Word Roots and Origins polygenic from the Greek poly, meaning "many," and the Greek genesis, meaning "origin"

figure 12-10

Many characters, such as height, weight, hair color, and skin color, are polygenic. Often, the environment strongly influences polygenic characters.

Patterns of Inheritance

Biologists learn about genetic diseases by analyzing patterns of inheritance, the expression of genes over generations. Pedigrees help to interpret patterns of inheritance. For example, if a trait is autosomal, it will appear in both sexes equally. If a trait is sex-linked, it is usually seen only in males. Most sex-linked traits are recessive.

If a trait is autosomal dominant, every individual with the trait will have a parent with the trait. If the trait is recessive, an individual with the trait can have one, two, or neither parent exhibit the trait.

If individuals with autosomal traits are homozygous dominant or heterozygous, their phenotype will show the dominant characteristic. If individuals are homozygous recessive, their phenotype will show the recessive characteristic. Two people who are heterozygous carriers of a recessive mutation will not show the mutation, but they can produce children who are homozygous for the recessive allele.

The pedigree in Figure 12-9 shows that the condition of cystic fibrosis is inherited as an autosomal recessive. Individuals such as the four people in the fifth generation in the pedigree are called carriers because they have one copy of the recessive allele but do not have the disease. Although carriers do not express the recessive allele, they can pass it to their offspring.

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