Name:_______________________
Exception to Mendel’s Laws
Per:___________ Date:__________
Incomplete Dominance and Codominance
In all of Mendel’s experiments, he worked with traits where a single gene controlled the trait and
where one allele was always dominant to the other. Although the rules that Mendel derived from
his experiments explain many inheritance patterns, the rules do not explain them all. There are
in fact exceptions to Mendel’s rules, and these exceptions usually have something to do with the
dominant allele.
One exception to Mendel’s rules is that one allele is always completely dominant over a
recessive allele. Sometimes an individual has an intermediate phenotype between the two
parents, as there is no dominant allele. This pattern of inheritance is called incomplete
dominance.
An example of incomplete dominance is the color of snapdragon flowers. One of the genes for
flower color in snapdragons has two alleles, one for red flowers and one for white flowers. A
plant that is homozygous for the red allele will have red flowers, while a plant that is
homozygous for the white allele will have white flowers. On the other hand, the heterozygote
(offspring) will have pink flowers. Neither the red nor the white allele is dominant, so the
phenotype of the offspring is a blend of the two parents. It is pink.
Pink snapdragons are an example of incomplete dominance.
An example of a co-dominant trait is ABO blood types, named for the carbohydrate attachment
on the outside of the blood cell. In this case, two alleles are dominant and completely expressed
(designated IA and IB), while one allele is recessive (i). The IA allele encodes for red blood cells
with the A antigen, while the IB allele encodes for red blood cells with the B antigen. The
recessive allele (i) doesn’t encode for any antigens. An antigen is a substance that provokes an
immune response, your body’s defenses against disease, which will be discussed further in the
Diseases and the Body's Defenses chapter. Therefore a person with two recessive alleles (ii)
has type O blood. As no dominant (IA and IB) allele is present, the person cannot have type A or
type B blood.
There are two possible genotypes for type A blood, homozygous (IAIA) and heterozygous (IAi),
and two possible genotypes for type B blood (IBi and IBIB). If a person is heterozygous for both
the IA and IB alleles, they will express both and have type AB blood with both antigens on each
red blood cell. This pattern of inheritance is significantly different than Mendel’s rules for
inheritance because both alleles are expressed completely and one does not mask the other.
Name:_______________________
Exception to Mendel’s Laws
Per:___________ Date:__________
Questions: Answer using complete sentences below.
1) What are the two exceptions to Mendel’s Laws described in the article?
2) Compare the two exceptions.
3) In snapdragons, flower color is controlled by incomplete dominance. The two alleles are red
(R) and white (W). The heterozygous genotype is expressed as pink.
a) What is the phenotype of a plant with the genotype RR? ___________
b) What is the phenotype of a plant with the genotype WW? ___________
c) What is the phenotype of a plant with the genotype RW? ___________
4) What phenotypes would you expect from a cross between a red bull (RR) and a white cow
(WW) in codominance? Show the Punnett Square.
5) If two alleles are dominant and completely expressed (designated IA and IB), what is this
blood group?
6) Give an example of incomplete dominance and complete dominance seen in the
environment. Explain.