What is Incomplete Dominance?
Incomplete dominance is a form of Gene interaction in which both alleles of a gene at a locus are partially expressed, often resulting in an intermediate or different phenotype. It is also known as partial dominance.
For eg., in roses, the allele for red color is dominant over the allele for white color. However, the heterozygous flowers with both alleles are pink in color.
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 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.
Mechanism of Incomplete Dominance
Incomplete dominance occurs because neither of the two alleles is completely dominant over the other. This results in a phenotype that is a combination of both.
Gregor Mendel conducted experiments on pea plants. He studied on seven characters with contrasting traits and all of them showed a similar pattern of inheritance. Based on this, he generalized the law of inheritance.
Later, researchers repeated Mendel’s experiment on other plants. Shockingly, they noted that the F1 Generation showed variation from the usual pattern of inheritance.
The monohybrid cross resulted in F1 Progeny which didn’t show any resemblance to either of the parents, but an intermediate progeny.
Let’s understand the incomplete dominance with the example of Snapdragon flower (Antirrhinum sp).
A Monohybrid cross was done between the red and white colored flowers of the Snapdragon plant. Consider, the pure breed of the red flower has an RR pair of alleles, and that for the white flower is rr.
Firstly, true-breeding red (RR) and white (rr) colored flowers of snapdragons were crossed. The F1 generation produced a pink-colored flower with an Rr pair of alleles.
Then the F1 progeny was self-pollinated. This resulted in red (RR), pink (Rr), and white (rr) flowers in the ratio of 1:2:1.
Recollect that the genotype ratio of F2 generation in the monohybrid cross by Mendel also gave the same ratio of 1:2:1. However, the phenotype ratio has changed from 3:1 to 1:2:1. The reason for this variation is the incomplete dominance of the allele R over the allele r. This led to the blending of color in flowers.
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 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 the 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
Incomplete dominance and codominance are different from each other.
In codominance, both the alleles present on a gene are expressed in the phenotype. A flower showing codominance will have patches of red and white instead of a uniformly pink flower.
In incomplete dominance, the F2 generation from heterozygous plants will have a ratio of 1:2:1 with the phenotypes red, white, and spotted flowers.
The humans with AB blood type also show codominance where the alleles for both blood types A and B are expressed.
Examples of Incomplete Dominance
Examples of incomplete dominance are mentioned below:
The child of parents each with curly hair and straight hair will always have wavy hair. Carriers of Tay-Sachs disease exhibit incomplete dominance.
In Other Animals
The Andalusian chicken shows incomplete dominance in its feather color.
When the rabbits with long and short furs are bred, the offspring produced will have medium fur length.