Penetrance versus Expressivity
Egg colour of the rare Hawai'ian Oo'Aa bird is 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 that differ from expectation
If penetrance 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
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 genome.
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?