Unit 2: Genetic Processes
Lesson 7: Incomplete Dominance and Codominance
Complete Dominance:
Complete Dominance: a situation where an allele will determine the phenotype, regardless of the
presence of another allele.
Ex: PP – purple
Pp – purple
pp – white
Mendel’s work with what he called “typical plants” (homozygous dominant) and “atypical plants”
(homozygous recessive) allowed him to see complete dominance and refute the idea of why traits of
parents did not blend in the offspring. Yet, blended inheritance is common in nature. In snapdragons,
one of the genes that that controls flower colour (C) has one allele for red flowers (CR) and another
allele for white flowers (CW). The homozygous plant (CRCR) produces red pigment flowers, and the
homozygous plant (CWCW) produces white pigment flowers; however, the heterozygous plant (CRCW)
produces both the red and white pigment making pink flowers. The heterozygous phenotype is a
blending of the two homozygous phenotypes and known as incomplete dominance.
Incomplete Dominance:
Incomplete Dominance: a situation where neither allele dominates the other and both have an
influence on the individual; results in partial expression of both traits
Codominance:
Codominance: a situation where both alleles are expressed fully to produce offspring with a third
phenotype.
Ex: With shorthorn cattle, when a red bull is crossed with a white cow, it will produce a roan calf.
Roan calves have intermingled pure white and pure red hair.
ABO Blood Types – Codominance and Dominance:
Human blood type is both a codominant and dominant genetic trait. There are four possible blood
types and different alleles.
If you have the allele IA, you will have the antigen A present on your red blood cells.
If you have the allele IB, you will have the antigen B present on your red blood cells.
If you have the allele i, you will have no antigens present on your red blood cells.
If you have blood type
blood type A, it means you
have the antigen A on your
red blood cells
(erythrocytes) and you
have the Anti-B antibodies
in your blood plasma. The
antibodies will produce an
immune response against
antigen B. Alternatively,
blood type B will produce an
immune response against
blood types containing the
A antigen. It is because of
this reason, that you must
be careful when doing blood
transfusions.
PHENOTYPE
GENOTYPE
Blood Group A
Blood Group AB
IAIA
IAi
IBIB
IBi
IAIB
Blood Group O
ii
Blood Group B
Which type is the universal donor?
Which type is the universal recipient?
ABLE TO RECEIVE
BLOOD FROM:
Type A or O
ABLE TO GIVE BLOOD
TO:
Type A or AB
Type B or O
Type B or AB
Type AB, A, B, or O
Type AB
Type O
Type A, B, AB or O
The I is used to represent
immunoglobin which the
protein on the surface of
the RBC.
Homework Questions:
1. Explain in your own words the meaning of dominance, codominance, and incomplete dominance.
2. In some chickens, the gene for feather colour is controlled by codominance. The allele for
black is FB and the allele for white is Fw. The heterozygous phenotype is known as erminette.
a) What is the genotype for black chickens?
b) What is the genotype for white chickens?
c) What is the genotype for erminette chickens?
d) If two erminette chickens are crossed, what is the probability that they would have a black
chick? A white chick?
3. A geneticist notes that crossing a round radish with a long radish produces oval radishes. When
oval radishes were crossed with oval radishes, the F2 generation had these phenotypes: 100
long, 200 oval, and 100 round radishes. Use symbols to explain the results obtained for the F1
and F2 generations.
4. How would Mendel’s conclusions have differed if he had worked with plants whose alleles were
incomplete dominant?
5. Thalassemia is an inherited anemic disorder in humans. Individuals can exhibit major anemia,
minor anemia, or neither. Assume only one gene is involved with two alleles in the inheritance of
this condition. What type of inheritance is thalassemia governed by? What are the
corresponding gentoypes to the three scenarios?
6. List the possible genotypes for an individual with type A blood.
7. Suppose a father of blood type A and a mother of blood type B have a child of type O. What
are the possible blood types of the mother and father?
8. Suppose a father of blood type B and a mother of blood type O have a child of type O. What
are the chances that their next child will be blood type O? Type B? Type A? Type AB?
9. Why is blood type inheritance an example of both codominace and complete dominance?
10. Another characteristic of human blood is the presence of absence of a blood protein called the
Rh factor. People with the protein are Rh+ and those without it are Rh-. Use the Internet and
other resources to answer the following questions:
a) What are the genotypes of individuals who are Rh- and Rh+? Is this an example of complete
dominance, incomplete dominance, or codominace?
b) How can the Rh blood type of two parents be of concern during a pregnancy?how can
possible harmful complications be avoided?
SOLUTIONS: