Figure 12.6 Alkaptonuria is a recessive genetic disorder in which two amino acids, phenylalanine and tyrosine,
are not properly metabolized. Affected individuals may have darkened skin and brown urine, and may suffer joint
damage and other complications. In this pedigree, individuals with the disorder are indicated in blue and have the
genotype aa. Unaffected individuals are indicated in yellow and have the genotype AA or Aa. Note that it is often
possible to determine a person’s genotype from the genotype of their offspring. For example, if neither parent has
the disorder but their child does, they must be heterozygous. Two individuals on the pedigree have an unaffected
phenotype but unknown genotype. Because they do not have the disorder, they must have at least one normal
allele, so their genotype gets the “A?” designation.