Human genetic diseases that result from mutation in one specific gene, referred to as monogenic diseases, display different inheritance patterns depending on the nature and chromosomal location of the alleles that cause them. One characteristic pattern is that exhibited by a dominant allele in an autosome (that is, one of the 22 human chromosomes that is not a sex chromosome). Because an autosomal dominant allele is expressed in the heterozygote, usually at least one of the parents of an affected individual will also have the disease. Diseases caused by dominant alleles often appear later in life, after reproductive age. If this were not the case, natural selection would have eliminated these alleles during human evolution. An example of an autosomal dominant disease is Huntington’s disease, a neural degenerative disease that generally strikes in mid- to late life. If either parent carries a mutant HD allele, each of his or her children (regardless of sex) has a 50 percent chance of inheriting the mutant allele and being affected (Figure 6-32a).

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A recessive allele in an autosome exhibits quite a different segregation pattern. Both parents must be heterozygous carriers of an autosomal recessive allele in order for their children to be at risk of the disease. Each child of heterozygous parents has a 25 percent chance of receiving both recessive alleles, and thus being affected; a 50 percent chance of receiving one normal and one mutant allele, and thus being a carrier; and a 25 percent chance of receiving two normal alleles. A clear example of an autosomal recessive disease is cystic fibrosis, which results from a defective chloride-channel gene known as CFTR (Figure 6-32b). Related individuals (e.g., first or second cousins) have a relatively high probability of being carriers for the same recessive alleles. Thus children born to related parents are much more likely than those born to unrelated parents to be homozygous for, and therefore affected by, a rare autosomal recessive disorder.

The third common pattern of inheritance is that of an X-linked recessive allele. A recessive allele on the X chromosome will most often be expressed in males, who receive only one X chromosome from their mother, but not in females, who receive an X chromosome from both their mother and their father. This leads to a distinctive sex-linked segregation pattern in which the disease is exhibited much more frequently in males than in females. For example, Duchenne muscular dystrophy (DMD), a muscle degenerative disease that specifically affects males, is caused by a recessive allele on the X chromosome. DMD exhibits the typical sex-linked segregation pattern in which mothers who are heterozygous, and therefore phenotypically normal, can act as carriers, transmitting the DMD allele, and therefore the disease, to 50 percent of their male progeny (Figure 6-32c).