Test analysis: Close relatives refer to lineal blood relatives and collateral blood relatives within three generations. China's marriage law clearly stipulates that it is forbidden to marry lineal blood relatives and collateral blood relatives within three generations. Why? The reason is that close relatives are more likely to have the same recessive genetic disease-causing gene, and children born to close relatives are more likely to have genetic diseases. For example, the incidence of single-gene recessive genetic diseases of children born to close relatives is 7.8 ~ 62.5 times that of non-close relatives. The probability of congenital malformation and stillbirth is 3-4 times higher than that of the general population. Children's mental decline, suffering from many congenital diseases, such as congenital stupidity, is very harmful. It is necessary to do a good job in pre-marital examination according to the marriage law and maternal and child health care law promulgated by our government, and do a good job in pre-marital and pre-pregnancy eugenics. Therefore, parents prohibit marriage in order to reduce the probability of genetic diseases in offspring, not to ensure that offspring have no genetic diseases.