Disorder caused by the presence of two recessive (homozygous) mutant (abnormal) genes on an autosome. If only one mutant recessive (heterozygous) gene is present the person is not affected with the disease but is a carrier of it. One fourth of the children of two heterozygous (carrier) parents will be affected and another one half will be carriers. All of the children of two homozygous affected persons will be affected. The children of one normal and one affected with the disease will all be carriers. When close relatives (e.g., first cousins) marry, the chance that an offspring will inherit an abnormal recessive gene from both parents is increased, as it is among persons marrying within certain ethnic groups (e.g.,Tay- Sachs disease among Ashkenazic Jews).