Pedigree of PTC taste sensitivity
The chemical phenylthiocarbamide (PTC) (OMIM 17120) tastes
bitter to most people ("tasters")
but is tasteless for others
Inability to taste PTC is sometimes
described as an autosomal
recessive trait: persons with the TT
or Tt genotypes have the "taster" phenotype, persons
with the tt 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 filled symbols. 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 grandchildren of
two unrelated grandparents, who happen to share the trait.
Among the "tasters",
identify which are homozygous (TT) and which heterozygous (Tt).
Figure ©2002 by Griffiths et al.; all text material ©2016 by
Steven M. Carr