Incomplete Dominance & Mendel's Experiment: Definition and Examples

Incomplete dominance is a genetic phenomenon where neither allele is completely dominant over the other. As a result, offspring show a blended phenotype or intermediate expression of both traits. This differs from what is typically observed in Mendelian genetics, where one dominant allele masks the expression of the recessive one. For example, crossing red and white snapdragon flowers results in pink offspring.

This Story also Contains

1. What is Incomplete Dominance?
2. Mendel's Experiments
3. Distinction between Complete Dominance and Codominance
4. Mechanism of Incomplete Dominance
5. Examples and Case Studies
6. MCQs on Incomplete Dominance

This concept challenges the traditional dominance-recessive patterns and helps explain more complex inheritance. Incomplete dominance highlights the diversity and variation in gene expression, which cannot be explained by Mendel’s Laws of Inheritance alone. It plays an important role in understanding how traits are passed down and expressed in genetics.

What is Incomplete Dominance?

Incomplete dominance of alleles is when the dominant allele does not hide the recessive one. Instead of fully expressing one allele, intermediate phenotypes of both alleles are observed. Charles Darwin's theory of evolution by natural selection in the 19th century laid the groundwork for understanding how traits change over time in populations. However, Darwin did not know how these traits were inherited. Gregor Mendel performed experiments on pea plants. He made numerous observations and mathematical interpretations that provided the foundation of modern genetics. The three laws of inheritance: Law of Dominance, Segregation and Independent Assortment.

It is important to study incomplete dominance because it highlights the complexity of genetics. Beyond simple dominant and recessive genes, there are variations in how genes work and appear in organisms. In this type of inheritance, both alleles contribute to the phenotype which deepens the understanding of how genes are expressed. It has applications in various fields such as medicine, agriculture, evolutionary biology, biodiversity conservation and many more.

Mendel's Experiments

Gregor Mendel also made important experiments using the pea plant as his model organism and laid the great foundation for genetics. His experiments led to the formulation of three fundamental laws of inheritance:

Law of Dominance

Mendel’s Law of Dominance states that when two different alleles of a gene, one allele (dominant) will mask the expression of the other (recessive). This means the dominant trait is observed in the offspring, while the recessive trait remains hidden unless both recessive alleles are present (homozygous recessive).

Law of Segregation

According to Mendel’s Law of Segregation, alleles for a particular trait separate during the formation of gametes and gametes get only one allele of the trait from either of the two parents.

Law of Independent Assortment

According to Mendel’s Law of Independent Assortment, alleles for different genes assort independently of one another during gamete formation. The genes should be located on different chromosomes or are far apart on the same chromosome. This means the inheritance of one trait does not influence the other.

These laws are based on genotypes such as the seed shape, seed color, flower position, and height of the plant. They have helped in understanding heredity and marked the foundation for modern day genetics.

Distinction between Complete Dominance and Codominance

Complete dominance and codominance are two different patterns of genetic inheritance that reflect how alleles influence a trait. These follow principles of inheritance and variations that are different from that of Mendel’s. Incomplete dominance differs from complete dominance and codominance in how alleles interact.

• Complete Dominance

In the heterozygous condition, one allele completely masks the expression of the second allele in the genotype. For instance, in Mendel’s pea plants, the yellow seed colour concealed the green seed because yellow seed is a dominant characteristic and the green 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 tells us that the genes for these traits are equally active, allowing both to contribute to the organism’s observable characters. A classic example is the human Blood groups where the AB group has both A and B antigens and no antibody against each other, demonstrating codominance.

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

A good example of incomplete dominance 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, intermediate of the two extremes, the pure red and white flowers. This example clearly shows that incomplete dominance, neither allele is fully dominant or completely hidden. Instead, the traits blend and appear as a mix in the phenotype.

Mechanism of Incomplete Dominance

Incomplete dominance occurs when neither allele is completely dominant over the other. This occurs because the dominant allele does not produce enough protein to completely mask the effect of the recessive allele. The mechanism of incomplete dominance is explained below-

Genetic Basis

The term incomplete dominance results from alleles in which both alleles are equally active. This leads to a heterozygous genotype 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. As a result, the heterozygous individuals will exhibit pink flowers.

Punnett squares describing Incomplete Dominance

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 heterozygous (Rr) snapdragon is crossed with another heterozygous (Rr) snapdragon, the Punnett square shows:

Genotype Ratio: 1 RR: 2 Rr: 1 rr

Phenotype Ratio: 1 Red: 2 Pink: 1 White

Examples and Case Studies

Incomplete dominance can be observed in various organisms. The examples and case studies help to study how this pattern of inheritance differs from complete dominance. Some of the examples are explained below-

Snapdragon Flower Color:

Genetic Basis: In snapdragons (Antirrhinum species), the red allele dominates the 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.

Recommended Video for Incomplete Dominance

MCQs on Incomplete Dominance

Q1. The inheritance of flower colour in Antirrhinum (dog flower ) is an example of

1. Incomplete dominance

2. Co-dominance

3. Multiple alleles

4. Linkage

Correct Answer: 1) Incomplete dominance

Explanation:

In the cross between true-breeding red-flowered dog flowers (RR) and true-breeding white-flowered dog flowers (rr), the F1 generation shows a pink phenotype (Rr). This is an illustration of incomplete dominance, where neither of the alleles completely dominates the other. Thus, the heterozygous offspring show a mix of the two parental traits, and therefore, pink-coloured flowers are formed instead of red or white. This genetic interaction shows the complexity of inheritance patterns in flowering plants.

Hence, the correct answer is Option (1) incomplete dominance.

Q2. Statement 1- F1 of snapdragon When experiments do not resemble either of the two parents and are in between the two of the parent phenotypes.

Statement 2- In snapdragon, Antirrhinum majus offspring produced are of pink flowers (Rr) when cross between a homozygous parent with white flowers (rr) and a homozygous parent with red flowers (RR) is done.

1. Statement 1 and statement 2 both are right.

2. Statement 1 is wrong , statement 2 is right

3. Statement 2 is wrong , statement 1 is right.

4. Both the statements are wrong

Correct Answer: 1) Statement 1 and Statement 2 both are right

Explanation:

F1 of snapdragon When experiments do not resemble either of the two parents and are in between the two of the parent phenotypes because one parent has a completely white phenotype and another parent has a red phenotype but the offspring produced in F1 has a pink phenotype. Antirrhinum majus a cross between a homozygous parent with white flowers (rr) and a homozygous parent with red flowers (RR) will produce offspring with pink flowers (Rr).

Hence, Option 1 is the correct answer. Statement 1 and statement 2 both are right.

Also Read:

• MCQs on Incomplete Dominance

• Polygenic Inheritance

• Gene Mapping