Incomplete dominance, a phenomenon that is responsible for variations in different life forms leading to enhanced and better reforms through the genetics used by humans.
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Incomplete Dominance Definition
After Gregor Mendel discovered inheritance laws, the term ”incomplete dominance” was proposed by the German botanist, Carl Correns (1864–1933). Carl Correns continued research and conducted an experiment on four o’clock flowers. This experiment leads to the discovery of incomplete dominance–a condition in which a heterozygous individual doesn’t show a dominant allele but rather shows an intermediate phenotype of the phenotypes of the dominant and recessive alleles.
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What is incomplete dominance?
The phenomenon in which two true-breeding parents crossed to produce an intermediate offspring (also known as heterozygous) is called incomplete dominance. It is also referred to as partial dominance or intermediate inheritance.
In incomplete dominance, the variants (alleles) are not expressed as dominant or recessive; rather, the dominant allele is expressed in a reduced ratio.
To further understand the basic concept of incomplete dominance, some terms are defined briefly as follows:
- An allele is a form, version, or set of gene expressions. An organism consists of two alleles from each parent for one gene. The allele that masks or suppresses other alleles and becomes prominent in the offspring is called a dominant allele. The effect of an allele that is suppressed by the dominant allele and does not appear in the offspring is referred to as a recessive allele. This is in contrast to multiple allelic inheritance, where multiple alleles (various alleles (two or more) for the same gene) determine a trait, or polygenic inheritance, where the phenotype is influenced by the combined effects of multiple genes.
- An organism that possesses two same alleles for a specific gene and can truly breed for the allele is described as homozygous. An organism that possesses two different alleles for a specific gene is described as heterozygous.
- A set of genes in an organism that is inherited by the offspring and that determines the offspring’s observable physical features is called the genotype. Phenotype is determined by the genotype and refers to the organism’s appearance, characteristics, behavior, and development (physically observable features).
- The number of times of trait appearance in the offspring after crossing the genes or alleles of the specific trait identified through the genotypic ratio. The genotypic or genotype ratio is better understood through the Punnett square. Punnett square shows all the possible traits (genotypes) of the new offspring in graphical or table form after the crossing of homozygotes.
Defining incomplete dominance
Incomplete dominance is defined differently as follows:
- The incomplete dominance is referred to as the dilution of the dominant allele with respect to the recessive allele, resulting in a new heterozygous phenotype. For example, the pink color of flowers (such as snapdragons or four o’clock flowers), the shape of hairs, hand sizes, and voice pitch in humans.
- The intermediate trait appearance in between the phenotypes of homozygous traits in the heterozygote is called incomplete dominance.
- The formation of a third phenotype specifically with traits that results from the combination of parent alleles is known as incomplete dominance or
- The incomplete dominance is referred to as intermediate inheritance in terms of trait expression, and none of the alleles from the paired alleles expressed over the other for a particular trait.
According to some definitions, there are several assumptions about incomplete dominance; incomplete dominance occurs due to the combination of parent alleles, both dominant and recessive. Whereas, several definitions define incomplete dominance as a phenomenon in which the heterozygote possesses an intermediate trait between the two homozygous traits. Moreover, some definitions show incomplete dominance where the new offspring has a specific trait with less intensity than the dominant trait among the paired alleles. In other words, the trait is not completely dominant — or, it is neither dominant nor recessive.
The situation in which the phenotype of the heterozygous organism is clearly manifested is a cross between two homozygous phenotypes. After the combination of homozygous alleles (F1 generation), the heterozygote will have the intermediate trait. At F2 generation, it, then, shows a phenotype ratio of 1:2:1 phenotype in which the two are intermediate traits and the others are either dominant or recessive traits.
In incomplete dominance, both alleles of the homozygous genotypes are not expressed over one another; rather, an intermediate heterozygote is formed. Incomplete dominance is a key role factor in the variation of an organism’s features or characteristics.
Mechanisms of Incomplete Dominance
Mendel’s experiment shows complete dominance after crossing the pea plants’ traits (round and wrinkled), meaning the pea plants with specific traits; round and wrinkled peas were crossed. This results in pea plants with round peas showing round as a dominant allele. Thus, the dominant allele was expressed over the recessive allele which is wrinkled peas.
Keeping Mendel’s work under consideration, Carl Correns performed an experiment on four o’clock flowers. He took two true-breeding flower traits (red color as the dominant allele and white color as a recessive allele) of four o’clock flowers and crossed them. The results show an intermediate heterozygote with pink phenotype color flowers (none of the alleles get dominant). This situation in inheritance is known as incomplete dominance.
How does incomplete dominance work?
To understand the mechanism of incomplete dominance, the botanists use the Punnett square. The Punnett square predicts the genotype of the breeding experiment. In this case, one plant producing red flowers and another plant producing white flowers are crossed.
The above Punnett square results in heterozygous offspring with an intermediate trait of pink color, showing that no allele gets dominated over the other. The two alleles are not expressed in a way to hide the effect of the other allele; instead, the phenotype is in between the two, thus, resulting in an intermediate. Thus, the heterozygote is one that produces flowers with a pink color. This example illustrates how the presence of three alleles and their interaction through incomplete dominance can lead to a range of phenotypic outcomes, allowing for various coat colors that don’t strictly adhere to the characteristics of any single allele.
For F2 generation, the heterozygotes are crossed to see the respective phenotypes.
The phenotype in the F2 generation results in the same ratio as proposed by Mendel, i.e., 1:2:1. The offspring phenotypes were 25% red flowers, 25% white flowers, and 50% pink flowers. This shows that incomplete dominance does not necessarily involve absolute blending because the heterozygote contains both distinct traits or alleles, i.e., red and white color, which after crossing the heterozygotes in the F2 generation, the red and white color traits still appear. Moreover, in incomplete dominance, the offspring contain both alleles, but the alleles’ expression gets a blended phenotype of the two parental traits.
Incomplete Dominance and Codominance
The laws of inheritance proposed by Mendel (Mendelian inheritance or Mendelian genetics) defined the dominance factors in inheritance and the effects of alleles on the phenotypes of diploid organisms. Codominance and incomplete dominance are different types of inheritance (specifically genetic). However, both incomplete dominance and codominance types of dominance were not identified by Mendel. However, his work leads to their identification. Several botanists worked in the inheritance field and found these respective dominance types. Both incomplete dominance and codominance are often mixed up. Therefore, it is important to see the primary factors that lead to differing from each other.
Incomplete dominance
As mentioned earlier, incomplete dominance is partial dominance, meaning the phenotype is in between the genotype dominant and recessive alleles. In the above example, the resulting offspring has a pink color trait despite the dominant red color and white color trait due to incomplete dominance. The dominant allele does not mask the recessive allele resulting in a phenotype different from both alleles, i.e., pink color. The incomplete dominance carries genetic importance because it explains the fact of the intermediate existence of phenotype from two different alleles. Moreover, Mendel explains the Law of dominance that only one allele is dominant over the other, and that allele can be one from both. The dominating allele will reduce the effect of the recessive allele.
Whereas in incomplete dominance, the two alleles remain within the produced phenotype, but the offspring possess a totally different trait. Mendel did not study incomplete dominance because the pea plant does not show any incomplete dominance (intermediate traits). However, Mendel’s proposed ratio of 1:2:1 tends to be accurate for incomplete dominance, as seen in the example of the four o’clock flower, where the F1 generation results in red, pink, and white flowers’ genotypic ratio of 1:2:1, respectively.
These results show the Law of inheritance where alleles are inherited from parents to offspring still occurs in the incomplete dominance described by Mendel. In research on quantitative genetics, the possibility for incomplete dominance requires the resulting phenotype to be partially related to any of the genotypes (homozygotes); otherwise, there will be no dominance.
Codominance
Codominance refers to the dominance in which the two alleles or traits of the genotypes (of both homozygotes) are expressed together in offspring (phenotype). There is neither a dominant nor recessive allele in cross-breeding. Rather the two alleles remain present and formed as a mixture of both of the alleles (each allele has the tendency to add phenotypic expression during the breeding process).
In some cases, the codominance is also referred to as no dominance due to the appearance of both alleles (of homozygotes) in the offspring (heterozygote). Thus, the phenotype produced is distinctive from the genotypes of the homozygotes.
The upper case letters are used with several superscripts to distinguish the codominant alleles while expressing them in writing. This writing style indicates that each allele can express even in the presence of other alleles (alternative).
The example of codominance can be seen in plants with white color as a recessive allele and red color as the dominant allele producing flowers with pink and white color (spots) after cross-breeding. Similarly, Mendel also did not consider the codominance factor due to the pea plant’s limited traits. However, further research revealed the codominance in plants and vice versa. The genotypic ratio was the same as Mendel described. They produced offspring that resulted in the F1 generation including red, spotted (white and pink), and white with the same genotypic ratio.
Codominance can be easily found in plants and animals because of color differentiation, as well as in humans to some extinct, such as blood types. The incomplete dominance produces offspring with intermediate traits whereas the codominance involves the mixing of allelic expressions. However, in both types of dominance, the parent alleles remain in the heterozygote. Nonetheless, no allele is dominant over the other.
Table 1: Incomplete dominance vs. Codominance
| Incomplete dominance | Codominance |
|---|---|
| Incomplete dominance occurs in the heterozygote, in which the dominant allele is incompletely dominant, meaning it does not dominate the recessive allele entirely; rather, an intermediate trait appears in the offspring. | Codominance occurs when the alleles do not indicate any dominant and recessive allele relationship. However, each allele from homozygote is able to add phenotypic expressions in the offspring or simply the “mix” of each allele. |
| The offspring’s phenotype is an intermediate of the parents’ homozygous traits. | The phenotypic expression of homozygous in codominance is independent. |
| The expression of alleles in incomplete dominance is conspicuous, meaning none of the alleles dominates over the other. | The expression of alleles in codominance is uniformly conspicuous, meaning both alleles have an equal chance of expressing their effects. |
| The formed trait (phenotype) is different due to mixing both parents’ phenotypes and genotypes. | The formed trait (phenotype) is not different due to the no mixing of both parents’ phenotypes and genotypes. |
| The offspring do not show the parental phenotype. | The offspring shows both parental phenotypes. |
| The dominant allele does not dominate over the recessive allele. | The offspring phenotype produced possesses the combination of two alleles and, thus, shows two phenotypes together. |
| The dominant allele does not dominate over the recessive allele. | None of the alleles is dominant or recessive, and the dominating relationship does not occur. |
| The quantitative approach can be used for the analysis of incomplete dominance in organisms (including the analysis of both non-dominating alleles). | The quantitative approach can be used for the analysis of codominance in the organism (only including the analysis of gene expressions). |
| Incomplete dominance examples include Pink flowers of four o’clock flowers (Mirabilis jalapa), and physical characteristics in humans, such as hair color, hand size, and height. | Codominance can be seen in humans as well as in animals. The red blood cells/blood type (or groups A, B, and O) in humans and the spots on the feathers or hairs of livestock are examples of codominance. |
READ: Non-Mendelian Inheritance Tutorial
Incomplete Dominance Examples
Incomplete dominance is a widely studied phenomenon in genetics that leads to morphological and physiological variations. The pink flower color trait, which is an example of incomplete dominance, occurs in nature, such as those found in pink-flower-bearing angiosperms. Turns out the dominant allele is not expressed “completely” as shown in Figures 1 and 2 – incomplete dominance (Punnett square). Apart from plants, incomplete dominance also occurs in animals and humans. For example, hair color, eye color, and skin color traits are determined by multiple alleles in humans. Take a look at the examples below for the incomplete dominance in plants, humans, and other animals.
In plants
The Carnation plant (which is an example of incomplete dominance) has true-breeding white flowers and true-breeding red flowers. A cross between white- and red-flowering carnation plants may result in offspring with a phenotype of pink flowers.
Four o’clock flowering plants are an example of incomplete dominance. Red and white flowering plants breed to produce offspring with pink color flowers.
Snapdragon also shows incomplete dominance by producing pink-colored snapdragon flowers. The cross-pollination between red and white snapdragons leads to pink color flowers because none of the alleles (white and red) is dominant.
Incomplete dominance is used to improve corn crops as the partially dominating traits of corn are generally high yielding and healthier than the original ones with fewer traits.
The multiple alleles occupy the same locus of the chromosome within an organism which causes varying organisms’ varying characteristics. In plants, the self-sterility n is an example of multiple alleles that causes the rapid growth of pollen tubes.
In humans
Despite the concept of adaptation of incomplete dominance by humans in genetics to increase better living, incomplete dominance can also be seen in humans genetically. The crossing of two different alleles in the genetic process produces human offspring either with different or intermediate forms between the two traits. Thus, it can be said that incomplete dominance is as old as human life that leads to variation with time.
Most of the physical characteristics of humans, including hair, eye color, height, skin color, sound pitch, and hand size, show incomplete dominance. Children born with semi-curly or wavy hair are an example of individuals exhibiting incomplete dominance because of the crossing of parents’ alleles both straight and curly hairs to produce such offspring. Thus, incomplete dominance occurs to produce an intermediate trait between the two parent traits. The eye color of humans is a more common example of incomplete dominance. However, understanding incomplete dominance for eye color is quite complicated.
Human height patterns also show incomplete dominance. Parents with different heights have offspring that show height in between the parent’s height range rather than similar to any one of the parents.
Human skin color is another example of incomplete dominance because the genes that produce the melanin (pigment) for either dark or light skin cannot show dominance over the other. Thus, the offspring produced have an intermediate skin color between the parents.
Usually, male humans have high-pitched sounds, and other homozygotes have reduced sound pitches. The resulting heterozygote individual would have an intermediate voice pitch rather than high or low sound pitches.
Similar to the above characteristics of humans, hand sizes also show incomplete dominance in the same manner. The offspring will have intermediate or medium-sized hands as compared to his/her parents.
Also, carriers of Tay-Sachs disease show incomplete dominance. In Tay-Sachs, the individuals do not have enzymes responsible for breaking down the lipids, leading to the accumulation of lipids all over the body, especially in the brain and nervous system. The lipid accumulation leads to the loss of abilities, both physical and mental, due to nerve deterioration. Another disease named familial hypercholesterolemia (FH) shows incomplete dominance. One type of allele causes the generation of liver cells either normally or without the receptors of cholesterol. Thus, incomplete dominance causes these cells unable to fully remove the excess cholesterol from the blood.
In other animals
In some animals or birds, the phenomenon of incomplete dominance is also visible. Several examples of incomplete dominance can be seen in chickens, rabbits, dogs (Labradoodles), cats, and horses. Below are the ways that show how incomplete dominance occurs in these animals.
An Andalusian chicken (found in Spain) is an example of incomplete dominance. An offspring produced shows incomplete dominance in its feathers as the parents (a white-feathered male and a black-feathered female chicken) breed to produce an offspring with blue and tinged feathers. This incomplete dominance occurs due to a diluting gene that reduces the intensity of the effect of melanin (a pigment) and lightens the color of feathers in the offspring.
When long and short-furred rabbits are bred together, the offspring produced have varying lengths of fur (medium). Usually, the breeding of a short-furred Rex and a long-furred Angora produces medium-length furs.
Similarly, the dog’s tail’s length also shows incomplete dominance. When a long-tailed dog parent is bred with a short-tailed dog parent, the offspring produced has a medium-sized tail. Another example is the labradoodle. They have wavy hairs that result when the straight and curly-haired parent dogs are bred.
The other example includes the spots on animals’ bodies more visible in cats, dogs, and horses. When bred (a more spotted animal with a less spotted animal), these animals will produce offspring with varying spots (less than more spotted parent and more than less spotted parent).
Summary
Now you are able to identify the incomplete dominance examples in different life forms due to a better understanding of the respective term. Plus, next time you will go out somewhere, you will see which flowers show incomplete dominance and other small pets.
Moreover, try to explore yourself first, look at the characteristics you differ from your parents, and find if any one of those features shows incomplete dominance, such as your hair, sound, hand size, or height.
You can also practice crossing different alleles and seeing what characteristics the offspring will have by using the Punnett square.
Lastly, think about what you can add for better life forms by using the concept of incomplete dominance.
Try to answer the quiz below to check what you have learned so far about incomplete dominance.
References
- Anthony JF Griffiths, Miller, J. H., Suzuki, D. T., Lewontin, R. C., & Gelbart, W. M. (2018). An Introduction to Genetic Analysis. Nih.Gov; W. H. Freeman. https://www.ncbi.nlm.nih.gov/books/NBK21766/
- Boris, M. Volodymir, F. & Diana, B. (2006). “Genes interaction. Dominance, incomplete dominance, codominance, and lethal alleles”. Medical Biology Practicals. Genetics.
- Bagheri, H.C. (2006). Unresolved boundaries of evolutionary theory and the question of how inheritance systems evolve: 75 years of debate on the evolution of dominance “Journal of Experimental Zoology Part B: Molecular and Developmental Evolution”, Volume 306B, Issue 4, pp. 329–359
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