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Not all genes have dominant and recessive alleles. Some have alleles that are both expressed together in the heterozygote individuals.
Co-dominant alleles have three phenotypes, one for each genotype.
In humans there are a number of conditions that are co-dominant.
Example: Sickle Cell Anaemia
Sickle cell anaemia is a genetic disease which affects the heamoglobin of the red blood cells. Haemoglobin is normally a ball-shaped molecule but the sickle cell allele makes it form long strands. The red blood cell carrying these molecules distorts into characteristic long shapes.
The shape of the haemoglobin molecule is controlled by two alleles:
There are three phenotypes
Symbols for codominant alleles
Because both alleles are expressed in the heterozygote they are considered codominant and both take a capital case letter. An index letter identifies the allele.
Because the heterozygotes have their own phenotype this gives rise to different proportions amongst their offspring compared with crosses between heterozygotes for dominant and recessive alleles.
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