Pedigree of PTC tasting
The chemical compound phenylthiocarbamide (PTC) (OMIM 17120) tastes
bitter to most people ("tasters")
but is tasteless to others ("non-tasters"). Inability to taste PTC
is traditionally described as an autosomal
recessive trait: persons with the TT
or Tt genotypes have the "taster" phenotype, persons
with the t t genotype have the "non-taster"
phenotype is therefore described as a "recessive" trait.
The pattern of inheritance of PTC
sensitivity is shown in the above pedigree of two related
families. Squares are males, circles are females; "tasters"
are indicated by open symbols, "non-tasters"
by closed symbols. Note that the "non-taster" phenotype shows the
classic pattern of a recessive
trait: it is present in generation I, "skips" generation II, and reappears in generation III in two of the
grandchildren of two unrelated grandparents who happen to share
Among the "tasters",
identify which are homozygous (TT) and which heterozygous (Tt).
Figure ©2002 by Griffiths et al.; all text material ©2013 by
Steven M. Carr