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Not all inheritance follows simple dominant-recessive patterns. In codominance, both alleles are expressed simultaneously in the heterozygote, producing a distinct phenotype. When a gene has more than two alleles in the population, we speak of multiple alleles. The ABO blood group system is the classic example that combines both concepts. This lesson covers these patterns in depth as required by the Edexcel A-Level Biology specification.
In codominance, both alleles in a heterozygous individual are fully expressed. The heterozygote shows both phenotypes simultaneously, rather than a blend.
| Pattern | Heterozygote phenotype | Example |
|---|---|---|
| Complete dominance | Same as homozygous dominant | Bb mice are brown (same as BB) |
| Incomplete dominance | Intermediate blend | Red × white snapdragons → pink |
| Codominance | Both phenotypes expressed simultaneously | Red × white cattle → roan (red AND white hairs) |
Important distinction: In incomplete dominance, neither allele is fully expressed (a blend occurs). In codominance, both alleles are fully expressed side by side. Examiners frequently test whether students can distinguish these two patterns.
When alleles are codominant, we cannot use upper and lower case of the same letter (because neither is dominant). Instead, we use a superscript notation:
In shorthorn cattle:
Cross: C^R C^W × C^R C^W (Roan × Roan)
Punnett Square:
| C^R | C^W | |
|---|---|---|
| C^R | C^R C^R | C^R C^W |
| C^W | C^R C^W | C^W C^W |
Offspring phenotypes: 1 Red : 2 Roan : 1 White
Key point: The 1:2:1 phenotypic ratio (not 3:1) is the hallmark of codominance. Every genotype has a unique phenotype.
The sickle cell allele (H^S) is codominant with the normal allele (H^A):
At the molecular level, both alleles produce their respective protein products in the heterozygote, which is the definition of codominance.
Exam context: Sickle cell trait (H^A H^S) confers heterozygote advantage — resistance to malaria. This is why the sickle cell allele is maintained at high frequency in populations where malaria is endemic, despite being harmful in the homozygous state.
A farmer has red cattle (C^R C^R) and wants to know what offspring to expect from different crosses:
| Cross | Offspring phenotypes | Ratio |
|---|---|---|
| C^R C^R × C^R C^R | All red | — |
| C^R C^R × C^W C^W | All roan | — |
| C^R C^R × C^R C^W | 1 red : 1 roan | 1:1 |
| C^R C^W × C^R C^W | 1 red : 2 roan : 1 white | 1:2:1 |
| C^R C^W × C^W C^W | 1 roan : 1 white | 1:1 |
This table is useful for quickly predicting outcomes in codominance problems.
Most of the crosses we study involve genes with only two alleles. However, many genes in a population have three or more alleles — these are called multiple alleles. Although a population may contain many alleles of a gene, any one diploid individual can carry a maximum of two alleles (one on each homologous chromosome).
The ABO blood group is controlled by a single gene (gene I) on chromosome 9, with three alleles:
| Allele | What it produces | Dominance relationship |
|---|---|---|
| I^A | A antigen on red blood cell surface | Codominant with I^B |
| I^B | B antigen on red blood cell surface | Codominant with I^A |
| i | No antigen (O) | Recessive to both I^A and I^B |
| Blood group (phenotype) | Possible genotype(s) | Antigens on RBCs | Antibodies in plasma |
|---|---|---|---|
| A | I^A I^A or I^A i | A antigen | Anti-B |
| B | I^B I^B or I^B i | B antigen | Anti-A |
| AB | I^A I^B | Both A and B antigens | Neither |
| O | ii | Neither antigen | Both anti-A and anti-B |
Why is this codominance? In the I^A I^B genotype, both alleles are expressed: both A and B antigens are present on the red blood cell surface. Neither allele masks the other. Meanwhile, both I^A and I^B are dominant over i.
Parents: Mother is blood group A (I^A i) × Father is blood group B (I^B i)
Punnett Square:
| I^A | i | |
|---|---|---|
| I^B | I^A I^B | I^B i |
| i | I^A i | ii |
Offspring:
All four blood groups are possible from this single cross — a common exam question.
A child has blood group O (genotype ii). The mother has blood group A.
Both parents are blood group A. What is the probability of their child being blood group O?
This depends on the parents' genotypes:
The question must specify or imply the parental genotypes. If both parents are stated to be heterozygous carriers, the answer is ¼.
A couple are both blood group A heterozygous (I^A i) and both carriers for cystic fibrosis (Ff). What is the probability of their child having blood group O AND cystic fibrosis?
| Recipient blood group | Can receive from | Reason |
|---|---|---|
| A | A, O | Has anti-B; cannot receive B or AB |
| B | B, O | Has anti-A; cannot receive A or AB |
| AB | A, B, AB, O (universal recipient) | Has no antibodies |
| O | O only (universal donor) | Has both anti-A and anti-B |
The rule: a recipient must not receive blood containing antigens against which they carry antibodies. Group O blood has no antigens, so it can be given to anyone. Group AB plasma has no antibodies, so AB individuals can receive any blood type.
Exam questions may combine codominance with:
Cattle coat colour (C^R, C^W — codominant) and horn presence (H = horns, dominant; h = polled/no horns, recessive).
Cross: C^R C^W Hh × C^R C^W Hh
For coat colour (codominance): 1 Red : 2 Roan : 1 White For horn presence (complete dominance): 3 Horned : 1 Polled
Combined offspring: 3 Red Horned : 6 Roan Horned : 3 White Horned : 1 Red Polled : 2 Roan Polled : 1 White Polled
This gives a 3:6:3:1:2:1 ratio (12 phenotypic classes from 16 genotypic combinations).
The ABO system is the most commonly examined example, but multiple alleles occur in many other systems:
| Gene/system | Organism | Number of alleles | Notes |
|---|---|---|---|
| ABO blood group (gene I) | Human | 3 (I^A, I^B, i) | Codominance + dominance |
| Self-incompatibility (S gene) | Plants | 50+ in some species | Prevents self-pollination |
| MHC/HLA genes | Human | Hundreds per locus | Immune system diversity |
| Coat colour (C gene) | Rabbit | 4 (C, c^ch, c^h, c) | Dominance hierarchy |
The C gene in rabbits has four alleles with a dominance hierarchy: C > c^ch > c^h > c
| Genotype | Phenotype |
|---|---|
| CC, Cc^ch, Cc^h, Cc | Full colour (wild type) |
| c^ch c^ch, c^ch c^h, c^ch c | Chinchilla |
| c^h c^h, c^h c | Himalayan (colour on extremities only) |
| cc | Albino (no pigment) |
This shows that multiple alleles can form a dominance series, not just simple dominant-recessive pairs.
Codominance occurs when both alleles are fully expressed in the heterozygote, producing a 1:2:1 phenotypic ratio. Multiple alleles mean a gene has more than two allele forms in the population. The ABO blood group system illustrates both concepts: I^A and I^B are codominant with each other and both dominant over i. Correct notation (superscripts) and clear Punnett squares are essential for answering exam questions on these topics.
Codominance and multiple alleles together extend monohybrid inheritance beyond the simple dominant–recessive 3 : 1 framework. Codominance is the case in which both alleles of a heterozygous diploid are expressed simultaneously at the protein level, generating a heterozygote phenotype that displays both parental traits side by side rather than a blended intermediate. Multiple alleles is the population-level fact that a single locus can have more than two allelic forms — though any one diploid individual still carries only two. The canonical synthesis of both concepts is the ABO blood-group system (alleles I^A, I^B, i), which simultaneously demonstrates codominance (I^A I^B → AB blood) and a dominance hierarchy (I^A and I^B both dominant over i). Sickle-cell heterozygotes (Hb^A Hb^S), the MN blood-group system and roan cattle round out the standard A-Level codominance examples.
This material sits in Edexcel 9BI0 Topic 8 (Grey Matter — Coordination, Response and Gene Technology) under the inheritance strand, where candidates construct and interpret monohybrid genetic diagrams governed by codominance and multiple alleles. The required content covers the ABO blood-group system as the key worked example — three alleles I^A, I^B, i, with I^A and I^B codominant to each other and both dominant over the recessive i, generating four phenotypes A (I^A I^A or I^A i), B (I^B I^B or I^B i), AB (I^A I^B) and O (ii). Candidates must also distinguish codominance from incomplete dominance (both alleles fully expressed vs. blended intermediate), recognise the 1 : 2 : 1 phenotypic ratio generated by a heterozygote × heterozygote cross, and apply the framework to sickle-cell anaemia (Hb^A Hb^A unaffected; Hb^A Hb^S trait with heterozygote advantage in malaria zones; Hb^S Hb^S sickle-cell anaemia), the MN blood-group system, and roan coat colour in Shorthorn cattle. Synoptic links run backwards to lessons 4–5 (mono- and dihybrid) and forwards to lesson 7 (linkage), lesson 8 (chi-squared) and lesson 10 (Hardy–Weinberg). Refer to the official Pearson Edexcel 9BI0 specification document for exact wording.
Question (8 marks):
(a) A woman with blood group A (genotype I^A i) has a child with a man whose blood group is AB (genotype I^A I^B). Construct the Punnett square, list all possible offspring genotypes and phenotypes, and state the probability of each blood-group phenotype. (4)
(b) A maternity ward records a baby with blood group O. The mother's blood group is A and the supposed father's blood group is AB. Use a genetic argument to evaluate whether the supposed father can be the biological father of the child. (2)
(c) Explain in molecular terms why the I^A I^B genotype produces blood group AB rather than blood group A or blood group B alone, referring to the polypeptides encoded by each allele. (2)
Solution with mark scheme:
(a) M1 (AO1) — symbols and gametes. Let I^A = allele encoding A antigen (codominant with I^B), I^B = allele encoding B antigen (codominant with I^A), i = recessive allele encoding no antigen. The mother I^A i produces gametes I^A and i at frequency 1/2 each; the father I^A I^B produces gametes I^A and I^B at frequency 1/2 each.
A1 (AO2) — Punnett square.
| I^A | I^B | |
|---|---|---|
| I^A | I^A I^A | I^A I^B |
| i | I^A i | I^B i |
M1 + A1 (AO2) — phenotype probabilities. Mapping each genotype to its phenotype: I^A I^A = A; I^A I^B = AB; I^A i = A; I^B i = B. Collecting by phenotype: 2/4 A : 1/4 AB : 1/4 B, i.e. P(A) = 1/2, P(AB) = 1/4, P(B) = 1/4, P(O) = 0. There is no possibility of a group O child from this cross because neither parent contributes an i gamete that can pair with another i.
(b) M1 (AO3.1) — analyse the cross. A child of blood group O has genotype ii and must therefore have inherited an i allele from each parent. The mother (group A) could be I^A i and could donate i, but the supposed father (group AB) is I^A I^B and carries no i allele, so he cannot donate i.
A1 (AO3.2) — conclusion. The supposed father cannot be the biological father on this evidence, because an AB father (I^A I^B) cannot produce a group O child (ii) regardless of the mother's genotype. (Caveat: extremely rare blood-group variants such as the Bombay phenotype can mimic group O at the antigen level despite I^A or I^B genotypes, so a definitive exclusion would require molecular genotyping.)
(c) M1 (AO1) — molecular basis. The I^A allele encodes a glycosyltransferase enzyme that adds N-acetylgalactosamine to the H antigen on the surface of red blood cells, producing the A antigen; the I^B allele encodes a different glycosyltransferase that adds galactose to the same H antigen, producing the B antigen; the i allele encodes a non-functional enzyme (no sugar added).
A1 (AO3.1) — codominance at the molecular level. In an I^A I^B heterozygote, both functional enzymes are produced and both sugar-modifications occur on the H antigens of the same red blood cell, so the cell carries both A and B antigens simultaneously — hence blood group AB. This is a clean molecular illustration of codominance: both alleles are independently expressed at the protein level, and both protein products are detectable in the heterozygote phenotype.
Total: 8 marks (M3 A5).
Question (6 marks): Two parents heterozygous for sickle-cell haemoglobin (genotype Hb^A Hb^S each) have a child. (i) Construct the Punnett square and state the expected phenotypic ratio of unaffected : sickle-cell trait : sickle-cell anaemia offspring, (ii) explain why this is described as codominance rather than complete dominance or incomplete dominance, and (iii) explain why the Hb^S allele persists at high frequency in regions of sub-Saharan Africa where falciparum malaria is endemic.
Mark scheme decomposition by AO:
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