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Incomplete Dominance, Codominance, Practice Problems and FAQs

Incomplete Dominance, Codominance, Practice Problems and FAQs

We are all aware of Mendel’s laws of inheritance and the first law proposed by Mendel, also known as the law of Dominance, states that out of a pair of factors or alleles (alternative forms of a gene) representing different traits of a character only one is dominant while the other one is recessive.

But did you know that this law is not absolute? When Mendel experimented with pea plants he could not see any blending of traits but when the same experiments were performed with other species of plants, blending and other deviations from Mendel’s law of dominance.

Sometimes alleles are not related as dominant and recessive and may equally express themselves as an intermediate character or be dominant simultaneously. An example for the first scenario will be a horse with a creamish white fur coat being born to parents, one of whom has a white fur coat while the other has a brown fur coat. On the contrary, an example for the second scenario will be that a cow with brown and white patches of fur is born to parents, one of whom has white fur while the other has brown fur. In this article we are going to discuss such deviations from Mendelism.

Table of contents:

Concept of Dominance

Genes are the specific sequences of nucleotides that code for specific proteins that control particular traits of an organism. Diploid organisms have two copies of each gene which are known as alleles. The alleles may be similar or dissimilar. Dissimilar alleles arise due to mutational changes in the nucleotides of a gene. The change in nucleotide sequence can result in an altered expression of the gene, that is, it may express a defective or non functional protein, altered protein with less efficiency or no protein at all. Consequently, the mutant allele fails to express the original trait and as it expresses a defective gene product, the protein expressed by the original allele dominates. The latter is therefore called the dominant allele or factor. The modified allele with a defective product will be the recessive allele. In case the modified allele gives rise to an altered but functional product, it behaves as a codominant or incompletely dominant allele. In some cases, the modified allele expresses the same trait as the original allele and such pairs are called equivalent allele pairs.

What is Incomplete Dominance?

Incomplete dominance is the phenomenon of neither of the two alleles of a gene being dominant over each other, such that when both of them are present together, a new, somewhat intermediate phenotype, between the independent expression of the two alleles, is expressed. 

Fig: Incomplete dominance of fur coat colour

Incomplete dominance in Antirrhinum majus and Mirabilis jalapa

Both 4 O’ clock flowers (Mirabilis jalapa) and snapdragon (Antirrhinum majus) have two pure breeding plants, red flowered and white flowered. Consider, the pure red flowered plant has an RR pair of alleles and the pure white flowered plant has an rr pair of alleles. A monohybrid cross between the red (RR) and white (rr) flowered plants of Snapdragon and Mirabilis was performed. The F1­ generation produced a pink coloured flower with an Rr pair of alleles.

Fig: F1 generation of Mirabilis

When the F1 progeny was self-pollinated to obtain the F2 generation, it resulted in the expression of red (RR), pink (Rr) and white (rr) flowers in the ratio of 1:2:1. Both the genotypic and phenotypic ratios in case of incomplete dominance are 1:2:1

Fig: F2 generation of Mirabilis

This was because of the blending of characters because neither of the two alleles were dominant over the other and resulted in the appearance of an intermediate phenotype due to incomplete dominance. 

These results were in contrast with the results obtained from Mendel’s monohybrid crosses of Pisum sativum because in Mendel’s crosses the all offspring of the F1 generation expressed the dominant trait whereas in F2 generation the ratio of phenotypic expression of dominant to recessive trait was 3:1. However, even Mendelian crosses showed a genotypic ratio of 1:2:1 in the F2 generation.

Fig: Incomplete dominance in Mirabilis and Antirrhinum

Incomplete dominance in Pisum sativum

Dominance is not an autonomous feature of an allele or the product it expresses. Sometimes, a single gene can influence multiple phenotypes in the body. For example, the alleles B and b of a gene control starch synthesis in pea plants. The genotype BB results in better starch synthesis and starch grains produced are large. Scratch synthesis is inefficient with the bb genotype and smaller grains of starch are produced. After maturation of seeds, the plants with the BB genotype have round seeds with large starch grains and the ones with the bb genotype have wrinkled seeds with smaller starch grains. Heterozygotes with Bb alleles produce round seeds with intermediate sizes of starch grains. Thus, if the phenotype of seed shape is considered, B seems to be dominant over b. However, if we consider the size of starch grain as the phenotype then the alleles show incomplete dominance and express an intermediate phenotype.


Codominance is the phenomenon in which two contrasting alleles of the same gene lack a dominant recessive relationship and express themselves simultaneously in a heterozygous condition.

The alleles for coat colour in short-horned cattle are codominant. The short-horned cattle have two pure breeding varieties - red hair having RR alleles and white hair having WW alleles. When these two pure breeding varieties are crossed, all offspring in the F1 progeny have a brownish or roan colour coat which occurs due to fine intermixing of small patches of red and white coloured hair. This occurs because both the alleles for red hair (R) and white hair (W) are expressed simultaneously in the F1 heterozygote having the alleles (RW)

Fig: F1 generation of short-horned cattle

Interbreeding of the F1 hybrids having a roan coat colour results in the appearance of three varieties of the animal - red haired (RR), roan haired (RW) and white haired (WW) in the ratio 1:2:1. Thus, both genotypic and phenotypic ratios in the F2 generation for the codominant alleles is 1:2:1

Fig: F2 generation of short-horned cattle

Codominance of alleles that control human blood group

Alleles which determine ABO blood grouping in human beings show codominance. ABO blood groups are controlled by multiple alleles of the gene I. This gene regulates the type of sugar polymers (antigens) that protrude from the plasma membrane of the red blood cells in our blood plasma. The three alleles of the gene (I) are IA, IB, and i. The allele IA expresses the A antigen on the surface of the RBCs, IB expresses B antigen on the surface of RBCs and i does not express any antigen on the surface of RBCs. Humans being diploid organisms possess any two of the three alleles of the gene I. 

IA and IB are completely dominant over i. When IA and i are present together in an allele, only IA expresses itself and the red blood cells will have A antigen on their surface and the person will have blood group A. When IB and i are present together in an allele, only IB expresses itself and the red blood cells will have B antigen on their surface and the person will have blood group A.

However, IA and IB are codominant and when both the alleles are present together in an individual, the RBCs in his/her plasma show the expression of both A antigen and B antigen on their surface and the person exhibits AB blood group. Six different combinations of genotypes are possible with these three alleles.

Allele from Parent 1

Allele from Parent 1

Genotype of offspring

Blood type of offspring





























A cross between a homozygous individual with A blood group (IAIA) and a homozygous individual with B blood group (IBIB) results in all offspring being of AB blood group (IAIB).

Fig: F1 generation showing progeny having all offspring with AB blood group

When two people with AB blood group mate, they produce two types of haploid gametes - 50% carrying the IA allele and the other 50% carrying the IB gene. A Punnett square for this cross can tell us that the progeny may have blood groups A (IAIA), AB (IAIB)or B (IBIB) in the ratio 1:2:1.

Fig: F2 generation showing progeny having blood group A, AB and B

Practice problems

Q1. What is the phenotypic ratio of F2 generation in a monohybrid cross between two organisms which are pure varieties for two traits which show incomplete dominance?

A. 1:2:1
B. 3:1
C. 9:3:3:1
D. 1:3

Solution: The alleles responsible for red flower colour (R) and white flower colour (r) in Mirabilis show incomplete dominance. When a purebred plant with red flowers (RR) is crossed with a purebred plant with white flowers (rr), the F1 generation had all pink flowers (Rr) as both the alleles expressed themselves so as to express an intermediate phenotype. When pink flowers were self-pollinated, the offspring of the F2 generation show the expression of red, pink and white flowers with genotypes RR, Rr and rr, respectively, in the ratio 1:2:1. Thus, in F2 generation both genotypic and phenotypic ratio is 1:2:1.

Hence, the correct option is a.

Q2. If a mother has A blood group and is heterozygous for this trait and a father has AB blood group, then what is the probability that the offspring will have blood group O?

A. 25%
B. 50%
C. 75%
D. 0%

Solution: The genotype of the heterozygous mother with A blood group - IAi

The genotype of the father with AB blood group - IAIB

Let us draw a Punnett square to understand the probability of blood group O in progeny.

Fig: Punnett square

As you can see, there is no probability that the offspring expresses O blood group. Thus, the correct answer is option d, that is, zero.

Q3. If the allele for red coat colour in short-horn cattle is R and that for white coat colour is W, what will be the phenotype for the genotype RW?

A. Red coat colour
B. White coat colour
C. Roan coat colour
D. Black coat colour

Solution: The alleles controlling the coat colour in short hor cattle show codominance. The short-horned cattle have two pure breeding varieties - red hair having RR alleles and white hair having WW alleles. When these two pure breeding varieties are crossed, all offspring in the F1 progeny have a brownish or roan colour coat which occurs due to fine intermixing of small patches of red and white coloured hair. 

Q4. After observing a plant in his garden, Rajesh hypothesised that the stem height exhibited incomplete dominance. In order to prove his hypothesis, he developed true-breeding lines of tall and short plants and crossed them. He obtained only plants with intermediate height in F1 progeny. He then self-pollinated the F1 hybrids and obtained the F2 progeny. Out of the F2 progeny, he sampled 2000 plants. Which of the following cases would hold true if his hypothesis were true?

a) 1000 tall plants, 500 intermediate plants, and 500 small plants
b) 500 tall plants, 1000 intermediate plants, and 500 small plants
c) 1500 intermediate plants, and 500 small plants
d) 500 tall plants, 500 intermediate plants, and 1000 small plants

Solution: The phenotypic ratio of the progeny that exhibits incomplete dominance is 1:2:1. Therefore, of 2000 plants sampled from the progeny, 500 should be tall, 500 should be small, and 1000 should be intermediate plants if incomplete dominance holds.


Question 1. Does eye colour show incomplete dominance?
Answer: Eye colour is an example of polygenic inheritance in which different alleles of different genes influence a particular phenotype. Characters such as eye colour, skin colour, etc which show polygenic inheritance also show incomplete dominance between the alleles that control them.

Question 2. Who was the first person to discover incomplete dominance?
Answer: Carl Correns was the first person to discover incomplete dominance in Mirabilis jalapa during his experiments.

Question 3. Does codominance occur in plants?
Answer: Codominance is observed in plants such as Rhododendron in which both the alleles for the expression of red and white petal colour are codominant and are expressed simultaneously. Thus flowers are seen to express both red and white petals.

Question 4. How do you determine if there is codominance?
Answer: If an offspring expresses the phenotypes of both the parents then the concerned alleles can be said to be codominant. If a black and white spotted puppy is born to parents one of whom has white fur and another who has black fur then we can say that the alleles expressing white and black fur coat are codominant. 

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Related Topics

Law of Dominance 

Multiple Allelism 

Law of Segregation 

The Law of Independent Assortment 

Mendelian disorders 

What Is Polygenic Inheritance? 

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