Biology · Principles of Inheritance and Variation · NEET
You cannot tell from blood group A alone - both IAIA and IAi show group A because IA is dominant over i. You only find out from the children or family. If an A parent has an O (ii) child, that parent MUST be IAi, because the child needs an i allele from each parent. If you only know the person is A, always write A = IAIA OR IAi and test both in the cross.
Yes. If both A parents are heterozygous (IAi x IAi), each can pass the hidden i allele. The cross gives IAIA : IAi : IAi : ii = 3 A : 1 O. So there is a 25% chance of an O child. This surprises many students because both parents look like group A. The hidden recessive i is the key.
AB is always IAIB. This parent has NO i allele to give. An O child needs ii - one i from each parent. Since the AB parent can only pass IA or IB, the child will always get at least one of these and can never be ii. This exact idea was tested in NEET 2026: father IAIB x mother IAi cannot give an O child.
Combine gametes IA, i (from father) with IB, i (from mother). You get IAIB (AB), IAi (A), IBi (B), and ii (O) in a 1:1:1:1 ratio. So all four blood groups A, B, AB, and O appear, each with 25% chance. NEET 2026 asked the O-child probability from this cross - the answer is 25%.
The O child is ii, so it took one i from each parent. The B father must be IBi (not IBIB, which has no i). The A mother must be IAi (not IAIA). So father = IBi, mother = IAi, child = ii. This is NEET 2024 - homozygous parents like IBIB or IAIA are impossible here.
Gametes IA, IB (father) x IA, i (mother) give IAIA, IAi, IAIB, IBi = 4 different genotypes. Their phenotypes are A (from IAIA and IAi), AB (from IAIB), and B (from IBi) = 3 phenotypes. So the answer is 4 genotypes, 3 phenotypes (NEET 2017). Note: genotype count and phenotype count are different - count carefully.
In codominance both alleles show fully and separately at the same time. In AB blood group, IA makes A sugar AND IB makes B sugar - the RBC carries BOTH sugars, not a blend. Incomplete dominance would give a mixed, in-between form (like a pink flower). AB is a clear mix of both, so it is codominance.
As per ABO blood grouping system, the blood group of father is B+, mother is A+ and child is O+. Their respective genotype can be: A. IBi/IAi/ii B. IBIB/IAIA/ii C. IAIB/iIA/iBi D. IAi/IBi/IAi
What is the probability of having children with 'O' blood group, where both mother and father are heterozygous for 'A' and 'B' blood group, respectively?
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?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
A = IAIA or IAi; B = IBIB or IBi; AB = IAIB only; O = ii only. Groups A and B have two possible genotypes each, while AB and O have just one.
The gene I has three alleles: IA, IB, and i. This is called multiple alleles. But each person is diploid, so any one person carries only TWO of these three alleles.
The i allele is recessive. Both IA and IB are completely dominant over i, so IAi shows group A and IBi shows group B. Only ii (no dominant allele) shows group O.
Step 1: write each parent's possible genotypes. Step 2: for unknown A or B parents, test both options. Step 3: split each genotype into gametes. Step 4: make a Punnett square. Step 5: read off genotypes, then convert to phenotypes (blood groups).
Yes, very often - almost every year. NEET asked genotype crosses in 2017, 2019, 2024, and 2026. It is a high-yield, scoring topic, so master the Punnett square method.
