According to relevant data, there may be 1/2 genes identical between each child and his parents. And 1/2 gene may be the same in all siblings. There are 1/4 possible similarities between grandchildren and uncles. However, there is a possible similarity of 1/8 between cousins. In some hereditary diseases, the pathogenic genes are recessive. If one of the two sides carries this gene and the pathogenic gene of the other side is hidden, then the offspring will not get sick. Only when both husband and wife carry this recessive gene will their offspring get sick. Therefore, inbreeding increases the probability of recessive genetic diseases, such as congenital deafness's disease, cerebellar malformation and phenylketonuria. It will also increase the incidence of polygenic diseases, such as hydrocephalus, spina bifida, anencephaly, schizophrenia, congenital heart disease and epilepsy.
Therefore, Article 6 of China's Marriage Law stipulates: "lineal blood relatives and collateral blood relatives within three generations are forbidden to get married." Many countries have also passed laws prohibiting consanguineous marriage. Lineal consanguinity refers to the relationship between parents and children, grandparents and grandchildren, and grandparents and grandchildren. Collateral blood relatives within three generations include compatriots, uncles (aunts) nephews (female), uncles (aunts) nephews (female) and cousins.
In ancient times, not all children born of close relatives were normal, and they were only probabilistic events in terms of physical structure and mental health. Natural marriage will greatly increase the probability of illness, but a few lucky people are healthy. After all, the ancient things are a little far away from us, and we can't fully understand the previous things.