Dr Praveen Kumar Agrawal is teaching live on Unacademy Plus
COURSE Principles of inheritance and variations Lesson 12 A Critical evaluation of Mendel's Laws Co-dominance Dr. Praveen Kumar Agrawal M.Sc., Ph.D., CSIR NET (JRF). SRF, GATE 21Years of Pre-medical teaching experience
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CO-DOMINANCE Co-dominance of alleles is another example against Mendel's principle of dominance. In some cattle, the inheritance of coat colour (skin colour) offers an example of co-dominance When some red coloured cattle (RR) are crossed with white coloured cattle (rr), the F1 hybrids have roan (Rr) colour (small patches of both colours simultaneously). According to Mendel's principle of dominance, F1 hybrids should be either red or white depending upon the dominant nature of red and white alleles.
White Red F1 generation R r R r . Roan Roan Roan Roan
Selfing of F1 individuals R r Roan Roan a o F 2 generation White Roan Roan Red
On selfing, these F1 hybrids produce following results in F2 generations: F2 results Red-1, Roan-2, White-1 Therefore, in co-dominance, both alleles exist simultaneously. (Note there is no blending of character as in incomplete dominance). This type of inheritance where the two alleles of a character are equally dominant and exist simultaneously is called codominance. Other Examples of Co-dominance A and B blood group genes in human beings are also codominant. When present separately these genes form blood group A and blood group B respectively. But when present in heterozygous condition (present together in same gamete), they result in the formation of AB blood group. Similarly, in MN blood group system in humans, M and N alleles show codominance. In this case, the person has two alleles, one for M antigen (on RBCs) and other for N antigen (on RBCs). Person can have MM, MN or NN groups. In MN blood group case, both alleles are in heterozygous condition, and show co-dominance.
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