Incomplete Dominance vs Codominance: Differences, Examples and Crosses for MDCAT

"Pink flowers or AB blood?" is the quickest way to tell incomplete dominance from codominance, and MDCAT tests the difference every year in some form. Both patterns break Mendel's simple rule that one allele hides the other, and both give the same 1:2:1 ratio in F2. That is exactly why students mix them up. This guide sets them side by side with complete dominance, works through the crosses, and gives you practice MCQs.

The short answer: in incomplete dominance, neither allele is fully dominant, so the heterozygote shows an intermediate phenotype: red × white four o'clock (or snapdragon) flowers give pink. In codominance, both alleles are fully expressed together in the heterozygote: a person with I^AI^B has both A and B antigens (blood group AB). In both cases the F2 phenotypic ratio is 1:2:1, the same as the genotypic ratio, instead of the 3:1 of complete dominance.

Start with complete dominance (Mendel's pattern)

Mendel worked on the garden pea, Pisum sativum. In his seed-shape cross, round (RR) × wrinkled (rr) gave all round (Rr) plants in F1. Selfing the F1 gave:

• Genotypic ratio: 1 RR : 2 Rr : 1 rr
• Phenotypic ratio: 3 round : 1 wrinkled

The phenotypic ratio is 3:1 because RR and Rr look the same: one dominant allele is enough to give the full dominant phenotype. This is complete dominance, and it is the law of dominance Mendel described.

Incomplete dominance and codominance are the two main exceptions. In both, the heterozygote looks different from both homozygotes, so all three genotypes can be seen. That is why the phenotypic ratio becomes 1:2:1.

Incomplete dominance: the blended-looking heterozygote

In incomplete dominance, one allele is not completely dominant over the other. The heterozygote shows a phenotype in between the two parents.

The classic example: four o'clock plant and snapdragon

Flower colour in the four o'clock plant (Mirabilis jalapa) and the snapdragon (Antirrhinum majus) is the standard FSc example. Because neither allele is recessive, it is clearer to write them as C^R (red) and C^W (white). Some books simply write RR for red, rr for white and Rr for pink; the result is the same.

Cross 1: P generation red (C^RC^R) × white (C^WC^W)

• Gametes: C^R and C^W
• F1: all C^RC^W, pink

Cross 2: F1 × F1 pink (C^RC^W) × pink (C^RC^W)

F2 of red × white four o'clock plants

Gametes	CR	CW
CR	CRCR red	CRCW pink
CW	CRCW pink	CWCW white

F2: 1 red : 2 pink : 1 white. Genotypic and phenotypic ratios are the same, so you can tell every genotype just by looking at the flower.

Why this is not "blending"

Pink looks like red paint mixed with white paint, but the alleles have not mixed. The proof is in F2: pure red and pure white flowers reappear. If the alleles had blended, pink × pink could only ever give pink. The alleles keep their identity and separate in gamete formation, exactly as Mendel's law of segregation says. Only the phenotype of the heterozygote is intermediate.

Two more crosses MDCAT likes

• Pink × white (C^RC^W × C^WC^W) → 1 pink : 1 white.
• Pink × red (C^RC^W × C^RC^R) → 1 red : 1 pink.

Codominance: both alleles show fully

In codominance, both alleles are expressed fully and at the same time in the heterozygote. There is no intermediate. You see both parental characters side by side.

Example 1: AB blood group

The ABO blood group is controlled by one gene with three alleles: I^A, I^B and i. I^A makes antigen A on red blood cells, I^B makes antigen B, and i makes neither.

• I^A and I^B are each dominant over i (complete dominance).
• I^A and I^B are codominant with each other: an I^AI^B person makes both A and B antigens and has blood group AB.
• Because the gene has more than two alleles in the population, ABO is also an example of multiple alleles: 3 alleles give 6 genotypes and 4 phenotypes.

ABO genotypes and blood groups

Blood group	Genotype(s)	Antigen on red cells
A	IAIA or IAi	A
B	IBIB or IBi	B
AB	IAIB	A and B (codominance)
O	ii	Neither

Example 2: MN blood group

The MN blood group has two alleles, L^M and L^N. L^ML^M people have M antigen, L^NL^N have N antigen, and L^ML^N people have both (blood group MN). This is a neat two-allele example of codominance.

Example 3: roan cattle

Roan coat colour in cattle is often quoted as codominance: a cross of red and white cattle gives roan offspring that have red hairs and white hairs growing side by side, not pink hairs. Some older books describe roan as incomplete dominance, so if it appears in an MCQ, read the options carefully. The blood group examples are the safest to remember.

Worked codominance crosses

Cross 1: AB × AB (I^AI^B × I^AI^B)

AB × AB

Gametes	IA	IB
IA	IAIA group A	IAIB group AB
IB	IAIB group AB	IBIB group B

Result: 1 A : 2 AB : 1 B, the same 1:2:1 pattern as incomplete dominance. No child can be group O, because neither parent has i.

Cross 2: heterozygous A × heterozygous B (I^Ai × I^Bi)

I^Ai × I^Bi

Gametes	IB	i
IA	IAIB group AB	IAi group A
i	IBi group B	ii group O

Result: 1 AB : 1 A : 1 B : 1 O. This is a favourite MDCAT question: parents of groups A and B can have a child of any of the four blood groups, each with a 25% chance.

Cross 3: AB × O (I^AI^B × ii)

Gametes: I^A or I^B from one parent, only i from the other. Children: I^Ai (group A) and I^Bi (group B) in a 1:1 ratio. They can be neither AB nor O. MDCAT often asks this one as "which blood group is not possible?"

Cross 4: MN × MN

L^ML^N × L^ML^N gives 1 M : 2 MN : 1 N.

Incomplete dominance vs codominance vs complete dominance

The three dominance patterns compared

Feature	Complete dominance	Incomplete dominance	Codominance
Heterozygote looks like	The dominant parent	Intermediate between parents	Both parents at once
Is either allele hidden?	Yes, the recessive one	No, but each is only partly expressed	No, both fully expressed
F2 phenotypic ratio	3:1	1:2:1	1:2:1
F2 genotypic ratio	1:2:1	1:2:1	1:2:1
Can you read genotype from phenotype?	Not for the dominant phenotype (RR or Rr)	Yes	Yes
Examples	Mendel's peas: round/wrinkled, tall/dwarf	Four o'clock and snapdragon flower colour	AB blood group, MN blood group, roan cattle
Picture it as	One colour covers the other	Red + white → pink	Red and white spots side by side

Remember this: In-complete = in-between (pink). Co-dominance = co-operation, both show (AB). Same F2 ratio (1:2:1), different heterozygote.

How MDCAT asks about these patterns

• Identify the pattern from a description: "The heterozygote shows both parental phenotypes" → codominance. "The heterozygote is intermediate" → incomplete dominance.
• Give the F2 ratio: red × white snapdragons, F2 phenotypic ratio? (1:2:1). Watch for an option of 3:1, which is the complete-dominance trap.
• Blood group possibilities: which blood group is impossible in the children of AB × O? (AB and O).
• Combine with multiple alleles: how many genotypes and phenotypes does the ABO system have? (6 and 4).
• Test the blending idea: "Reappearance of red and white in F2 shows that…" (alleles do not blend).

Common mistakes

• "Incomplete dominance is blending inheritance." Wrong. Only the heterozygote's phenotype is intermediate; the alleles stay separate and red and white come back in F2.
• "Codominance gives a 3:1 ratio." Wrong. Both incomplete dominance and codominance give 1:2:1 phenotypically in a monohybrid F2.
• "AB blood group is incomplete dominance." Wrong. AB people make both antigens fully; nothing is intermediate.
• "I^A is codominant with i." Wrong. I^A and I^B are codominant with each other; each is completely dominant over i.
• Writing pink as Rr and then calling r "recessive". If you use Rr, remember r is not recessive here; that is why C^RC^W notation is clearer.
• Forgetting that AB × O can never give AB or O children. Each child gets I^A or I^B plus i.

MDCAT-style practice MCQs

1. When red-flowered and white-flowered four o'clock plants are crossed, all F1 plants have pink flowers. This is an example of:
A. Complete dominance
B. Codominance
C. Incomplete dominance
D. Multiple alleles

Show answer

C. The heterozygote is intermediate (pink) between the two parents, which is incomplete dominance.

2. The F2 phenotypic ratio of a monohybrid cross showing incomplete dominance is:
A. 3:1
B. 1:2:1
C. 9:3:3:1
D. 1:1

Show answer

B. The heterozygote has its own phenotype, so the phenotypic ratio equals the genotypic ratio, 1:2:1.

3. A person with blood group AB has the genotype I^AI^B. The relationship between I^A and I^B is:
A. I^A dominant over I^B
B. Incomplete dominance
C. Codominance
D. I^B recessive to I^A

Show answer

C. Both A and B antigens are made fully on the same red cells, so the alleles are codominant.

4. A man of blood group AB marries a woman of blood group O. Which blood groups are NOT possible in their children?
A. A and B
B. AB and O
C. A and O
D. B and AB

Show answer

B. Every child gets i from the mother and I^A or I^B from the father, so children can only be A (I^Ai) or B (I^Bi).

5. Parents with genotypes I^Ai and I^Bi have a child. The probability that the child has blood group O is:
A. 0
B. 1/4
C. 1/2
D. 3/4

Show answer

B. The cross gives 1 AB : 1 A : 1 B : 1 O, so the chance of ii is 1 in 4.

6. A pink snapdragon is crossed with a white snapdragon. The expected offspring are:
A. All pink
B. 1 red : 2 pink : 1 white
C. 1 pink : 1 white
D. 3 pink : 1 white

Show answer

C. C^RC^W × C^WC^W gives half C^RC^W (pink) and half C^WC^W (white).

7. The reappearance of red and white flowers in the F2 of a red × white snapdragon cross shows that:
A. The alleles blended in F1
B. Red is completely dominant
C. The alleles kept their identity and segregated
D. A mutation occurred in F1

Show answer

C. If the alleles had blended, pink × pink could give only pink; the return of both parental colours shows segregation.

8. How many genotypes and phenotypes are possible for the ABO blood group system?
A. 3 genotypes, 3 phenotypes
B. 4 genotypes, 4 phenotypes
C. 6 genotypes, 4 phenotypes
D. 6 genotypes, 6 phenotypes

Show answer

C. Three alleles give 6 genotypes (I^AI^A, I^Ai, I^BI^B, I^Bi, I^AI^B, ii) but only 4 blood groups (A, B, AB, O).

What ePrepare's notes cover for this topic

ePrepare's Biology conceptual notes cover the base you need for these questions in the Inheritance chapter: Mendel's experiments, his laws of dominance, segregation and independent assortment, monohybrid and dihybrid crosses with Punnett squares, and probability rules. Incomplete dominance and codominance are not separate lessons in the notes, so use this post for them and the notes for the Mendelian groundwork they build on.

Read how the notes work in ePrepare's MDCAT conceptual notes, and see why Inheritance is worth the effort in MDCAT Biology high-yield chapters. For the other big genetics pattern, go to sex-linked inheritance MCQs. To drill the topic under time pressure, build a chapter test with the Quiz Builder.

Quick revision table

Dominance patterns in one look

Point	Remember
Complete dominance	Heterozygote = dominant parent; F2 3:1
Incomplete dominance	Heterozygote intermediate; F2 1:2:1
Codominance	Both alleles fully shown; F2 1:2:1
Incomplete example	Four o'clock (Mirabilis) and snapdragon: red × white → pink
Codominance examples	AB blood group (IAIB), MN blood group, roan cattle
ABO alleles	IA, IB codominant; both dominant over i; 6 genotypes, 4 phenotypes
IAi × IBi	1 AB : 1 A : 1 B : 1 O
AB × O	1 A : 1 B; no AB, no O
Not blending	Parental types reappear in F2

آسان الفاظ میں: incomplete dominance میں کوئی allele پوری طرح dominant نہیں ہوتا، اس لیے heterozygote درمیان والا رنگ دکھاتا ہے، جیسے red اور white سے pink پھول۔ Codominance میں دونوں alleles ایک ساتھ پوری طرح نظر آتے ہیں، جیسے AB blood group میں A اور B دونوں antigens۔ دونوں میں F2 کا ratio 1:2:1 ہوتا ہے، جبکہ complete dominance میں 3:1۔

Frequently asked questions

What is the main difference between incomplete dominance and codominance?

In incomplete dominance the heterozygote is a mix in between the parents (pink from red and white). In codominance the heterozygote shows both parental traits fully at the same time (both A and B antigens in group AB).

Why is the F2 ratio 1:2:1 in both?

Because in both patterns the heterozygote looks different from either homozygote. Each genotype has its own phenotype, so the phenotypic ratio is the same as the genotypic ratio, 1:2:1.

Is the AB blood group codominance or multiple alleles?

Both. The ABO gene has three alleles in the population (multiple alleles), and two of them, I^A and I^B, are codominant with each other in an AB person.

Is incomplete dominance the same as blending inheritance?

No. The heterozygote looks blended, but the alleles stay separate. Red and white flowers reappear in F2, which could not happen if the alleles had truly mixed.

What are the standard examples for MDCAT?

Incomplete dominance: flower colour in the four o'clock plant and snapdragon. Codominance: the AB blood group and the MN blood group. Roan cattle is also quoted for codominance.

Which chapter are these in for MDCAT?

They are part of the Inheritance chapter of Biology, with Mendel's laws, sex linkage, and linkage and crossing over. See the full MDCAT syllabus guide.