as a preliminary stage of the genital organs.

These predispositions can still develop in a feminine and masculine direction, with the development towards the feminine organism being the basic form. Additional genetic information is necessary for the development of a male appearance. If this information is missing, the embryo automatically develops into a female organism.

In X, Y embryos, the SRY gene is switched on from around the seventh week of pregnancy. The SRY gene sets in motion a number of processes that lead to the male embryo. The testicles develop. It is in them that the most masculine of all hormones is produced for the first time: testosterone. From now on the points are on man.

Sex-linked hereditary diseases

In addition to their role in determining sex, the sex chromosomes, mainly the larger X chromosome, carry a lot of genetic information that has nothing to do with gender development. For example, genes for various hereditary diseases can also be found on the X chromosome. One of these hereditary diseases is hemophilia.

If you follow the pedigrees of European royal families in which hemophilia occurs more frequently, you will quickly see that it is predominantly male family members that are affected by the disease. From this one can conclude that it is a so-called X-linked inheritance of the disease.

There is a gene on the X chromosome that is responsible for the formation of a blood clotting factor. In hemophilia this gene is defective and therefore blood clotting is disrupted.

Women have two X chromosomes. If the blood coagulation gene is defective on one of these X chromosomes, a sufficient amount of the blood coagulation factor can still be produced by the corresponding gene on the second X chromosome.

The likelihood that the gene is defective on both chromosomes is very small. However, if a man has inherited a defective gene on the X chromosome, the disease will definitely break out because he does not have a second X chromosome to compensate for the defect. The corresponding gene is missing on the small Y chromosome.

In the noble houses of Europe, Leopold, the son of Queen Victoria (1819 to 1901) of England, was probably the first hemophiliacs. He had inherited the mutated and therefore defective gene from his mother. Viktoria's daughters did not suffer from the disease due to their second healthy X chromosome, but carried the defective gene on the second X chromosome.

Through their marriages, the bleeding gene came into the royal families of Prussia, Russia and Spain. The old custom of strengthening political alliances through royal marriages helped the hemophilia to spread noticeably across several royal families.

Other X-linked diseases are color blindness and Duchenne muscular dystrophy. People suffering from Duchenne muscular dystrophy only have an average life expectancy of around 20 years. Those affected suffer from increasing muscle weakness and coordination disorders.

Sexual development disorders

A maldistribution of the sex chromosomes occurs in around 0.3 percent of all newborns. The pattern of the sex chromosomes then deviates from the normal XX for "female" and XY for "male". These disorders are not hereditary diseases, but errors in the distribution of chromosomes on the sex cells during their formation. The disorders are therefore innate and not inherited.

If an X chromosome is missing, one speaks of the X0 monosomy or Turner syndrome. Those affected appear feminine, but do not develop functional ovaries or secondary sexual characteristics. In addition, symptoms such as short stature, heart, vascular, kidney and bone malformations can appear.

Another example is the so-called triple X syndrome. The X chromosome occurs three times here. XXX women are usually physically inconspicuous and fertile.

If the X chromosome appears twice when the Y chromosome is present, one speaks of the XXY state or Klinefelter syndrome. Affected people develop into men through the Y chromosome. Their secondary sexual characteristics may be normal, but Klinefelter males cannot produce sperm.

The Y chromosome can also be present in duplicate. The Diplo-Y men are taller than average, but otherwise physically inconspicuous. In most cases your fertility is unrestricted.