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Incomplete Dominance Definition
The term “Incomplete dominance” was first proposed by the German Botanist, Carl Correns (1864-1933). Based on his experiments conducted on the four o’clock plant, he described the concept of incomplete dominance.
Incomplete dominance can be defined as a condition in which none of the alleles shows complete dominance over others. The condition results in the phenotype that intermediates of the phenotype of both the alleles.
What is Incomplete Dominance?
Incomplete dominance is the phenomenon in which a heterozygous offspring produced by crossing two true-breeding parents intermediate the phenotype of both the parents. It is also described as partial dominance or intermediate inheritance.
The condition arises when any of the alleles are not expressed as dominant or recessive. In other words, the dominant allele cannot show complete dominance.
To understand the phenomenon of incomplete dominance, let’s understand some terms-
A set or version of gene expressions is called an allele. There are two alleles present in all organisms inherited from each parent for a specific gene. The allele can be categorized as dominant or recessive.
The prominent allele is called dominant whereas suppressed allele is called the recessive allele. The dominant allele shows its effect on the phenotype of the organism.
Based on different sets of alleles, an organism can be heterozygous or homozygous.
A genotype is a set of genes that is responsible for the features of an organism. The genotype determines the phenotype which refers to the physical appearance of the organism.
Defining incomplete Dominance
There are different ways to define incomplete dominance- The dilution of the dominant allele over the recessive allele is referred to as the incomplete dominance that results in the formation of a new heterozygous phenotype.
In heterozygous organisms, the intermediate trait appears between homozygous dominant and homozygous recessive alleles as termed as incomplete dominance.
The condition occurs due to a combination of both dominant and recessive alleles in an offspring. Whereas some definitions also show that incomplete dominance arises due to a specific trait in the offspring which is neither dominant nor recessive.
In that situation, the phenotype of the heterozygous offspring resembles intermediate characters. In incomplete dominance, an intermediate heterozygote is formed. This phenomenon plays a crucial role in the variation of an organism’s features.
Mechanisms of Incomplete Dominance
Mendel did his experiment in the pea plant and proposed laws of inheritance. He also described dominant and recessive traits. Carl Correns considered Mendel’s work and performed experiments in the four o’clock plant.
He crossed red flower-producing plants with white flower-producing plants and observed results. He got an intermediate pink-colored flower. Correns termed this phenomenon incomplete dominance.
How Does Incomplete Dominance Work?
The punnet square is used to understand the mechanism of incomplete dominance. It predicts the genotype of the offspring in the experiment. In this experiment, the red flower-producing plant is crossed with a white flower-producing plant. In the given picture, we can see that the result shows an intermediate phenotype that is a pink-colored flower.
It produces due to expressions of a dominant allele that cannot be expressed completely. So in incomplete dominance, the offspring contain both alleles but none of them are expressed completely therefore the phenotype gets intermediate between parent traits.
Incomplete Dominance vs Codominance
Mendel described the effects of alleles on the phenotype of an offspring. Based on phenotypic expressions, dominance can be categorized into different types. Codominance and incomplete dominance are also different types of inheritance. Mendel did not define both the term.
1. Incomplete Dominance
The phenomenon where the offspring show the phenotype that is intermediate between both the parents is called incomplete dominance.
For example- when a plant producing red flowers is crossed with the plant producing white flowers, the resulting offspring produces pink flowers, which is intermediate between red and white.
In incomplete dominance, the dominant allele cannot mask the recessive allele completely which results in the formation of a new phenotype. The garden pea plant does not show incomplete dominance therefore Mendel did not study this phenomenon. It can be seen in the plant of four o’clock flower
The phenomenon where both of the alleles for a specific trait are expressed together in the offspring is called codominance. In this condition none of the alleles is dominant or recessive, therefore both alleles remain present and express in the phenotype.
In the results, the heterozygotic offspring produced by crossing two homozygotes produce a distinctive phenotype. There are several examples of codominance that can be seen in plants as well as in animals.
For example- When the plant with a white flower crossed with a red flower it produces flowers with red and white spots. Codominance was also not studied by Mendel. Another example of codominance is blood type in humans.
|The condition in which the dominant allele does not show complete dominance over the recessive allele.||The condition, in which none of the alleles show dominant and recessive characters.|
|The phenotype of the offspring is intermediate between both parents’ traits.||The phenotype of the offspring expresses traits of both alleles.|
|Mixing of traits occur.||No mixing of traits occur.|
|Examples- pink flowers of Mirabilis jalapa.||Examples- The blood type (A, B, and O) in humans.|
Incomplete Dominance Examples
Several morphological and physiological variations occur due to incomplete dominance. Examples of incomplete dominance include the pink flower color trait in Mirabilis jalapa and other species. Incomplete dominance also occurs in some animals and humans.
Hair color, eye color, and skin color are some common examples of incomplete Dominance in humans.
The common example of incomplete dominance is the four o’clock plant. In Mirabilis jalapa, the red and white flowers are crossed to produce pink flowers. The Carnation plant is also an example of incomplete dominance in which a cross between a red flowering plant and a white flowering plant produces pink flowers in its phenotype.
The phenomenon have also some significant roles in crop improvement. A corn plant has partially dominating traits that are high yielding than original ones therefore the crop is improved by using incomplete dominance.
The phenomenon of incomplete dominance can also be seen in humans. The common examples are hair color, eye color, skin color, height, and sound pitch, etc.
For example, when a person having curly hair marries a female having straight hair, their offspring may be born with wavy hairs. The eye color of humans also shows incomplete dominance. Incomplete dominance can also be seen in height patterns and skin color. However, skin color is determined by genes that produce melanin therefore parents with dark and light skin produce offspring with intermediate skin color.
In Other Animals
The phenomenon is also visible in some other animals and birds. Chicken, rabbits, dogs, cats, and horses also show incomplete dominance. The common example of incomplete dominance is the Andalusian chicken. Feathers of Andalusian chicken show the intermediate color of both the parents.
When a white feathered male crosses with black feathered female chicken, the offspring results in blue and tinged feathers. Similarly, a dog’s tail is also an example of incomplete dominance. The length of the tail shows an intermediate phenotype when a long-tailed dog is crossed with a short-tailed dog. The spots in cats, dogs, and horses are also examples of incomplete dominance.
Incomplete Dominance Citations
- Incomplete dominance of the low antibody response to Cryptosporidium parvum antigens in mice selected for high and low antibody responsiveness. Braz J Med Biol Res . 1996 Nov;29(11):1479-83.
- An Introduction to Genetic Analysis. 7th edition. Griffiths AJF, Miller JH, Suzuki DT, et al. New York: W. H. Freeman; 2000.
- Incomplete dominance of deleterious alleles contributes substantially to trait variation and heterosis in maize. PLoS Genet. 2017 Sep; 13(9): e1007019.