Close relatives (or kinship) refer to ancestors who have the same ancestry within three generations. If they marry, it is called consanguineous marriage. Couples who are closely related may get the same genes from their ancestors and pass them on to their children. If this gene is autosomal recessive, its children may get sick because they are homozygotes of the mutation. So inbreeding will increase the risk of some autosomal recessive genetic diseases. In addition, the incidence of polygenic genetic diseases is high, such as hypertension, schizophrenia, congenital heart disease, anencephaly, epilepsy and so on. The mortality rate of children born of consanguineous marriage is also significantly higher than that of children born of non-consanguineous marriage. To this end, many countries have passed laws prohibiting consanguineous marriage.
However, Australian data also show that cousins can give birth to healthy babies after marriage. However, China's laws prohibit it.