102 ❯ STEP 4. Review the Knowledge You Need to Score High
One genetic condition in humans that exhibits incomplete dominance is
hypercholesterolemia—a recessive disorder (hh) that causes cholesterol levels to be many
times higher than normal and can lead to heart attacks in children as young as two years old.
Those who are HH tend to have normal cholesterol levels, and those who are Hh have
cholesterol levels somewhere in between the two extremes. As with many conditions, the
environment plays a major role in how genetic conditions express themselves. Thus, people who
are HH do not necessarily have normal cholesterol levels if, for example, they have poor
diet or exercise habits.
One important side note—try not to confuse the terms blending “hypothesis” and blend-
ing “inheritance.” The latter is another name for incomplete dominance, whereas the former
was the theory on heredity before Mendel worked his magic. The blending “hypothesis” says
that the HH and hh extremes can never be retrieved. In reality, and according to blending
inheritance, if you were to cross two Hh individuals, the offspring could still be HH or hh,
which the blending “hypothesis” says cannot happen once the blending has occurred.Codominance
Codominanceis the situation in which both alleles express themselves fully in a heterozy-
gous organism. A good example of codominance involves the human blood groups: M, N,
and MN. Individuals with group M blood have the M glycoprotein on the surface of the
blood cell; individuals with group N blood have N glycoproteins on the blood cell; and
those with group MN blood have both.This is not incomplete dominance because both
alleles are fully expressed in the phenotype—they are codominant.Other Forms of Inheritance
Polygenic Traits
Another interesting form of inheritance involves polygenic traits,or traits that are affected
by more than one gene. Eye color is an example of a polygenic trait. The tone(color),
amount(blue eyes have less than brown eyes), and position(how evenly distributed the pig-
ment is) of pigment allplay a role in determining eye color. Each of these characteristics is
determined by separate genes. Another example of this phenomenon is skin color, which is
determined by at least three different genes working together to produce a wide range of
possible skin tones.Multiple Alleles
Many monogenic traits (traits expressed via a single gene) correspond to two alleles, one
dominant and one recessive. Other traits, however, involve more than two alleles. A classic
example of such a trait is the human blood type. On the most simplistic level, there are four
major blood types: A, B, AB, and O. They are named based on the presence or absence of
certain antigens on the surface of the red blood cells. The gene for blood type has three pos-
sible alleles (multiple alleles): IA, which causes antigens A to be produced on the surface of
the red blood cell; IB, which causes antigens B to be produced; and i, which causes noanti-
gens to be produced. The following are the possible genotypes for human blood type: IAi
(type A), IAIA(type A), IBi (type B), IBIB(type B), IAIB(type AB), ii (type O). Type AB blood
displays the codominanceof blood type. As we saw in MN blood groups, both the A and the
B alleles succeed in their mission—their antigens appear on the surface of the red blood cell
(Figure 10.3). Analyzing blood type can be really complex because human blood types
involve not only multiple alleles (IA, IB, and i) and codominance (type AB blood), but clas-
sic dominance of IAand IBover i as well.