Question
If siblings engage in that kind of activity, will the child born be mentally retarded?
Answer
Hello, marriage between individuals who are closely related, often referred to as cousin marriage. When a couple has a common ancestor within the great-grandparent generation, they are considered to have engaged in cousin marriage. For example, first cousin marriage is a common form of cousin marriage. Most countries do not encourage or prohibit cousin marriage, as this type of marriage can increase the risk of congenital defects and genetic diseases in offspring. After cousin marriage, the mortality rate of offspring is higher, and there is often a prevalence of intellectual disability, malformed children, and genetic disease patients. This is because cousin marriage couples inherit more identical genes from their common ancestors, and the chance of harmful recessive genes meeting and pairing increases, thus raising the birth rate of unhealthy offspring. The World Health Organization estimates that each person carries an average of 5 to 6 recessive genetic disease-causing genes. In non-cousin marriages, since there is no blood relationship between the couple, they typically carry different recessive disease-causing genes, making it less likely to form a recessive disease-causing gene pair (patient). However, in cousin marriages, the likelihood of carrying the same recessive disease-causing genes is higher, which increases the incidence of genetic diseases in offspring. The table below lists several recessive genetic diseases, showing that the incidence rate in offspring of cousin marriages is several times higher than that in non-cousin marriages. Additionally, cousin marriage is also associated with polygenic genetic diseases such as hypertension and schizophrenia.