The problem of the inheritance of genes
It is believed that closely-related marriages dramatically increase the likelihood of hereditary genetic diseases in children of such parents. Marriage is akin to if men and women have a common ancestor in the previous 3-4 generations. The higher the degree of relationship, the higher the risk. Parents and their children, brothers and sisters have the highest degree of kinship.
Each child receives from the mother and father a certain set of genes. When a child is conceived as the result of relations of relatives, defective gene in one parent can't be replaced by a healthy gene of another because close relatives have a fold up set of genes. Thus, the risk of inheriting a child damaged genes and hereditary genetic diseases increases, as well as the risk of a child with a birth defect, the stillborn child and the risk of miscarriage.
The influence of different factors
Often carriers of the recessive gene damaged themselves healthy and recognize the presence of it until they have a sick child, and that such a gene is inherited. In ordinary marriage, the probability of having a child with disorders is 2-4%, and in the related marriage increased to 4-6%. At first glance, the difference is not so great. But if the family previously had a hereditary genetic disorder, the risk increases to an average of 12-14%. To identify such a possibility can advice vracha genetics that will recommend to pass the required tests.
Overall, statistics shows that close relatives often are born talented and gifted children as mentally and physically retarded, and normal children less. It's kind of a lottery. In addition, there is no guarantee that a healthy child in the future and the offspring will be healthy.
The danger of incest between the family brother and sister is much higher than between cousins and second cousins. Latest in more ancient times it was quite common, especially in the birth of nobles and rulers.
Especially dangerous are the relationships between relatives for several generations. Over time, genetic material is deteriorating, it accumulated mutations, damage, and requires new genes to prevent degeneration.
If the family incest has occurred once, the probability of having a normal child is quite high. But if it is practiced from generation to generation, the likelihood of deformities and birth defects may reach 50% or even 100%. The more genetic diseases in the family, the more complicated the situation.