BiologyISC Class 12 · Principles of Inheritance and Variation

Incomplete Dominance, Co-dominance and Multiple Alleles: ISC Class 12

Updated 15 October 2026 · Fact-checked

In incomplete dominance the heterozygote is intermediate, as pink snapdragons from red and white parents, so F₂ is 1 red : 2 pink : 1 white. In co-dominance both alleles show fully, as Iᴬ and Iᴮ in blood group AB. ABO blood groups also show multiple alleles: three alleles giving six genotypes.

Understand Incomplete Dominance, Co-dominance and Multiple Alleles

Deviations from Mendel's dominance. Mendel's pea traits showed complete dominance. Later work found other relationships between the two alleles of a gene. The law of segregation still holds in all of them; only the expression in the heterozygote changes.

Why dominance varies. A gene carries information for a product, often an enzyme. In a heterozygote, the normal allele makes the normal enzyme; the other allele may make a normal, a less efficient or a non-functional product. If the single normal allele makes enough product, the phenotype matches the normal homozygote (complete dominance); if not, the phenotype is intermediate.

Incomplete dominance. Neither allele completely dominates, so the F₁ (heterozygote) is intermediate and does not resemble either parent. ISC example: snapdragon (Antirrhinum) flower colour.

P: red (RR) × white (rr) → F₁ all pink (Rr).
F₂ (Rr × Rr): 1 RR red : 2 Rr pink : 1 rr white.
Phenotypic ratio = genotypic ratio = 1 : 2 : 1 (not 3 : 1), because the heterozygote can be recognised.
In any colour combination, match the ratio to the colours: for red × yellow → orange F₁, F₂ is red : orange : yellow = 1 : 2 : 1, i.e. red : yellow : orange = 1 : 1 : 2.
Pea starch grains (NCERT): BB gives large starch grains and round seeds, bb small grains and wrinkled seeds. For seed shape B is dominant (Bb round), but starch grain size in Bb is intermediate: incomplete dominance. So dominance depends on which phenotype is considered. The same gene affecting both is an example of pleiotropy.

Co-dominance. Both alleles are expressed fully and together in the heterozygote, so F₁ resembles both parents. ISC example: ABO blood groups.

Gene I controls the sugar polymers on the surface of red blood cells. Allele Iᴬ makes sugar A, Iᴮ makes sugar B, i makes no sugar.
Iᴬ and Iᴮ are co-dominant with each other; both are dominant over i.
Iᴬ Iᴮ individuals carry both A and B sugars: group AB.

Genotype	Blood group
IᴬIᴬ, Iᴬi	A
IᴮIᴮ, Iᴮi	B
IᴬIᴮ	AB
ii	O

Multiple alleles. When a gene has more than two alleles in a population, they are multiple alleles. ABO has three alleles (Iᴬ, Iᴮ, i), so there are six genotypes and four phenotypes. Any one person still carries only two alleles, so multiple alleles are recognised only by studying a population. In general, n alleles give n(n + 1)/2 genotypes.

Comparing the three.

	Incomplete dominance	Co-dominance	Multiple alleles
Heterozygote	intermediate, blended	both traits fully	(relationship varies among the alleles)
F₂ ratio	1 : 2 : 1	1 : 2 : 1	depends on the cross
Example	snapdragon, starch grain size	AB blood group	ABO (3 alleles)

Uses of blood-group genetics. Blood transfusion, and settling disputed parentage: a man can be excluded as a father if he cannot supply an allele the child must have received (an AB man cannot be the father of an O child, since an O child is ii).

Draw: Punnett square for snapdragon F₂. Checklist: gametes R and r along both sides; four boxes RR (red), Rr (pink), Rr (pink), rr (white); ratio written below as 1 : 2 : 1 with phenotypes named.

Incomplete Dominance, Co-dominance and Multiple Alleles: key facts

Incomplete dominance F₂: $R r \times R r \to 1 R R (red) : 2 R r (pink) : 1 r r (white)$; Phenotypic ratio equals genotypic ratio.
ABO genotypes: $A : I^{A} I^{A}, I^{A} i; B : I^{B} I^{B}, I^{B} i; A B : I^{A} I^{B}; O : ii$; Six genotypes, four phenotypes.
Genotypes from n alleles: $number of genotypes = \frac{n ( n + 1 )}{2}$; n = 3 gives 6 (ABO); n = 4 gives 10.
AB × O: $I^{A} I^{B} \times ii \to \frac{1}{2} I^{A} i (A) : \frac{1}{2} I^{B} i (B)$; No child of this couple can be AB or O.
Paternity exclusion: $child’s alleles - mother’s contribution = father’s contribution$; If the man cannot give it, he is excluded (never proved to be the father by this alone).

How to answer Incomplete Dominance, Co-dominance and Multiple Alleles questions

ISC asks you to identify the pattern from F₁ and F₂ data, give ratios with phenotypes, and solve blood-group crosses and parentage problems.

1Look at F₁: intermediate → incomplete dominance; both parental traits fully → co-dominance; like one parent → complete dominance.
2Write genotypes and gametes, draw a Punnett square, and give the ratio with phenotype names in the order the question asks.
3For blood groups, write every possible genotype of each parent (group A could be IᴬIᴬ or Iᴬi) and use the child's group to decide.
4For parentage, ask what allele the child must have received from the father; if the man cannot supply it, he is excluded.
5For probabilities over several children, multiply the probability for each child.
6State that the law of segregation still applies; only expression in the heterozygote differs.

Incomplete Dominance, Co-dominance and Multiple Alleles in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Blood groups and a hospital mix-up

Suppose two babies are born on the same night in a small hospital and their tags fall off. Baby P is group O and Baby Q is group AB. Couple 1 are both group AB; Couple 2 are group A and group O.

Which baby belongs to which couple?

Try it first, then show the solutionSolution

Couple 1 (IᴬIᴮ × IᴬIᴮ) can only have children IᴬIᴬ, IᴬIᴮ or IᴮIᴮ: groups A, AB or B. They cannot have an O child (ii), because neither parent has i.
Couple 2 (group A, IᴬIᴬ or Iᴬi, × group O, ii) can have only group A or group O children; they cannot have an AB child, because neither parent carries Iᴮ.
Baby P (O) can belong only to Couple 2; Baby Q (AB) can belong only to Couple 1.
This works because Iᴬ and Iᴮ are co-dominant and i is recessive, so each blood group limits the possible genotypes.

Answer: Baby Q (AB) goes to Couple 1 (AB × AB); Baby P (O) goes to Couple 2 (A × O).

Memory hook: An O child needs an i from both parents; an AB child needs an Iᴬ from one and an Iᴮ from the other.

Tricks to remember Incomplete Dominance, Co-dominance and Multiple Alleles

Short memory aids. Say each one aloud once, then use it on a question the same day.

Pink is a blend, AB is a band
Incomplete dominance blends (pink); co-dominance shows both side by side (A and B both on the cell).
Use it when: Telling the two apart.
1-2-1 is the giveaway
If an F₂ shows 1 : 2 : 1 phenotypes, the heterozygote is visible: not complete dominance.
Use it when: Identifying the pattern from data.
Three alleles, six genotypes, four groups
3-6-4 for ABO.
Use it when: Multiple allele facts.

Incomplete Dominance, Co-dominance and Multiple Alleles: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Snapdragon F₂ · 2 marks · Short answerBoard level

A pure red-flowered snapdragon is crossed with a pure white-flowered one; F₁ is pink. Show the F₂ by a Punnett square and give the phenotypic and genotypic ratios.

Try it first, then show the solutionSolution

P: RR × rr → F₁ Rr (pink); F₁ gametes R and r (½ mark).
F₂: RR, Rr, Rr, rr (1 mark).
Phenotypes red : pink : white = 1 : 2 : 1; genotypes RR : Rr : rr = 1 : 2 : 1 (½ mark).

Answer: 1 red : 2 pink : 1 white; genotypic ratio the same.

Example 2: Co-dominance defined · 2 marks · Short answerBoard level

Define co-dominance and explain it with the blood group AB.

Try it first, then show the solutionSolution

Co-dominance: both alleles of a gene are fully expressed in the heterozygote, so it resembles both parents (1 mark).
In IᴬIᴮ persons, Iᴬ makes sugar A and Iᴮ makes sugar B, so red blood cells carry both sugars: group AB (1 mark).

Answer: Both alleles express fully; IᴬIᴮ gives both A and B sugars on RBCs.

Example 3: Group A father, group AB mother · 2 marks · Short answerHarder board level

A group A man whose mother belonged to group O has children with a woman who is group AB. Work out which blood groups the children may show, and in what proportions.

Try it first, then show the solutionSolution

His mother (ii) gave him i, so he is Iᴬi; she is IᴬIᴮ (½ mark).
Children: IᴬIᴬ, IᴬIᴮ, Iᴬi, Iᴮi (1 mark).
A (IᴬIᴬ, Iᴬi) 1/2; AB 1/4; B 1/4; no O (½ mark).

Answer: A 1/2, AB 1/4, B 1/4.

Example 4: Starch grains: one gene, two answers · 3 marks · Short answerHarder board level

In pea, Bb seeds are round like BB seeds but have starch grains of intermediate size. Is B dominant or incompletely dominant? Explain, and give the expected ratios for seed shape and for starch grain size from Bb × Bb.

Try it first, then show the solutionSolution

For seed shape B is dominant: Bb is round like BB (1 mark).
For starch grain size the alleles show incomplete dominance: Bb grains are intermediate; so dominance depends on the phenotype being considered (1 mark).
Bb × Bb → BB : Bb : bb = 1 : 2 : 1; seed shape round : wrinkled = 3 : 1; grain size large : intermediate : small = 1 : 2 : 1 (1 mark).

Answer: Dominant for seed shape (3 : 1), incompletely dominant for grain size (1 : 2 : 1).

Example 5: A paternity question · 3 marks · Short answerHarder board level

A child is of blood group O and the mother is group A. Two men claim to be the father: Mr X is group AB and Mr Y is group B. Which man can be excluded? Can blood groups prove that the other man is the father?

Try it first, then show the solutionSolution

A group O child is ii: it received i from each parent; the mother must be Iᴬi (1 mark).
Mr X (IᴬIᴮ) has no i allele, so he cannot be the father: excluded (1 mark).
Mr Y could be Iᴮi and so could be the father, but many other men of groups A, B or O could too; blood groups can exclude but cannot prove paternity (1 mark).

Answer: Mr X (AB) is excluded; Mr Y is possible but not proved.

Example 6: Four children, four groups · 4 marks · Short answerHOTS / competency

Suppose a couple have the genotypes Iᴬi and Iᴮi. (a) What is the chance that one child is group O? (b) If they have four children, what is the probability that all four have different blood groups? (c) How many genotypes would be possible in the population if a fourth allele of gene I existed?

Try it first, then show the solutionSolution

(a) Children IᴬIᴮ, Iᴬi, Iᴮi, ii are equally likely: P(O) = 1/4 (1 mark).
(b) Each blood group has probability 1/4. The first child can be any group; the second must differ (3/4), the third must differ from both (2/4), the fourth must be the last group (1/4): 1 × 3/4 × 2/4 × 1/4 = 6/64 = 3/32 (2 marks).
(c) n(n + 1)/2 with n = 4: 10 genotypes (1 mark).

Answer: (a) 1/4 (b) 3/32 (c) 10.

Where students lose marks in Incomplete Dominance, Co-dominance and Multiple Alleles

The slips examiners see most often, and what to write instead.

Mistake: Calling a blended F₁ co-dominance
In 2024 examiners noted that for red × yellow giving orange flowers, candidates wrote co-dominance or 'masking of colours' instead of incomplete dominance.
Do this: Blend (intermediate colour) = incomplete dominance; both colours side by side = co-dominance.
Usually costs: 1 mark.
Mistake: Quoting 1 : 2 : 1 without matching phenotypes
Examiners (2024) found the textbook 1 : 2 : 1 written for red : yellow : orange, when the correct order gives 1 : 1 : 2.
Wrong working
Red : yellow : orange = 1 : 2 : 1.
Right working
Red : yellow : orange = 1 : 1 : 2 (the heterozygote, orange, is 2).
Do this: Write the ratio in the order of the phenotypes named in the question.
Usually costs: 1 mark.
Mistake: Assuming group A means IᴬIᴬ
Students forget Iᴬi.
Do this: List both genotypes for groups A and B; use relatives' groups to decide.
Usually costs: 1 to 2 marks.
Mistake: Saying blood groups prove paternity
Students overstate exclusion tests.
Do this: Blood groups can exclude a man; they cannot prove he is the father.
Usually costs: 1 mark.

Exam tips

Use Iᴬ, Iᴮ and i notation, not A, B and O, for genotypes.
Always name phenotypes beside a ratio.
Mention that segregation still occurs in incomplete dominance and co-dominance.
Learn the NCERT starch-grain example: it shows that dominance depends on the trait examined.
In parentage problems, start from the child and work backwards.

Practice questions on Incomplete Dominance, Co-dominance and Multiple Alleles

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
When one gene shows incomplete dominance, the F₂ of a monohybrid cross has a phenotypic ratio of:
(a) 1 : 1 (b) 9 : 3 : 3 : 1 (c) 1 : 2 : 1 (d) 3 : 1
Hint
The heterozygote is visible.
Answer
(c) 1 : 2 : 1.
Q2 · 2 marks · Short answerBoard level
Why is the ABO blood group system called an example of multiple alleles? How many alleles can one person carry?
Hint
Population versus individual.
Answer
Gene I has three alleles (Iᴬ, Iᴮ, i) in the human population, more than two, so they are multiple alleles. Any one person carries only two of them, one on each homologous chromosome.
Q3 · 1 mark · Assertion-reasonHarder board level
Assertion (A): In snapdragon, the F₂ phenotypic and genotypic ratios are the same.
Reason (R): In incomplete dominance the heterozygote has a phenotype different from both homozygotes.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Can you tell Rr from RR by looking?
Answer
(a).
Q4 · 2 marks · Short answerHarder board level
A pink snapdragon is crossed with a white one. Give the phenotypic ratio of the offspring. If 400 plants are raised from a pink × pink cross, how many of each colour are expected?
Hint
Rr × rr; then Rr × Rr.
Answer
Rr × rr → 1 pink : 1 white. Rr × Rr → 1 red : 2 pink : 1 white, so about 100 red, 200 pink and 100 white.
Q5 · 3 marks · Short answerHarder board level
Give three differences between incomplete dominance and co-dominance.
Hint
Phenotype of the heterozygote; examples.
Answer
Incomplete dominance: heterozygote intermediate (blend), resembles neither parent fully, e.g. pink snapdragon. Co-dominance: both alleles fully expressed together, heterozygote resembles both parents, e.g. blood group AB. In incomplete dominance one allele's product is insufficient on its own; in co-dominance each allele makes its own distinct product (A and B sugars).
Q6 · 2 marks · Short answerHarder board level
A woman of group B, whose father was group O, marries a man of group AB. What blood groups can their children have, and which group is impossible?
Hint
She must be Iᴮi.
Answer
She is Iᴮi; he is IᴬIᴮ. Children: IᴬIᴮ (AB), Iᴬi (A), IᴮIᴮ (B), Iᴮi (B): AB 1/4, A 1/4, B 1/2. Group O is impossible because the father cannot give i.
Q7 · 3 marks · Short answerHOTS / competency
Suppose in a flower, a cross between a pure purple-flowered plant and a pure white-flowered plant gives F₁ flowers with purple and white patches side by side. (a) Name the pattern of inheritance. (b) Predict the F₂ phenotypes and ratio. (c) How would the F₁ have looked if the pattern were incomplete dominance?
Hint
Both colours shown fully.
Answer
(a) Co-dominance: both alleles are expressed fully in the heterozygote. (b) 1 purple : 2 purple-and-white patched : 1 white. (c) A uniform intermediate shade (such as lilac/pale purple) in every F₁ flower.
Q8 · 3 marks · Case-basedHOTS / competency
Read and answer.
Suppose a family's blood groups are: father A, mother B, first child O, second child AB.
(a) Give the genotypes of the father and mother, with reasons. (1)
(b) What is the probability that their third child is group A? (1)
(c) Which two genetic principles does the second child illustrate? (1)
Hint
The O child tells you both parents carry i.
Answer
(a) Father Iᴬi, mother Iᴮi: the O child (ii) needs i from each parent. (b) 1/4 (Iᴬi). (c) Co-dominance (Iᴬ and Iᴮ both expressed) and multiple alleles (three alleles of gene I in the family).

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Give the scientific name of snapdragon.Show answer
Antirrhinum.
2.Which alleles of gene I are co-dominant?Show answer
Iᴬ and Iᴮ.
3.How many genotypes produce group A?Show answer
Two: IᴬIᴬ and Iᴬi.
4.What does allele i produce?Show answer
No sugar on the RBC surface.
5.What is the F₁ colour of red × white snapdragon?Show answer
Pink.
6.In pea, which trait of the B gene shows incomplete dominance?Show answer
Starch grain size.

Incomplete Dominance, Co-dominance and Multiple Alleles: frequently asked questions

What is incomplete dominance?

A condition in which neither allele is completely dominant, so the heterozygote has an intermediate phenotype, such as pink snapdragons from red and white parents.

What is co-dominance?

A condition in which both alleles are fully expressed in the heterozygote, as in blood group AB, where both A and B sugars are present.

What are multiple alleles?

More than two alleles of a gene in a population, such as Iᴬ, Iᴮ and i of the ABO gene; each person carries only two.

How many genotypes and phenotypes does the ABO system have?

Six genotypes and four phenotypes (A, B, AB, O).

Can parents of groups A and B have a child of group O?

Yes, if both are heterozygous (Iᴬi and Iᴮi); the child (ii) has a 1/4 chance.