Incomplete Dominance & Mendel’s Experiment

Incomplete Dominance is a form of intermediate inheritance where neither of the two alleles for a particular trait is completely dominant over the other. In this type of dominance, the heterozygous phenotype is a blend or intermediate between the two homozygous phenotypes. The concept of incomplete dominance emerged as a significant discovery following Gregor Mendel’s initial work in genetics. Incomplete dominance plays an important role in contributing to the diversity and variation of an organism’s traits or characteristics.

What is Incomplete Dominance?

Incomplete dominance is a post-Mendelian discovery. Carl Correns, a German botanist, conducted an experiment on four o’clock flowers and proposed the term “incomplete dominance”. Incomplete dominance refers to a condition where a heterozygous individual does not exhibit the dominant allele but instead shows a phenotype that is intermediate between the phenotypes of the dominant and recessive alleles. It is also known as partial dominance or intermediate inheritance. For example: In four o’clock plant there are two types of pure breeding plants, red flowered and white flowered. On crossing the two, F1 plants produced pink flowers. The pink flower is due to incomplete dominance.

Mechanism of Incomplete Dominance

Generally, genes are present in pairs in an individual, with one copy inherited from each parent. In complete dominance, one allele is fully expressed, masking the effects of the other allele. However, in incomplete dominance, the heterozygous genotype (having two different alleles) results in a phenotype that is a combination or blend of the phenotypes associated with each allele. The mechanism of incomplete dominance is as follows:

The presence of two dominant red alleles is represented by RR, while the presence of two recessive white alleles is denoted by rr. In a test cross between red and white flower-bearing plants, the F1 generation produced pink flowers, so it is evident that the heterozygous individuals (Rr) show an intermediate phenotype. This means that neither allele is completely dominant, and the pink color is a blend of the red and white alleles rather than one allele fully overshadowing the other.

In the F2 generation, when the heterozygotes (Rr) are cross-pollinated to assess their traits, their progeny will exhibit the genotypes RR, Rr, and rr in a ratio of 1:2:1. This means that some individuals will inherit two copies of the dominant R allele, some will inherit two copies of the recessive r allele, and others will inherit both the dominant R and recessive r alleles. This genotypic ratio aligns with the classic Mendelian ratio observed in inheritance.

This clarifies that incoomplete dominance does not mean complete blending, as demonstrated by the fact that half of the F2 progeny still exhibit the parental homozygous alleles. Despite all the offspring in the F1 generation being heterozygous and displaying the intermediate phenotype, the presence of both the dominant and recessive alleles is still evident in the subsequent generation.

Concept of Dominance

The concept of dominance describes the relationship between alleles of one gene. It determines how traits are expressed in an organism’s phenotype based on the combination of alleles it inherits. In a pair of alleles for a specific trait, one allele may show its influence over the other, resulting in a dominant phenotype, while the other allele remains hidden or recessive. This means that the dominant allele is expressed in the phenotype, and its traits are observed, while the recessive allele is not phenotypically expressed in the presence of the dominant allele. A heterozygous gene consists of a pair of dissimilar alleles, whereas a homozygous gene carries two identical alleles. In heterozygous alleles, each allele contains distinct information regarding traits. When we describe one allele as dominant over the other, it can be due to two possible reasons:

1. The recessive allele may be non-functional, meaning it does not contribute to the expression of the trait.
2. The recessive allele may be less active compared to the dominant allele, resulting in the dominance of the trait associated with the dominant allele.

These mechanisms explain how dominance is established within alleles and the impact it has on the expression of traits in organisms.

Incomplete Dominance and Codominance

Incomplete dominance and codominance are two different types of inheritance that involve the expression of alleles and the resulting phenotypes in organisms. In incomplete dominance, neither allele completely dominates or masks the other allele. Instead, the heterozygous phenotype is an intermediate or blended expression of both alleles. For example, in snapdragons, a red allele and a white allele may result in pink flowers in heterozygous individuals.

In codominance, both alleles are expressed simultaneously in the phenotype. Each allele contributes its own observable trait without blending or compromising with the other. For example, in certain blood types, the A and B alleles are codominant, resulting in individuals with both A and B antigens on their red blood cells.

Examples of Incomplete Dominance

Example of incomplete dominance is as follows:

In Humans

• Parents with curly hair and straight hair will always have a child with wavy hair.
• Carriers of Tay-Sachs disease and Familial hypercholesterolemia (FH)  exhibit incomplete dominance.

In Animals

• When a long-tailed dog is bred with a short-tailed dog, the resulting offspring shows a medium-sized tail.
• When long-furred rabbits are bred with short-furred rabbits, the offspring shows medium fur length.

In Plants

• When deep purple eggplants are combined with white eggplants, the resultant offspring is light violet in color.
• When red and white carnation flowers are crossed,​ offspring with pink phenotypic flowers are produced.

FAQs on Incomplete Dominance

1. How does incomplete dominance differ from complete dominance?

Answer:

In complete dominance, one allele is completely dominant over the other, resulting in a clear dominant phenotype. In incomplete dominance, neither allele is completely dominant, and the heterozygous phenotype is an intermediate blend of the two alleles.

2. What is incomplete dominance?

Answer:

Incomplete dominance is an inheritance pattern where neither allele for a specific trait is completely dominant over the other. Instead of one allele fully masking the expression of the other, the heterozygous phenotype displays an intermediate or blended phenotype.

3. Does incomplete dominance occur in all organisms?

Answer :

Incomplete dominance can occur in various organisms, but its prevalence may vary. It is observed in plants, animals, and humans for a range of traits, such as flower color, hair texture, and blood type.

4. Give an example of incomplete dominance.

Answer:

Example of incomplete dominance is flower color in snapdragons. When a red-flowered snapdragon (RR) is crossed with a white-flowered snapdragon (WW), the resulting offspring (RW) have pink flowers. Pink is an intermediate phenotype between red and white, demonstrating incomplete dominance.

5. What is incomplete dominance called?

Answer:

Incomplete dominance is a genetic inheritance pattern in which neither of two alleles for a particular gene completely masks or dominates the other allele. Incomplete dominance is also known as partial dominance or semi dominance.