Pedigree of PTC taste sensitivity
The chemical phenylthiocarbamide (PTC) (OMIM 17120) tastes
bitter to most people ("tasters")
but is tasteless for others
("non-tasters").
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.
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.
Homework:
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