We typically think of dominant alleles as expressing their phenotype fully. In genetics, this is called complete penetrance. However, this is not always the case with the phenotypes of dominant alleles. Sometimes we observe incomplete penetrance. Penentrance is defined as the probability of an individual exhibiting a phenotype associated with its dominant allele.
Some alleles exhibit incomplete penetrance. This means that the individuals have a lower probability of exhibiting the phenotypic trait of a dominant allele. This is quite important for medical purposes. The BRCA1 gene, which is associated with breast cancer, exhibits incomplete penetrance. 70 percent of women carrying this allele will have breast cancer by the age of 70. There is a highly increased risk, but exhibiting the symptoms are uncertain. More specifically, this allele exhibits 70 percent penetrance.
Not only do different alleles exhibit different degrees of penetrance, but their expressivity, or degree of expression, differs, depending on the individual who has the gene. This is the case with polydactyly, for example. Some individuals have inchoate, barely functioning stubs for extra digits, and others have fully functional extra fingers.
There are other complications to the picture of simple dominance which Mendel worked with. The flowers of his pea plants came in two colors: white and purple. Purple was both fully dominant and fully penetrant. If any of the plants had any purple gene, therefore, it fully expressed itself without inferference from the recessive white color.
This is more complicated when there are more than two alleles, however. Let us consider the case of rabbits. They possess a C gene which has 4 alleles. These alleles determine the amount of pigment produced in the rabbit’s hair shaft.
We have C+, which produces a brown rabbit, c, which produces an albino rabbit when it is homozygous, c^ch, which produces a gray rabbit, and c^h, which produces white irabbits with dark hair on their noses, ears and feet.
The C+ brown rabbit is considered the ‘normal’ phenotype. Others are mutations. This phenotype is most common and dominant. Furthermore, although there are 4 different alleles that can be found among rabbits, each rabbit only contains two at any given time. It is dominant over all three of the other alleles. Any rabbit with the brown allele will be brown.
Gray is dominant over all other non-brown alleles. Heterozygous rabbits with a gray and a non-brown allele will therefore be gray. Heterozygous gray/albino rabbits, however, are lighter than homozygous gray rabbits. As noted before, albino is only straightforwardly expressed when the rabbit is homozygous for both alleles.
Robinson, Tara Rodden (2010-04-13). Genetics For Dummies (Kindle Location 1438). Wiley. Loc. 1241- Kindle Edition.