Traits Controlled by Genes with Three
or More Alleles
Genes with three or more alleles are said to have
.For example, in the human population, the ABO blood
groups (blood types) are determined by three alleles, IA, IB, and
i.The letters Aand Brefer to two carbohydrates on the surface
of red blood cells. In the iallele, neither carbohydrate is present.
The IAand IBalleles are both dominant over i. But neither IAnor
IBis dominant over the other. When IAand IBare both present
they are codominant. Even for traits controlled by genes with
multiple alleles, an individual can have only two of the possible
alleles for that gene. Figure 14shows how combinations of the
three different alleles can produce four different blood types—A,
B, AB, and O. Notice that a person who inherits two ialleles has
type O blood.
Traits with Two Forms Displayed
at the Same Time
For some traits, two dominant alleles are expressed at the same
time. In this case, both forms of the trait are displayed, a phe-
nomenon called .Codominance is different from
incomplete dominance because both traits are displayed.
The situation of human ABO blood groups, as discussed
above, is an example of co-dominance. The genotype of a person
who has blood type AB isIAIB,and neither allele is dominant
over the other. Type AB blood cells carry both A-and B-types of
carbohydrate molecules on their surfaces.
codominance
alleles
multiple
178 CHAPTER 8Mendel and Heredity
Different combinations of the three alleles IA, IB,and iresult in four different blood
phenotypes, A, AB, B, and O. For example, a person with the alleles IAand i
would have blood type A.
Figure 14 Multiple alleles control the ABO blood groups
IAIA IAIB IAi
IAIB IBIB IBi
IAi IBi ii
IA IB i
IA
IB
i
Possible alleles
Possible alleles
Blood types A AB B O
