Figure 1

Pedigrees. Here a square represents a male, and a circle a female. (a) For a dominant disorder, such as Huntington's disease, represented by the allele symbol H, an individual need only inherit the mutant allele from one parent (that is, be heterozygous) to be affected. In such cases, affected individuals almost always are in fact heterozygous for the mutant allele, and they transmit it, and the disease, to half the offspring. (b) For a recessive disorder, in this case sickle cell anemia, shown by the β^S allele symbol, the disease is seen only in individuals homozygous for the allele (that is, who have inherited the allele from both parents); they are typically the offspring of two heterozygotes (carriers of the disease).