A normal human being has 23 pairs of chromosomes, which contain 22 pairs of autosomes and one pair of sex chromosomes. Sex chromosomes are related to gender and autosomes are not related to gender and are common to both sexes. A genetic disease with an inherited gene located on an autosome is called an autosomal genetic disease. There are dominant and recessive modes of inheritance, and these genetic diseases are controlled by two genes, one from the father and one from the mother. Usually the dominant gene is represented by an uppercase letter, such as A, and the recessive gene is represented by a lowercase letter, such as a. If the gene is heterozygous for Aa and shows no disease and purely heterozygous for aa, the disease is controlled by a recessive gene, which is an autosomal recessive disorder, mostly associated with consanguineous marriage, such as albinism, phenylketonuria, hepatomegaly, galactosemia, and other diseases. If both parents are genetically heterozygous for Aa and do not have the disease, the chance of the child having the disease is 1 in 4.