Analysis of Pedigrees
The limited number of offspring in most human families means that clear Mendelian ratios are usually impossible to discern in a single pedigree. Pedigree analysis requires a certain amount of genetic sleuthing, which requires recognizing patterns associated with different modes of inheritance.
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RECESSIVE TRAITS Recessive traits normally appear with equal frequency in both sexes and appear only when a person inherits two alleles for the trait, one from each parent. If the trait is uncommon, most parents of affected offspring are heterozygous and unaffected; consequently, the trait often skips generations (Figure 3.15). Frequently, a recessive allele may be passed on for a number of generations without the trait appearing in a pedigree. When both parents are heterozygous, approximately ¼ of the offspring are expected to express the trait, but this ratio will not be obvious unless the family is large. In the rare event that both parents are affected by an autosomal recessive trait, all the offspring will be affected.
When a recessive trait is rare, most people outside the family are homozygous for the normal allele. Thus, when an affected person mates with someone outside the family (aa × AA), usually none of the children will display the trait, although all will be carriers (i.e., heterozygous). A recessive trait is more likely to appear in a pedigree when two people within the same family mate; in this case, there is a greater chance of both parents carrying the same recessive allele. Mating between closely related people is called consanguinity. In the pedigree shown in Figure 3.15, individuals III-
CONCEPTS
Autosomal recessive traits appear in pedigrees with equal frequency in males and females. Affected children are commonly born to unaffected parents who are heterozygous carriers of the gene for the trait, and the trait often skips generations. Recessive traits appear in pedigrees more frequently among the offspring of consanguineous matings. Autosomal dominant traits also appear in both sexes with equal frequency. An affected person has an affected parent, and the trait does not skip generations. Unaffected persons do not transmit the trait.
CONCEPT CHECK 8
Recessive traits often appear in pedigrees in which there have been consanguineous matings because these traits
tend to skip generations.
appear only when both parents carry a copy of the allele for the trait, which is more likely when the parents are related.
usually arise in children born to parents who are unaffected.
appear equally in males and females.
b
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DOMINANT TRAITS Autosomal dominant traits normally appear in both sexes with equal frequency, and both sexes are capable of transmitting these traits to their offspring. Every person with a dominant trait must have inherited the allele from at least one parent (unless the person carries a new mutation), so autosomal dominant traits do not skip generations (Figure 3.16).
Sex-