Question
Explain co–dominance taking an example of human blood groups in the population.
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Answer
Co–dominance:
- The alleles which are able to express themselves independently, even when present together are called co–dominant alleles and this biological phenomenon is called co–dominance. For example, ABO blood grouping in humans.
- ABO blood groups are controlled by gene I. Gene I has three alleles $I^A,I^B$ and $I^O/i.$
- $I^A$ and $I^B$ produce RBC surface antigens sugar polymer A and B, respectively, whereas i does not produce any antigen.
- $I^A$ and $I^B$ are dominant over i hence $I^A$ and $I^B$ are dominant alleles and i is recessive allele as in $I^Ai$ and $I^Bi.$
- When $I^A$ and $I^B$ are present together, both express equally and produce the surface antigens A and B, hence show co–dominance.
- Since humans are diploid, each person possesses any two of the three ‘I’ gene alleles, resulting into six different genotypic combinations and four phenotypic expressions.
| Allele fromParent 1 | Allele fromParent 2 | Genotype of offspring | Blood groups of offspring |
| $I^A$ | $I^A$ | $I^AI^A$ | A |
| $I^A$ | $I^B$ | $I^AI^B$ | AB |
| $I^A$ | i | $I^Ai$ | A |
| $I^B$ | $I^B$ | $I^BI^B$ | B |
| $I^B$ | i | $I^Bi$ | B |
| i | i | ii | O |
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