In the last article we looked at inheritance of a characteristic coded for by a single gene which has one dominant allele and one recessive allele. However, sometimes alleles can be codominant or there can be more than two alleles for a gene. This increases the number of possible phenotypes. Let’s look at each case individually.
If two alleles show codominance, it means that neither allele is recessive and that they are both expressed in the phenotype if the organism is heterozygous. A classic example of codominance is the inheritance of sickle-cell anaemia. Sickle-cell anaemia is an inherited condition which affects the haemoglobin protein. The sickle haemoglobin protein means that an affected person’s red blood cells are sickle-shaped instead of the usual biconcave-disc shape. The means that the oxygen-carrying capacity of red blood cells is decreased and they do not flow through blood vessels as easily.
The allele for normal haemoglobin (HN) and the allele for sickle haemoglobin (HS) are codominant. For someone to have full sickle-cell anaemia they must have two HS alleles. If a person is heterozygous, they will have the sickle-cell trait: they will produce some sickle haemoglobin, but the allele for normal haemoglobin is also expressed so they will generally stay healthy. They also have the ability to pass on the HS allele to their offspring, as shown below. In this specific case, the phenotypic ratio of the F1 generation is 1:2:1.
Another example of codominance could be flower colour. If there were codominant red flower and white flower alleles, the possible phenotypes would be red, pink (heterozygous), and white.
So far, we have only talked about genes where there are two possible alleles. Sometimes, there are more than two possible alleles, and therefore more possible genotypes and phenotypes. A good example of this is blood group in humans. There are three alleles for blood group: IO (blood group O), IA (blood group A), and IB (blood group B). And just to complicate things, IA and IB are codominant, meaning that there is a fourth possible blood group (blood group AB). IO is the recessive allele.
In the example below, a parent with blood group AB and a parent with blood group O have a 50% chance of having a child with blood group A and a 50% chance of having a child with blood group B. The phenotypic ratio is 1:1.
- Codominant alleles are both expressed in the phenotype if the organism is heterozygous.
- If there are more than two possible alleles for a gene, there are more possible phenotypes. A good example of this is human blood group.
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