Penetrance versus Expressivity
Egg colour of the extinct Hawai'ian Oo'Aa bird was controlled
by a single locus, BLU, where the B allele is dominant
to the b allele.
As part of the recovery strategy, a pure-breeding blue-egg bird (BB) from one island
was crossed with a pure-breeding white-egg bird (bb) from another
the offspring are all Bb
heterozygotes, they are all expected to show a uniform,
blue phenotype like that of the BB parent. However, if penetrance and / or expressivity vary,
three patterns of variation from expectation are possible.
varies [left], the Bb genotype sometimes does not penetrate
through to the phenotype: some Bb eggs are blue
just like those of their BB parents, others are white, like
those of their bb
parents, despite presence of the B allele*. "Lack of penetrance"
is historically used as an explanation for
individuals whose phenotypes do not reflect their
inferred genotypes, e.g., a dominant trait that
appears to "skip" a generation in a pedigree
If expressivity varies
[middle], the Bb genotype
will not be expressed uniformly in the
phenotype: all eggs are blue, but the exact shade of
blue varies among individuals with the same genotype.
Variable expressivity is frequently attributable to
environmental factors or variation elsewhere in the
If expressivity and
penetrance both vary [right], there
may be a continuous gradient of phenotypes between white
and blue eggs. Note that lack of penetrance can be
considered as an extreme from of expressivity, in
which the range of expression includes non-expression.
*HOMEWORK: How would
you distinguish the results of a Bb x Bb cross from
those of a cross Bb x bb
cross, where the parents are blue and white
respectively, and half the offspring are blue Bb eggs and half are
white bb eggs?