Incomplete Dominance & Mendel's Experiment: Definition and Examples

Incomplete dominance definition: Incomplete dominance is a genetic phenomenon where neither allele in a pair completely dominates the other, resulting in a blended phenotype in the offspring. Unlike complete dominance, where one trait is fully expressed, incomplete dominance leads to an intermediate trait. For example, crossing red and white snapdragon flowers results in pink offspring. In this article, incomplete dominance, Mendel's experiments, the mechanism of incomplete dominance, and examples and case studies are discussed. Incomplete dominance is a topic of the chapter Principles of Inheritance and Variation in Biology.

This Story also Contains

1. What is Incomplete Dominance?
2. Mendel's Experiments
3. Mechanism of Incomplete Dominance
4. Examples and Case Studies

What is Incomplete Dominance?

Incomplete dominance of alleles is when the second allele does not hide the first, and instead of fully expressing the predisposition of one allele, intermediate phenotypes of both alleles are observed. Charles Darwin published his theory of evolution in the 19th century; however, the initial study on the laws of heredity was conducted by Gregor Mendel on pea plants.

He made numerous observations and mathematical interpretations that provided the foundation of modern genetics; laws of Segregation and Independent Assortment. It is important to study incomplete dominance because, besides simple dominant and recessive genes, there are variations in how genes work and appear in things, and it is useful to know more about that. This is important in areas like medicine, agriculture and the study of other aspects of evolution because knowledge of genetic inheritance enables development in disease cure, improvement in food crops and the effort to conserve species

Mendel's Experiments

Gregor Mendel also made important experiments using the pea plant as his model with which he laid the great foundation for genetic science. His experiments led to the formulation of two fundamental laws of inheritance:

Law of Segregation

According to this law of segregation, alleles for a particular trait separate during the formation of gametes and gametes get only one allele on this trait from either of the two parents.

Law of Independent Assortment

Mendel also found that alleles for different genes do not get transmitted together to the next generation, during gamete formation if the two alleles are situated on different chromosomes or if they are far apart on the same chromosome.

These laws based on genotypes such as the shape of a seed, color, and height of the plant have helped in understanding heredity and marked the foundation for genetics as a science.

Law of Dominance

In Law of Dominance, there is complete dominance in which one allele masks the other allele and there is a dominance series where there is an intergradient of three phenotypes of the two genotypes. In complete dominance, one allele is dominant to another and thus can prevent the other from expression while in incomplete dominance, both alleles and neither is dominant to the other.

Distinction between Complete Dominance and Codominance

Incomplete dominance differs from both complete dominance and codominance in how alleles interact: Incomplete dominance differs from both complete dominance and codominance in how alleles interact.

Complete Dominance

In the heterozygous condition, one allele G completely masks the expression of the second allele g in the genotype. For instance, in Mendel’s pea plants, the yellow seed colour concealed the green colour trait since it is a dominant characteristic whereas the green colour seed is a recessive characteristic.

Codominance

In codominance, both alleles are dominant in the sense that both contribute to the phenotype of the heterozygous individual without the blending. This leads to the genes that code for these two traits being expressed fully, hence the organism will exhibit the particular traits. A well-documented example is blood groups in human beings where the AB group has both A and B antibodies.

Examples in Nature (E.g., Flower Colour In Snapdragons)

A good incomplete dominance example is a flower colour in snapdragons (Antirrhinum species) where F1 plants are blush-orange coloured flowers and the F2 plants have six different phenotypes, with no dominant phenotype. In this case, the R-genotype represents incomplete dominance over the r-genotype where R stands for the red flower colour and r for the white flower colour. Rr genotypes are considered heterozygous and the plants have pink flowers in the middle of the two extremes, the pure red and white flowers. This case clearly shows that the system of incomplete dominance leads not to the completely dominant or thoroughly recessive relation, but rather to the scales of phenotypic manifestation.

Also Read:

• MCQ Practice on Incomplete Dominance
• Principles of Inheritance and Variation
• Genetics
• Molecular Basis of Inheritance

Mechanism of Incomplete Dominance

The mechanism of incomplete dominance is explained below-

Genetic Basis

The term incomplete dominance results from alleles in which neither allele has a superior phenotype over the other. This leads to a heterozygous genotype that is that is an equal blend of the two homozygous genotypes. For instance in snapdragons, the R allele for red flowers is codominant with the r allele for white flowers, this is because heterozygous individuals will exhibit pink flowers.

Punnett squares describing Incomplete Dominance preemptively.

Example: Red Snapdragons, White Snapdragons and Pink Flowers

Genotypes:

• Homozygous Red: RR

• Homozygous White: rr

• Heterozygous Pink: Rr

Phenotypes:

• RR: Red flowers

• rr: White flowers

• Rr: The intermediate phenotype/ transition phase/ grade II is lavender flowers or pink flowers.

Genotype and Phenotype Ratios:

When a homozygous red (RR) snapdragon is crossed with a homozygous white (rr) snapdragon, the Punnett square shows:

Genotype Ratio: 1 RR: 2 Rr: 1 rr

Phenotype Ratio: Therefore the R alleles will all paint red and the r alleles paint white.

Examples and Case Studies

The incomplete dominance examples and case studies are explained below-

Snapdragon Flower Color:

Genetic Basis: In snapdragons (Antirrhinum species), the L: red allele dominates the L: white allele, but not completely. Dominant and recessive plants came out pink and pure red respectively; the Rr plants showed a pink flower which is an intermediate phenotype.

Andalusian Chickens (Feather Color):

Genetic Basis: In Andalusian chickens, the blue allele is an example of complete dominance to the black allele, marked B/b. Purebred chickens with BB genes for body colour are blue, those with a single pair of genes Bb are splash, a mottling of blue and white and those with bb genes are black.

Human Hair Texture:

Genetic Basis: Human hair texture includes the type of hair such as curly, wavy, or straight and this trait arises due to multiple genes and the type of dominance. There are dominant and recessive genes containing Curly (C), Wavy (W) and straight (S) hair textures combined to give a variability of hair textures.

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