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Both mother (heterozygous A) and father (heterozygous B) have the following genotypes.
What is the probability of having children with blood group O?
Options
1
50%
2
0%
3
75%
4
25%
Correct Answer
25%
Solution
1

Mother (heterozygous A): genotype = IAi

Father (heterozygous B): genotype = IBi

2

Cross: IAi × IBi → Offspring:

IAIB = AB (25%) | IAi = A (25%) | IBi = B (25%) | ii = O (25%)

All four blood groups equally probable!

IAi × IBi → 25% AB : 25% A : 25% B : 25% O
Probability of blood group O = 25%
Theory: Genetics
1. ABO Blood Group System — Genetics

The ABO blood group system is controlled by a single gene with three alleles. IA allele: codes for glycosyltransferase A enzyme which adds N-acetylgalactosamine to the H antigen → produces A antigen on red blood cell surface. IB allele: codes for glycosyltransferase B which adds galactose to H antigen → produces B antigen. i allele: codes for non-functional transferase → H antigen remains unchanged (blood group O). Dominance relationships: IA and IB are co-dominant (both expressed when present together in IAIB). Both IA and IB are dominant over i (recessive). Genotypes and blood groups: IAIA or IAi → blood group A. IBIB or IBi → blood group B. IAIB → blood group AB (co-dominance). ii → blood group O.

2. Punnett Square — IAi × IBi Cross

When mother (IAi) crosses with father (IBi): Mother produces gametes: IA (50%) and i (50%). Father produces gametes: IB (50%) and i (50%). Punnett square results: IA × IB = IAIB → Blood group AB (25%). IA × i = IAi → Blood group A (25%). i × IB = IBi → Blood group B (25%). i × i = ii → Blood group O (25%). All four ABO blood groups are equally possible — 1:1:1:1 ratio. This remarkable result means two parents, one with blood group A and one with B (both heterozygous), can have children with ALL four blood groups. The 25% probability for blood group O is the answer to this question.

3. Why ABO Shows Both Multiple Alleles AND Co-dominance

Multiple alleles: three alleles (IA, IB, i) exist in the population for a single gene locus. Any individual can have only two of these three alleles. This creates six possible genotypes (IAIA, IAi, IBIB, IBi, IAIB, ii) giving four blood groups. Co-dominance: when an individual has both IA and IB alleles (genotype IAIB, blood group AB), BOTH alleles are expressed simultaneously. Red blood cells carry BOTH A and B antigens. This is NOT incomplete dominance (which would produce an intermediate phenotype). In co-dominance: both original phenotypes are expressed at the same time in the heterozygote. The blood group AB clearly demonstrates co-dominance — not a blend of A and B, but the presence of both A and B antigens on the same cell.

4. Blood Group Antibodies and Transfusion

ABO antibodies are naturally occurring (do not require prior sensitisation): Blood group A: has anti-B antibodies in plasma. Blood group B: has anti-A antibodies. Blood group AB: has NO antibodies (neither anti-A nor anti-B) — universal recipient for red cells. Blood group O: has BOTH anti-A AND anti-B antibodies — universal donor for red cells (but not whole blood). If incompatible blood is transfused: recipient antibodies attack donor red cells → agglutination (clumping) → haemolysis → transfusion reaction → potentially fatal. Example: if Group A patient receives Group B blood: patient anti-B attacks donor B cells → massive haemolysis → acute kidney failure, shock. Before any transfusion: ABO typing AND crossmatch (mixing donor and recipient blood to detect any unexpected antibodies).

5. Rh Blood Group System

Rh (Rhesus) blood group system is second most important after ABO. D antigen is most clinically significant. Rh positive (Rh+): has D antigen on red blood cells (~85% of people). Rh negative (Rh-): does not have D antigen (~15%). Unlike ABO: Rh-negative people do NOT naturally have anti-D antibodies. They develop anti-D only AFTER exposure to Rh+ blood (sensitisation event). This matters in pregnancy: Rh-negative mother, Rh-positive father → Rh+ foetus possible. At delivery: foetal Rh+ RBCs enter maternal circulation → mother makes anti-D (sensitisation). In SECOND Rh+ pregnancy: maternal IgG anti-D crosses placenta → attacks foetal RBCs → haemolytic disease of newborn (HDN / erythroblastosis foetalis). Prevention: RhoGAM (anti-D immunoglobulin) injection to Rh- mother within 72 hours of delivery → destroys any foetal RBCs before they can sensitise the mother.

6. Genetic Basis of Blood Groups — Molecular Details

The ABO gene is on chromosome 9q34. It encodes a glycosyltransferase enzyme. The IA allele (699 bp coding sequence): differs from IB allele at only 4 nucleotide positions → 4 amino acid differences → different sugar specificity. The i allele: single nucleotide deletion near the start of the coding sequence → frameshift → non-functional truncated protein. So O type is essentially a loss-of-function allele. H antigen (produced by FUT1/H gene on chromosome 19): the substrate for both A and B enzymes. Without H antigen, neither A nor B antigens can be made. Bombay phenotype (hh genotype, Oh blood type): extremely rare. These individuals cannot make H antigen → cannot make A or B antigens even with functional IA or IB alleles → serum contains anti-H, anti-A, anti-B → can only receive blood from other Bombay phenotype individuals. First described in Bombay (Mumbai), India.

7. Blood Groups in Disease and Population Genetics

Blood group O: most common worldwide (~44-55%). Individuals with blood group O may have lower risk of venous thromboembolism and cardiovascular disease. Higher susceptibility to Helicobacter pylori infection (causing duodenal ulcers). Some protection against severe Plasmodium falciparum malaria (rosetting reduced in O cells). Blood group A: higher risk of stomach cancer, some higher risk of COVID-19 severity reported. Blood group B: higher in South Asian and East Asian populations. Blood group AB: rarest (~4%). Universal plasma donor (for plasma, not red cells). Native American populations: extremely high frequency of blood group O (90-100% in some isolated populations) — result of founder effect and genetic drift. Blood group forensics: historically used for paternity exclusion (blood groups can EXCLUDE but cannot CONFIRM paternity). Now replaced by DNA profiling (STR analysis).

8. Other Applications of Blood Groups

Organ transplantation: ABO compatibility required for kidney, liver, heart transplants. ABO-incompatible transplants cause hyperacute rejection (preformed antibodies attack graft within minutes to hours). ABO-incompatible transplants now possible with special desensitisation protocols. Haemolytic disease of newborn (HDN): ABO incompatibility can also cause HDN (usually mild, as anti-A and anti-B are IgM and do not cross placenta well). Rh HDN is more severe (IgG crosses placenta easily). Neonatal jaundice: bilirubin from haemolysed foetal RBCs. Paternity testing: blood groups used historically for exclusion. Example: if child has blood group O (ii), and alleged father has blood group AB (IAIB), paternity is EXCLUDED — an AB father cannot produce an i gamete. Secretor status: ~80% of people secrete ABO antigens in body fluids (saliva, tears, semen, urine). Used in forensic analysis of body fluid evidence.

Frequently Asked Questions
1. Why can heterozygous A x heterozygous B parents have O children? ⌄
Heterozygous A = IAi. Heterozygous B = IBi. Both parents carry the recessive i allele. When parent passes i gamete AND the other parent also passes i gamete: child gets ii genotype = blood group O. Probability: 50% chance of getting i from mother × 50% chance of getting i from father = 25% probability of O child. This follows standard Mendelian genetics for recessive traits. The key is that BOTH parents must carry i. If either parent were homozygous (IAIA or IBIB), they could NOT produce an i gamete → O child impossible.
2. What is the difference between co-dominance and incomplete dominance? ⌄
Co-dominance: BOTH alleles expressed simultaneously in heterozygote. Both phenotypes present at same time. Example: IAIB blood group AB — both A and B antigens present on same RBC. Each allele produces its own gene product. Incomplete dominance: heterozygote shows INTERMEDIATE phenotype between two parents. New phenotype — blend of two. Example: Antirrhinum (snapdragon) — RR (red) x WW (white) = RW (pink). Neither allele dominates. Key test: Is the heterozygote showing BOTH original phenotypes (co-dominance) or a NEW intermediate phenotype (incomplete dominance)? For ABO: IAIB individual has BOTH A and B antigens (not some new intermediate antigen) → co-dominance.
3. What is the Bombay blood group? ⌄
Bombay phenotype (Oh blood type) is an extremely rare blood type. These individuals lack the H antigen (due to mutations in the FUT1 gene encoding the H enzyme on chromosome 19). Without H antigen: the ABO transferase enzymes have no substrate → cannot make A or B antigens, regardless of ABO genotype. Serology: Bombay individuals have anti-H, anti-A, and anti-B antibodies in their serum. They can ONLY receive blood from other Bombay phenotype individuals. Even group O blood (which has H antigen) is incompatible. First identified in Bombay (now Mumbai), India. Prevalence: approximately 1 in 10,000 in South India, extremely rare elsewhere globally.
4. How does paternity testing use blood groups? ⌄
Blood groups can be used for paternity EXCLUSION (proving someone is NOT the father) but NOT for confirmation (proving they ARE the father). Examples of exclusion: Child is blood group AB. Alleged father is group O (ii). Since O father cannot produce IA or IB gamete, he cannot be the father of an AB child. Child is group O. Alleged father is group AB (IAIB). AB father cannot produce i gamete, so cannot be father of an O (ii) child. Limitation: blood groups cannot confirm paternity — many people have the same blood group. Modern DNA profiling (STR analysis) is far more definitive. Probability of random match at 20 STR loci is less than 1 in 10 billion.
5. What are the ABO blood group frequencies worldwide? ⌄
Approximate ABO frequencies globally: Blood group O: most common worldwide (44-55%). Very high in indigenous South/Central American populations (up to 90-100% in isolated groups). Blood group A: common in Europe (40-50%), North American indigenous people. Highest in some Scandinavian and Australian Aboriginal groups. Blood group B: more common in South Asia, East Asia, Africa (15-30%). Low in Western Europe. Blood group AB: rarest blood group globally (~4%). Rh-negative: highest in Basque people (35%), Western Europeans (~15%). Very rare in East Asians (<1%) and sub-Saharan Africans (<3%). These differences reflect population history including founder effects, genetic drift, migration patterns, and possible natural selection by diseases.
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