The concept of inheritance of mental disability .. The coupling of chromosomes, non-recombinant genes or recessive genes leads to a mental retardation and is likely to occur in conjugal marriages

Penrose sees that a child may inherit the mental disability from his parents and grandparents through dominant genes, which lead to mental disability, which appears in all generations by 1: 3 according to Mendel's inheritance law. This disability is produced by coupling chromosomes or non- Harmonious.
The child may inherit the mental disability from his / her parents and grandparents through retinal genes that lead to mental retardation (ie, recessive genes are not shown to either parent but are capable of inheriting them) (Dr. Al-Karouti, Dr. Al-Sartawi, It produces pathological properties and biochemical disorders that move from one parent to the fetus, damaging the brain and nervous system. The inheritance of these characteristics is of the highest order and is likely to occur in the marriage of relatives rather than the marriage of non-relatives, and causes about 20% to 40% of severe and moderate mental disabilities. (Dr.Marsi 1996).
From the above it becomes clear to us that the inheritance of mental disability means either the child has inherited a biochemical characteristic that damage the brain cells and nervous system, and thus lead to mental disability, and we see that this property is likely to appear significantly relatives.