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Codominance
Mendel’s original work seemed to show there were two types of genes: dominant and recessive.
We have learned that this description is overly simple. A case of this kind is referred to as
codominance.
MN Blood Types
An example of codominance is found in the MN blood group in humans. The two alleles for this
blood group, M an N are codominant, with MM producing type M blood, NN producing type N
blood, and MN producing type MN blood.
Steps to Solving Genetic Problems:
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What genetic relationship is shown? (i.e. incomplete dominance, codominance, dominant/recessive, multiple
alleles, sex-linked, etc.)
Write a key. (Dominant traits are underlined.)
Write the cross.
Show the Punnett square/ FOIL method.
Answer the question(s) and circle.
1. Suppose that a person who is homozygous for the M blood type marries a person who
is homozygous for the N type. Show the phenotypic and genotypic ratios of the F1
generation.
2. Now show the phenotypic and genotypic ratios of the F2 generation.
Sickle Cell Anemia
The genetic disease “sickle-cell anemia” in humans, is caused by two codominant alleles. Two
normal genes (N) produce normal red blood cells. Two sickle cell genes (S) cause the disease to
occur. Individuals afflicted with this disease have “sickled” red blood cells that are unable to
carry oxygen properly. The heterozygous is not fatal and seems to offer protection against
malaria infection.
1. If a man and women are tested and found to be carriers of the sickle allele, what is the
probability that they will have a child who has sickle cell anemia?
2. Show a cross between a man who has sickle cell anemia and a women who is a carrier of
the trait. What percentage of the offspring will have the trait?
3. Show a cross between a normal male and woman who has sickle cell anemia. What
percentage of their offspring will have sickle cell anemia?
Thalassemmia
An anemic condition in humans called thalassemmia is controlled by codominant alleles.
Homozygous for the defective allele (TT) have a severe anemia (thalassemia major), whereas
heterozygotes (TN) have a mild anemia (thalassemmia minor). Normal individuals are
homozygous (NN) and do not have anemia.
1. Show a cross between a man that is homozygous for thalassemmia and a woman that
does not have anemia. What are the percentages of all phenotypes and genotypes?
2. What are the genotypic and phenotypic ratios for the F2 generation?