The pea plants Mendel used in articulating his observations about dominance exhbiited the traits of simple dominance. This was fortunate for Mendel, since using eggplants intead of pea plants, for example, may have caused him to come to different conclusions about the nature of heritability.
That is, the trait he observed was that of 'simple dominance.' In his experiments, AA and Aa would produce the dominant trait A, and only the heterozygous aa would produce the recessive phenotype a. But this is not the case for all traits of all organisms.
The dominant trait of having dark purple skin, for example, in eggplants, is that of incomplete dominance. Dark purple (PP) and white skin (pp) are both the result of homozygous alleles. One might think that Pp would yield the phenotype of dark purple skin, but this is not correct. Instead, an intermediate phenotype of light purple or violet results.
This is the c ase when there are parent plants that consist of a dark purple (PP) and white (pp) plant. Both parents are homozygous; one is homozygous dominant and the other is homozygous recessive. This produces a heterozygote with an intermediate phenotype that is light purpole, rather than either dar purple or white.
Things become more interesting when two heterozygotes (Pp and Pp) are crossed with one another. A quarter of the offspring are dark purple( PP) and another quarter are white (pp). The combination of both heterozygotes produce two sets of homozygotes.
One would expect a Pp + Pp to have a 3:1 phenotypic ratio, with 3 dark purple eggplants and one white eggplant. Instead, however, there is a 1:2:1 ratio; a dark purple eggplant, two light purple eggplants, and a white eggplant, or PP, Pp, Pp, pp.
Sometimes what we observe is the phenomenon of "codominance." This is when two phenotypes fully express themselves rather than one dominating the other, or an intermediate phenotypic expression between the dominant and recessive. Both alleles, instead, express themselves fully.
This is the case with blood-types. A and B are both coded for by dominant alleles, and when an individual has both alleles, what is produced is the AB type. Those with the AB genotype therefore exhibit the AB phenotype. The individuals blood has A and B antigens in the same measure.
ABO blood-types are more complicated yet. There is AB codominance as well as AB paired with the O allele, which is a dominant-recessive relationship. O itself is only expressed homozygously, so it remains recessive in such a case.
Robinson, Tara Rodden (2010-04-13). Genetics For Dummies (Kindle Location 1438). Wiley. Kindle Edition.