Many pharmacogenetic traits are transmitted by autosomal recessive inheritance (also referred to as autosomal codominant inheritance) and a number of those traits are relatively common. Among autosomal recessive traits that cause disease, most are rare and harmful, but the severity of the idiosyncrasy of the susceptible pharmacogenetic phenotype depends on the nature and degree of exposure to environmental agents. Susceptible persons are offspring of phenotypically normal parents who are carriers of the trait. Other susceptible family members occur among siblings of the index case, and not among other relatives. In matings between two phenotypically normal heterozygotes, the probability of having a susceptible child (the segregation frequency) averages 1 /4 (Figure 5.3) and that of having a carrier among phenotypically normal children is 2/3. There is an elevated incidence of consanguinity among the parents. The incidence of susceptible persons is the same in males and females.
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