The secret of inheritance

Christiane Fux studied journalism and psychology in Hamburg. The experienced medical editor has been writing magazine articles, news and factual texts on all conceivable health topics since 2001. In addition to her work for, Christiane Fux is also active in prose. Her first crime novel was published in 2012, and she also writes, designs and publishes her own crime plays.

More posts by Christiane Fux All content is checked by medical journalists.

Why are men more likely to develop certain hereditary diseases? And why can two brown-eyed people have a blue-eyed child - but not the other way around? Our inheritance mechanisms provide the answer.

The genetic makeup of humans, animals and plants is twofold: Whether eye color, shoe size or the shape of the lips - we have almost every gene involved in duplicate. One comes from the father, one from the mother. Geneticists call alleles the two versions of a gene. The only exception are genes that are located on the male Y chromosome. Only one of them is contained in each male body cell.

Dominant or recessive

Most traits are inherited through a dominant-recessive inheritance. This means that one of the two alleles is stronger. When it comes to coloring the eyes, for example, the predisposition to brown eyes is stronger than that to blue. A child who received a gene for blue eyes from one parent but one from the other for brown eyes will therefore get brown eyes. However, this child can later have offspring with blue eyes if his partner also gives their child a gene for blue eyes. Two people with brown eyes who are carriers of blue eyes can therefore father a blue-eyed child. The probability of this is one in three. Two people with blue eyes, on the other hand, usually only have blue-eyed children.

For a few genes, none of the alleles are stronger. The result is a hybrid of both properties. This form of inheritance is called intermediary.
As early as 1865, the Augustinian monk Gregor Johann Mendel had determined the basic mechanisms of inheritance by crossing peas of different colors and sizes.

However, many properties do not depend on just one gene, but on many. This applies to body size, hair color and the color of the complexion. In this respect, inheritance of most traits is complicated.

Double execution, double security

The double genetic makeup has decisive advantages. On the one hand, the human genes are mixed up again and again, which benefits evolution. Many hereditary diseases can be balanced in this way: If one gene is broken, the other healthy gene alone takes on its intended tasks in the body.

Some diseases, such as hemophilia, are therefore more common in men than women: They are passed on via the X chromosome, of which a man only has one, while a woman has two. Men therefore cannot balance the diseased gene and the disease breaks out in them. A woman only gets sick if the defective gene is on both X chromosomes.