Incomplete Dominance: Definition, Concept, and Example

Incomplete dominance is a type of partial inheritance in which the phenotype of the phenotype of first filial (F1) generation resembles an intermediate character that neither dominates male nor the female parent. In simple words, when none of the factors of a gene is dominant, the phenotype of a heterozygous dominant individual is a blend of dominant and recessive traits. This is known as Incomplete Dominance.

This genetic phenomena was first proposed by a biologist & scientist, Gregor Mendel. The observable characters of an organism is called phenotype such as colour, hair etc. The examples of Incomplete dominance is seen in plants, animals and humans.

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What is Incomplete Dominance?

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Incomplete dominance is an important genetic phenomenon that happens when recessive alleles are not masked completely by dominant alleles. The concept of incomplete dominance is best understood by the law proposed by the German botanist-Carl Correns. The incomplete dominance theory was firstreported in a 4’o clock plant (Mirabilis jalapa). 

According to law of incomplete dominance,

“None of the two alleles are dependent upon each other resulting in the phenotype that is the combination of both.” 

• It was also discovered that Mendel's laws were not universal in occurrence, ie; the law was not true in all the cases. 
• There are characters or alleles that are not dominant or recessive.
• In such cases both the alleles of the contrasting conditions of a character expresses a mixture.
• As a result of which, the hybrid of the two pure parents does not resemble any of the parents but appears midway between them .

Mechanism of Incomplete Dominance

It is a phenomenon where none of the two contrasting alleles of a gene are dominant over each other. Mendel conducted a dihybrid cross between homozygous red and homozygous white-flowered plant. 

• In the first generation, he got the pink flowered plant which he considered intermediate character of both parent plants. 
• From the expirement, Mendel Incomplete dominance is an exception of Mendel’s laws of inheritance.

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Examples of Incomplete Dominance

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The examples of incomplete dominance are observed in all plants, animals and humans that are explained below in detail:

Flowers

In the case of Mirabilis jalapa ( Four O'clock plant) there are two types of flower in pure state: red and white. When a red flowered Mirabilis plant is crossed with a white flowered Mirabilis plant, all the F1 plants instead of producing the red flower, produce intermediate coloured flowers.

The effect is caused by partial or incomplete dominance of allele for red colour. On selfing the pink flowers of F1 plants in F2 generation , three phenotypes of the flowers are produced - red, pink and white flowers in the ratio 1:2:1. The pink colour of the flower is due to the interaction of the two alleles of red and white flowers.

Birds

Depending on the feather colour of an Andalusian fowl, there are three varieties of fowl - splashed white, black and blue. A cross between a pure splashed white male and a pure black female fowl produces hybrid fowls with blue colour in F1 generation . The blue appearance of the hybrid is due to very fine alternating black and white stripes on the feathers. The F1 blue hybrids on crossing with each other give the usual Mendelain ratio for monohybrid cross that is one splashed white, two blue and one black in the ratio 1:2:1.

Outcomes of the cross

1. The genes for black and splashed white colour do not mix-up in the F1 hybrids and segregate in the F2 generation
2. There is no specific gene for the blue colour of the fowl. 

Codominance

Co-dominance is closely related to Incomplete Dominance. Here, both the alleles are simultaneously expressed in the heterozygote. Even the co-dominance doesn't follow Mendel's law. When the two alleles present on the same gene locus of homologous chromosomes, neither show dominant-recessive relationship nor show intermediate condition but both of them express themselves simultaneously, then this condition is known as codominance. The alleles which express their individual effect together is called codominant alleles .

Let us take the same example of a red and white flower . In case of incomplete dominance we have seen that the colour of the hybrid flower turned pink as neither of the alleles were able to dominate upon each other but in codominance, we will see that the flower will be having patches of red and white instead of a uniformly pink flower . 

Let us take another example of AB blood groups in human beings. In human beings there are three types of alleles (IA, IB, & IO or I) for the four blood groups (A, B, AB & O). It is because of the fact that, out of these three alleles, IA and IB alleles are dominant over IO whereas IA and IB are co dominant alleles. If a person possesses both IA and IB alleles as the contrasting alleles of a gene, these will be able to produce both A antigen as well as B antigen and hence the blood group of the person becomes AB.

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Codominance v/s Incomplete Dominance

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In incomplete dominance, none of the parent traits are completely dominant and they blend to form a new character where the effect of one of them is slightly more than that of the other. The offspring generally displays a phenotype different from its parents. Snapdragon and Mirabilis are examples of incomplete dominance. 

In codominance, both the parent traits are dominant and they are equally expressed in the offspring and likewise, their effects are also visible . They don't show any new phenotype. Roan character in cattle is one example of this trait . 

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Dominance and Incomplete Dominance

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Following are some of the major differences between dominance and incomplete dominance: 

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Previous Years Questions

1.  Which of the following is an example of pleiotropy ?…..[ CUCET 2011]
2. Who proposed the chromosomal theory of inheritance ?….[CUCET 2010]
3. The children of a father with type 'O' blood and mother with type….[COMEDK UGET 2012]
4. Epicanthus skin fold above the eyes and transverse palmer….[COMEDK UGET 2007]
5. Distance between the genes and percentage of….[COMEDK UGET 2015]
6. Which one of the following is a sex-linked disease….​[CUCET 2010]
7. The science that deals with the application of principles….[CUCET 2010]
8. A 10 years old patient is found to have the following….[COMEDK UGET 2012]
9. Which of the following genes shows the heterozygous...[BHU UET 2008]
10. Which chromosomal constitution refers to Jacob's syndrome….[BHU UET 2012]
11. Which of the following is an example of intergenic gene….
12. Which of the following is not considered as a mutagen…
13. Haemophilia is an example of….
14. In order to lessen the suffering of phenylketonurics...[AMUEEE 2014]
15. In Mendel's experiments with garden pea, round seed shape..[JKCET 2015]
16. Genotypic and phenotypic ratio in monohybrid cross remains..[JKCET 2011]