The human X chromosome has the gene for Duchenne Muscular Dystrophy on its short arm. In mitosis the number of chromosomes is halved. Autosomal dominant inheritance has surviving affected individuals, in a heterozygous state. In X linked mental retardation the X chromosome shows a fragile site at the tip of its short arm. Familial hypercholesterolaemia is autosomal dominant.
Alpha thalassemia is autosomal recessive.
Adult polycystic kidney disease is autosomal dominant. Hurler syndrome is autosomal recessive. Sickle cell anaemia is inherited as an autosomal dominant condition. Most isolated neural tube defects belong to this category. The recurrence risk in his type, does not depend on the previous incidence of the same condition in the family.