Each cell has 46 chromosomes. The baby inherits 23 chromosomes from its father and 23 chromosomes from its mother. The diagram below shows the 23 chromosome pairs of a man, after extracting them from the nucleus of a cell and preparing them by a special method. If, in a chromosome pair, one chromosome has a bad gene and the other chromosome has a normal gene, then the baby will not have a defect. However, if both chromosomes in the pair have bad genes, then the baby will have a defect.
Close relatives share the same bad genes because of their common grandparents.
Therefore, in a blood marriage the chances of such bad genes coming together as a pair is greater than in an unrelated marriage. In fact, in a blood marriage, the risk of producing a deformed child is double that of an unrelated marriage. Educating your teenage children about the problems of blood marriages. A marriage between the children of a brother and sister does not produce children with birth defects. A marriage between children of two sisters does not produce children with birth defects.