ABO Blood Groups & Codominance — NEET Biology
✅ Asked in NEET 2024Till now we were discussing crosses where the F₁ resembled either of the two parents (dominance) or was in-between (incomplete dominance). But, in the case of co-dominance the F₁ generation resembles both parents. A good example is different types of red blood cells that determine ABO blood grouping in human beings. ABO blood groups are controlled by the gene I. The plasma membrane of the red blood cells has sugar polymers that protrude from its surface and the kind of sugar is controlled by the gene. The gene (I) has three alleles Iᴬ, Iᴮ and i. The alleles Iᴬ and Iᴮ produce a slightly different form of the sugar while allele i does not produce any sugar. Because humans are diploid organisms, each person possesses any two of the three I gene alleles. Iᴬ and Iᴮ are completely dominant over i, in other words when Iᴬ and i are present only Iᴬ expresses (because i does not produce any sugar), and when Iᴮ and i are present Iᴮ expresses. But when Iᴬ and Iᴮ are present together they both express their own types of sugars: this is because of co-dominance. Hence red blood cells have both A and B types of sugars. Since there are three different alleles, there are six different combinations of these three alleles that are possible, and therefore, a total of six different genotypes of the human ABO blood types (Table 4.2). How many phenotypes are possible?
NTA tests whether you understand that ABO blood groups have 4 phenotypes (A, B, AB, O) but 6 possible genotypes due to codominance and dominance relationships. The key trap: students confuse the 6 genotypes with 6 phenotypes. Remember—I^A and I^B are codominant to each other (AB phenotype), but both are dominant over i. So I^A I^A and I^A i both show phenotype A; I^B I^B and I^B i both show phenotype B; ii shows O; only I^A I^B shows AB. When a child has O blood (ii), both parents must carry at least one i allele, even if they appear as A or B phenotypes.
This paragraph was tested 3 times in NEET.
A child has blood group O. What could be the possible genotype of the parents? (NEET 2024)
Which of the following characteristics represent ‘inheritance of blood groups’ in humans? (NEET 2018) (i) Dominance (ii) Co-dominance (iii) Multiple allele (iv) Incomplete dominance (v) Polygenic inheritance
The genotypes of a husband and wife are IᴬIᴮ and Iᴬi. Among the blood types of their children, how many different genotypes and phenotypes are possible? (NEET 2017)
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