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 thought by some to be inherited 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.
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. Parents II-3 and II-4 are evidently heterozygous (Tt)
carriers.
Homework: Among the "tasters", identify which are
homozygous
(TT)
and which heterozygous (Tt).
Figure ©2002 by Griffiths et al.; all text
material ©2012
by Steven M. Carr