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Introduction to Pedigrees

Author(s): Lisa M. Meffert, PhD

Albinism: Genotype the Affected Individual

The first step in working out the pedigree of an autosomal recessive trait is to genotype the affected individuals. Here, we are using an arbitrary coding system of "A" for the dominant normal allele and "a" for the recessive allele that confers albinism. Since both "a" alleles are required to cause albinism, the genotype of any individual expressing the trait must be homozygous for "a."

Because the parents of the first generation did not express albinism, we know they must have at least one "A" each. And since we know those same parents also produced affected offspring, we know they also must have at least one "a" each. Thus, the parents in the first generation must be heterozygotes.