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Co-Dominance - Lesson Summary

This is a homozygous bull, white in colour, and this is a homozygous cow, red in colour. The calf is roan-coloured – its coat is a mix of red and white hair. Therefore, in this case, the F 1 generation doesn’t resemble either of the parents as in dominance nor is it in-between as in incomplete dominance. What the calf displayed was co-dominance. Co-dominance refers to a situation where a heterozygous organism has a phenotype that demonstrates traits from both dominant as well as recessive genes. A co-dominant trait is not blended but is independently and equally expressed.
The plasma membrane of the RBCs present in human blood has sugar polymers that protrude from its surface. It is these sugar polymers that determine the specificity of the major blood types. The difference between blood types A and B lie in a single sugar unit that protrudes from the end of a carbohydrate chain of a glycoprotein or glycolipid on the plasma membrane of an RBC. It is the gene ‘I’ that is responsible for determining the type of sugar produced by the RBCs. This gene has three different alleles , namely capital I A, capital I B and small i. Of these, alleles capital I A and capital I B produce sugars that are slightly different from each other, while the allele small i doesn’t produce any sugar. There are six genotypes and four phenotypes. Apart from being a good example for co-dominance, the ABO blood grouping system is also an ideal example of multiple alleles. As we just saw, the gene I has three alleles, capital I A, capital I B and small i that govern the same character, that is, blood group.

Therefore, dominance, incomplete dominance or co-dominance, depends on gene product and the production of a particular phenotype from this product. It also depends on the phenotype that we wish to examine in case the same gene influences more than one phenotype.


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