Consanguinity (especially between 2 first cousins) exposes the couple to various genetic diseases at a risk of 25% per each pregnancy. This phenomenon is based on Mendelian autosomal-recessive inheritance. When the couple is from the same family or clan, or even from the same ethnic origin , there is a probability that "dormant" abnormal genes will pass through generations of intermarriage, thus rendering the couple as "carriers" and producing a risk of 25% for the offspring to be affected.