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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.
The shape of the haemoglobin molecule is controlled by two alleles:
There are three phenotypes
Sickle cell anaemia, a severe form where all the red blood cells are affected. Sickle cell anaemia 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.
Symbols for codominant alleles
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.
© Paul Billiet 2014