Penetrance versus Expressivity
Egg colour in Hawai'ian Oo'Aa birds is controlled by a single locus, P, where P is dominant to p. A pure-breeding line of purple-egg birds (PP) is crossed with a pure-breeding line of white-egg birds (pp). Because the offspring are all Pp heterozygotes, they are therefore all expected to show a uniform, purple phenotype*. Two variations from expectation are possible. If expressivity varies [top right], the Pp genotype will not be expressed uniformly: all eggs are purple, but some are more or less purple than others. If penetrance varies [left], the Pp genotype sometimes does not manifest itself: some of the eggs are purple, like the parental eggs, others are white. If expressivity and penetrance both vary [bottom right], there may be a continuous gradient of phenotype between white and purple eggs.
Note that lack of penetrance is an extreme from of expressivity, in which the range of expression includes non-expression.
*HOMEWORK: How would you distinguish this result from a cross Pp x pp, where the parents are purple and white respectively, and half the offsping are purple Pp eggs and half are white pp eggs?