In most civilized countries, marriage unions between close relatives are prohibited: parents and children, sisters and brothers. Such marriages fall under the category of "incest" and lead to the birth of sick offspring with serious genetic abnormalities and deformities. But marrying distant relatives is not the best choice either. Marriages between cousins, second cousins, and even more distant relatives in the family tree can easily provoke the appearance of abnormalities in the offspring.
Elementary laws of genetics
Charles Darwin was the first to think about issues of heredity. He was married to his cousin Emma Wedgwood. The couple had 10 children. Three babies died in early childhood, others were in very poor health. Darwin grieved every death and illness of his son. He suspected that the reason for such poor health of almost all children was his relationship with his wife. Later, these thoughts were reflected in the writings of the scientist.
Much time has passed since then, and genetics has advanced far in its development. Now geneticists know for sure that each of us has dominant and recessive genes. In relatives, these genes are very similar, which gives a huge share of the likelihood of unhealthy offspring when crossing. And the most dangerous are not so much dominant genes as recessive genes. These are hidden, as it were, dormant signs of a particular quality (not only the color of the eyes or the length of the legs, but also various hereditary diseases).
The more different gene sets are crossed - which is possible only in unrelated marriages - the less chance a child has of getting a hereditary disease that is "encoded" in the recessive gene of one of the parents. And vice versa: the closer the relationship between husband and wife, the higher the chances of stillbirth, miscarriage or the birth of a baby with mental or physical deformities.
Facts as a percentage
In modern genetics, crossing of closely related individuals is called inbreeding. The higher the coefficient of inbreeding, the more chances the recessive genes of inherited diseases will meet. So for an uncle and niece, he is 1/8, a cousin and cousin - 1/16, second cousins - 1/32, fourth cousins - 1/64.
Even if the degree of kinship is greater, the inbreeding coefficient will still be sufficient for a child in a pair of distant relatives to die while still in the womb or be born mentally retarded or physically underdeveloped. This percentage and the related regularity was described by the geneticist of the highest category, professor and author of many scientific works on genetics Elena Leonidovna Dadali. [С-BLOCK]
Distant kinship is also dangerous because the spouses may not even be aware of their blood ties and the presence of an unwanted recessive gene. The birth of a sick child then becomes a complete surprise and a very painful blow to the well-being of the couple. When people who do not have any family ties are married, the percentage of coincidences of mutant recessive genes is negligible, and therefore children are born healthy.
Mutant recessive genes are a very dangerous thing, hidden from prying eyes like a time bomb. They can cause a variety of mutations, ranging from albinism and deafness to severe ailments such as hemophilia or Tay-Sachs disease. The latter is widespread among some peoples of the world and is fraught with severe lesions of the nervous system. A sick child never lives longer than 4 years.
As is known, representatives of the Russian royal family suffered from hemophilia. But these 2 ailments are just a tiny tip of the iceberg of hereditary diseases that can be triggered by the meeting of mutant recessive genes. It can only be prevented by genetic analysis, which is a normal pre-wedding practice in many civilized countries of the world.