Biology · Principles of Inheritance and Variation · NEET
Codominance means both alleles in a heterozygote express themselves fully and separately. In dominance, only one allele shows. In codominance, BOTH show at the same time, side by side. Example: a person with genotype I^A I^B has AB blood group. Their red blood cells carry A sugar AND B sugar together. Neither allele is hidden and neither is diluted. NCERT states: in codominance the F1 resembles BOTH parents (not one, not an in-between).
The gene I controls ABO blood groups. Allele I^A makes A sugar (antigen A) on the red blood cell, and I^B makes B sugar (antigen B). When a person is I^A I^B, both alleles work at once, so the red blood cells show BOTH A and B sugars. That is why the blood group is called AB. Because both alleles express fully together, this is codominance. This exact point is tested by NEET again and again.
In incomplete dominance the heterozygote is a BLEND or in-between phenotype (like a pink flower from red and white parents in snapdragon). In codominance there is NO blend; BOTH alleles show separately and fully (AB blood group shows both A and B, not a mixed 'in-between' blood). Remember: incomplete = mixing (one new middle trait); codominance = both appear at once (two traits side by side).
I^A and I^B are two different alleles of the same gene I. A person is diploid, so they carry only TWO of the three alleles (I^A, I^B, i). Genotype I^A I^B means one chromosome carries I^A and the partner chromosome carries I^B. Since both are codominant and both express, the phenotype is blood group AB. This person makes both A and B antigens.
No. Blending would give a mixed, middle result (that is incomplete dominance). In codominance nothing blends. The two alleles keep their own separate identity and both show up fully. In AB blood, you can still see A sugar AND B sugar as two distinct things on the same red blood cell. This is why a common NEET trap says 'both express the SAME sugar' which is WRONG; they express their OWN different sugars.
Yes, the ABO blood group shows BOTH codominance AND multiple alleles. Multiple alleles means one gene has more than two alleles in the population (here I has three: I^A, I^B, i). Codominance is about how I^A and I^B both express in one AB person. NEET 2018 asked which features blood group inheritance shows: the answer is dominance, co-dominance and multiple alleles (I^A and I^B are each dominant over i, and together they are codominant).
O group has genotype ii. The allele i makes NO sugar at all, and I^A and I^B are both completely dominant over i. So in genotypes like I^A i, only A sugar shows (group A), which is normal dominance, not codominance. Codominance only appears in the I^A I^B (AB) person, where two active alleles express together.
Which of the following characteristics represent 'Inheritance of blood groups' in humans? a. Dominance b. Co-dominance c. Multiple allele d. Incomplete dominance e. Polygenic inheritance
Identify the wrong statement with reference to the gene 'I' that controls ABO blood groups.
Match the following: Column I: (a) Dominance (b) Codominance (c) Pleiotropy (d) Polygenic inheritance Column II: (i) Many genes govern a single character (ii) In a heterozygous organism only one allele expresses itself (iii) In a heterozygous organism both alleles express themselves fully (iv) A single gene influences many characters
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Codominance is the phenomenon where both alleles of a gene express themselves fully and independently in a heterozygous organism, so the phenotype shows both traits at the same time. The standard example is the AB blood group (genotype I^A I^B).
Blood group AB. Both I^A and I^B express, so the red blood cells carry both A and B antigens (sugars).
The gene I controls ABO blood groups. It has three alleles: I^A, I^B and i. This is also an example of multiple alleles. Each person carries only two of these three alleles.
It is codominance. Both A and B antigens appear fully and separately on the red blood cells; there is no blending or in-between antigen, which would be incomplete dominance.
Codominance is an exception to the Law of Dominance because neither allele is dominant over the other; both express. However, the alleles still segregate normally during gamete formation, so segregation still holds.
