Incomplete dominance results from a cross in which each parental contribution is genetically distinct, giving rise to progeny with an intermediate phenotype. Semi-dominance and partial dominance are other terms for incomplete dominance. Mendel described dominance but not partial dominance. If he had worked with Mirabilis jalapa instead of a pea, he would have considered crossing a plant with red flowers with one with white flowers. Pink flowered progeny were all produced, indicating an intermediate phenotype. When a person is heterozygous for a gene, he or she has two different alleles, but this is not always enough to produce an intermediate phenotype. As heterozygotes, some genotypes that cause disease if both alleles are recessive show no phenotype. However, if both parents contribute the mutated form of the gene, the child will be homozygous, recessive, and sick. Other diseases show up as a phenotype intermediate between the parents. Incomplete dominance can also result from one gene’s effect masking the phenotype of another. Because of these epistatic interactions, many colour pathways are complicated.
Incomplete Dominance
Incomplete dominance refers to the phenomenon in which two true-breeding parents cross to produce an intermediate offspring (also known as heterozygous). Partial dominance or intermediate inheritance are other terms for it.
The variants (alleles) in incomplete dominance are not expressed as dominant or recessive; rather, the dominant allele is expressed in a reduced ratio.
To help you understand the fundamental concept of incomplete dominance, the following terms are defined briefly:
- A form, version, or set of gene expressions is referred to as an allele. An organism is made up of two alleles from each parent for a single gene. A dominant allele masks or suppresses other alleles and becomes prominent in the offspring. A recessive allele is the effect of an allele that is suppressed by the dominant allele and does not appear in the offspring. Multiple alleles are different alleles (two or more) for the same gene.
- A homozygous organism has two identical alleles for a specific gene and can truly breed for the allele. A heterozygous organism has two different alleles for a specific gene.
- The genotype is a set of genes in an organism that is inherited by the offspring and determines the offspring’s observable physical characteristics. The genotype determines the phenotype, which refers to the appearance, characteristics, behaviour, and development of the organism (physically observable features).
- The number of times a trait appears in the offspring as a result of crossing the genes or alleles of the specific trait identified by the genotypic ratio. The Punnett square provides a better understanding of the genotypic ratio. After homozygotes are crossed, Punnett square displays all possible traits (genotypes) of the new offspring in graphical or table form.
Defining incomplete dominance
Incomplete dominance is defined differently:
- The dilution of the dominant allele about the recessive allele results in a new heterozygous phenotype due to incomplete dominance. For example, the pink colour of flowers (such as snapdragons or four o’clock flowers), hair shape, hand sizes, and human voice pitch.
- Incomplete dominance refers to the appearance of intermediate traits in the heterozygote between the phenotypes of homozygous traits.
- Incomplete dominance is the formation of a third phenotype with traits that result from the combination of parent alleles.
- In terms of trait expression, incomplete dominance is referred to as intermediate inheritance, and none of the alleles from the paired alleles expressed more than the other a distinguishing feature
There are several assumptions about incomplete dominance, according to some definitions; incomplete dominance occurs as a result of the combination of parent alleles, both dominant and recessive. Incomplete dominance, on the other hand, is defined by several definitions as a phenomenon in which the heterozygote produced possesses an intermediate trait between the two homozygous traits. Furthermore, some definitions of incomplete dominance show that the new offspring has a specific trait that is less intense than the dominant trait among the paired alleles. To put it another way, the trait is neither dominant nor recessive.
A cross between two homozygous phenotypes occurs when the heterozygote’s phenotype is manifested. The heterozygote will have the intermediate trait after the combination of homozygous alleles (F1 generation). At the F2 generation, it has a 1:2:1 phenotype ratio, with two intermediate traits and the rest being either dominant or recessive traits.
In the case of incomplete dominance, neither allele of the homozygous genotype is expressed over the other; instead, an intermediate heterozygote is formed. Incomplete dominance plays an important role in the variation of an organism’s features or characteristics.
Mechanisms of Incomplete Dominance
Mendel’s experiment demonstrates complete dominance after crossing the traits of pea plants (round and wrinkled), implying that pea plants with specific traits; round and wrinkled peas were crossed. As a result, pea plants with round peas have round as the dominant allele. As a result, the dominant allele, wrinkled peas, was expressed over the recessive allele.
Carl Correns experimented on four o’clock flowers while keeping Mendel’s work in mind. He crossed two true-breeding flower traits (red as a dominant allele and white as a recessive allele) of four o’clock flowers. The findings show an intermediate heterozygote with pink flowers (none of the alleles gets dominant). In inheritance, this is known as incomplete dominance.
Incomplete Dominance Examples
Incomplete dominance is a well-studied genetic phenomenon that results in morphological and physiological variations. Pink flower colour trait occurs in nature, such as those found in pink-flower-bearing angiosperms, and is an example of incomplete dominance. The dominant allele is not expressed “completely” – this is known as incomplete dominance (Punnett square). Animals and humans, in addition to plants, exhibit incomplete dominance. In humans, for example, multiple alleles determine hair colour, eye colour, and skin colour. Consider the following examples of incomplete dominance in plants, humans, and other animals.
In plants
The Carnation plant has true-breeding white flowers and true-breeding red flowers (an example of incomplete dominance). A cross between white- and red-flowering carnation plants may produce pink-flowering offspring.
Plants that bloom at four o’clock are an example of incomplete dominance. Pink flowers are produced by crossing red and white flowering plants.
Snapdragon also demonstrates insufficient dominance by producing pink snapdragon flowers. Cross-pollination of red and white snapdragons results in pink flowers because neither allele (white or red) is dominant.
Incomplete dominance is used to improve corn crops because corn with partially dominating traits is generally higher-yielding and healthier than corn with fewer traits.
Multiple alleles occupy the same chromosomal locus within an organism, resulting in varying organism characteristics. Self-sterility n is an example of multiple alleles in plants that cause the rapid growth of pollen tubes.
In humans
Despite the concept of human adaptation of incomplete dominance in genetics to improve living conditions, incomplete dominance can also be seen in humans genetically. In the genetic process, the crossing of two different alleles results in human offspring with either different or intermediate forms of the two traits. As a result, it can be stated that incomplete dominance is as old as human life, resulting in variation over time.
Most human physical characteristics, such as hair colour, eye colour, height, skin colour, sound pitch, and hand size, exhibit incomplete dominance. Children with semi-curly or wavy hair are an example of an individual with incomplete dominance because the crossing of parents’ alleles results in such offspring having both straight and curly hair. As a result, incomplete dominance occurs, resulting in an intermediate trait between the two parent traits. Human eye colour is a more common example of incomplete dominance. However, comprehending incomplete dominance for eye colour is a difficult task.
Human height patterns exhibit partial dominance as well. Parents of different heights have offspring who are in the middle of their parents’ height range rather than similar to any of the parents.
In other animals
The phenomenon of incomplete dominance can also be seen in some animals or birds. Incomplete dominance can be seen in chickens, rabbits, dogs (Labradoodles), cats, and horses. The examples below demonstrate how incomplete dominance occurs in these animals.
An example of incomplete dominance is an Andalusian chicken (found in Spain). As the parents (a white feathered male and a black feathered female chicken) breed to produce an offspring with blue and tinged feathers, the offspring shows incomplete dominance in its feathers. This partial dominance is caused by a diluting gene, which reduces the intensity of the effect of melanin (a pigment) and lightens the colour of the offspring’s feathers.
Conclusion
Incomplete dominance results from a cross in which each parental contribution is genetically distinct, giving rise to progeny with an intermediate phenotype. Semi-dominance and partial dominance are other terms for incomplete dominance. Incomplete dominance refers to the phenomenon in which two true-breeding parents cross to produce an intermediate offspring (also known as heterozygous). Partial dominance or intermediate inheritance are other terms for it. There are several assumptions about incomplete dominance, according to some definitions; incomplete dominance occurs as a result of the combination of parent alleles, both dominant and recessive. Mendel’s experiment demonstrates complete dominance after crossing the traits of pea plants (round and wrinkled), implying that pea plants with specific traits; round and wrinkled peas were crossed. Incomplete dominance is a well-studied genetic phenomenon that results in morphological and physiological variations. Pink flower colour trait occurs in nature, such as those found in pink-flower-bearing angiosperms, and is an example of incomplete dominance.