1. CODOMINANCE
Some genes have alleles that are both expressed in the heterozygote individuals
© 2016 Paul Billiet ODWS

2. Codominant alleles have three phenotypes
Sickle cell anaemia is a genetic disease
Affects the hemoglobin of the red blood cells
Hemoglobin is normally a ball-shaped molecule
The sickle cell allele makes it form a long strands
The red blood cell carrying these molecules distorts into characteristic long shape.
© 2016 Paul Billiet ODWS

3. Blood cells Sickle cell anemia Blood smear (normal)
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Sickle cell anemia
© 2016 Paul Billiet ODWS

4. The genetics of sickle cell anemia
The shape of the hemoglobin molecule is controlled by two alleles
Normal Hemoglobin allele
Sickle Cell Hemoglobin allele
© 2016 Paul Billiet ODWS

5. The genetics of sickle cell anemia
There are three phenotypes
Normal
Normal individuals have two normal hemoglobin alleles
Sickle cell anemia, a severe form where all the red blood cells are affected.
Sickle cell anemia patients have two sickle cell alleles in their genotype
Sickle cell trait, a mild condition where 50% of the red blood cells are affected.
Sickle cell trait individuals are heterozygotes, having one of each allele.
© 2016 Paul Billiet ODWS

6. Symbols for codominant alleles
Both alleles are expressed in the heterozygote both take a CAPITAL CASE letter
An index letter identifies the allele
Therefore:
Normal hemoglobin allele is HbN
Sickle cell allele is HbS .
© 2016 Paul Billiet ODWS

7. Codominant genotypes Genotypes Phenotypes HbNHbN Normal hemoglobin
HbNHbS
Sickle cell trait
HbSHbS
Sickle cell anemia
© 2016 Paul Billiet ODWS

8. Unusual proportions
In codominance, heterozygotes have their own phenotype
This gives rise to different proportions amongst offspring of some genetic crosses
Phenotypes
Sickle cell trait x
Genotypes
HbNHbS
Gametes
HbN
HbS
HbNHbN
HbSHbS
Offspring
Normal
Sickle cell anemia
Proportions
25%
50%