Genetics is a phenomenon, and one such concept is incomplete dominance. In this article, we will explore incomplete dominance definition, and explain the difference between incomplete dominance and codominance. We will also touch upon who discovered incomplete dominance and offer unique insights not found elsewhere.

Introduction

Incomplete dominance is a type of genetic interaction where neither allele completely masks the effect of the other. In simple terms, when an organism has two different alleles for a trait, the resulting phenotype is an intermediate blend of the two. This means that the incomplete dominance definition involves partial expression of both alleles rather than one allele being fully dominant over the other.

For example, if you cross a pure-bred red flower (RR) with a pure-bred white flower (rr), the resulting offspring will exhibit a pink colour (Rr). This incomplete dominance example clearly shows that the phenotype is neither red nor white but a blend of both.

Also Check: Mendel’s Law of Inheritance

Mechanism of Incomplete Dominance

The mechanism behind incomplete dominance is based on the idea that each allele contributes to the phenotype, but neither is strong enough to completely override the other. In heterozygous individuals, both alleles are partially expressed, resulting in an intermediate phenotype. This is why, when we ask what is incomplete dominance, the answer is that it is a form of gene interaction where the heterozygote exhibits a mix of both parental traits.

For instance, in snapdragon plants, when you cross red and white flowers, the heterozygous progeny (Rr) display pink flowers. This example of incomplete dominance also demonstrates that the genotype ratio in the F₂ generation (resulting from self-pollination of the heterozygote) is 1:2:1, but the phenotype ratio differs from Mendel’s classic 3:1, highlighting the unique pattern of incomplete dominance.

Incomplete Dominance Examples in Nature

Incomplete dominance examples can be found across various species:

Snapdragons: As mentioned, red (RR) and white (rr) snapdragons produce pink (Rr) offspring.
Roses: While many assume red is completely dominant, some crosses yield intermediate hues.
Animals: In certain breeds of chickens, such as the Andalusian chicken, feather colour shows intermediate shades due to incomplete dominance. Similarly, when rabbits with long and short furs are mated, their offspring exhibit medium-length fur.

Comparing Incomplete Dominance and Codominance

It is crucial to distinguish between incomplete dominance and codominance. Although they might seem similar, the difference between incomplete dominance and codominance lies in how the alleles are expressed in the phenotype:

In incomplete dominance, the heterozygous phenotype is a blend of both alleles. A classic incomplete dominance example is the pink flower from red and white snapdragons.
In codominance, both alleles are fully expressed and visible simultaneously. For instance, in human AB blood type, both A and B antigens appear together. This marks the difference between codominance and incomplete dominance.

By understanding these differences, students can grasp the nuance of genetic expression. This section aims to clarify the difference between incomplete dominance and codominance so that learners can easily distinguish between these two genetic phenomena.

Who Discovered Incomplete Dominance?

A common question among budding geneticists is who discovered incomplete dominance. While Gregor Mendel’s experiments with pea plants laid the foundation of classical genetics, his work primarily demonstrated complete dominance. The phenomenon of incomplete dominance was later identified through studies on other plants, such as snapdragons. Researchers in the early 20th century observed that some traits did not follow Mendel’s strict dominant-recessive pattern, leading to the identification and further study of incomplete dominance.

Understanding who discovered incomplete dominance provides historical context to this genetic concept and enriches our appreciation of how genetic theories have evolved.

Additional Insights: The Molecular Basis and Educational Activities

Understanding the molecular basis of incomplete dominance not only deepens our grasp of genetic inheritance but also clarifies what is incomplete dominance at a biochemical level. At its core, incomplete dominance arises because neither allele at a gene locus is completely dominant over the other. This often occurs due to differences in gene dosage and variations in enzyme or protein activity.

The Molecular Basis of Incomplete Dominance

At the molecular level, the phenomenon of incomplete dominance can be explained by the following mechanisms:

Gene Dosage and Enzyme Activity: In many cases, each allele contributes a certain amount of functional protein or enzyme. For instance, in the classic incomplete dominance example of snapdragons, one allele might produce a fully active enzyme responsible for pigment synthesis, while the other allele produces a less active form. In heterozygous plants, the combination of these alleles leads to an intermediate level of pigment production, resulting in a pink colour rather than the pure red or white seen in the homozygous conditions. This scenario is a clear representation of the incomplete dominance definition in action.
Partial Protein Expression: Sometimes, the two alleles produce proteins that vary in their effectiveness. When both proteins are present, they do not fully complement each other to produce a dominant phenotype. Instead, the resulting phenotype is an intermediate blend, which further emphasises what is incomplete dominance at the molecular scale.
Regulatory Gene Expression: The regulation of gene expression can also contribute to incomplete dominance. If one allele is transcribed at a lower level than the other, the overall expression of the trait will be a mix of both alleles. This subtle interplay of gene regulation underpins the intermediate phenotypes observed in many organisms.

Interactive Quiz

1. What is incomplete dominance?
A) A condition where one allele completely masks the other.
B) A type of gene interaction where both alleles are partially expressed.
C) A scenario where alleles are not involved in trait expression.
Correct Answer: B

2. Which of the following is an example of incomplete dominance?
A) A heterozygous snapdragon produces a pink flower.
B) A person with AB blood type displaying both A and B antigens.
C) A pea plant showing the dominant tall phenotype.
Correct Answer: A

3. What is the main difference between incomplete dominance and codominance?
A) In codominance, only one allele is expressed, while in incomplete dominance both are expressed.
B) In incomplete dominance, the heterozygous phenotype is a blend of both alleles, while in codominance both alleles are fully expressed.
C) There is no difference between them.
Correct Answer: B

Conclusion

Incomplete dominance is an aspect of genetics where the heterozygous phenotype is a blend of both parental traits. With clear incomplete dominance definition and examples of incomplete dominance like the pink snapdragon, students can easily see the contrast with the difference between codominance and incomplete dominance. Additionally, understanding who discovered incomplete dominance adds historical depth to this topic.

FAQs on Incomplete Dominance in Genetics: Definition, Examples & Comparisons

1. What is incomplete dominance in genetics?

Incomplete dominance is a pattern of non-Mendelian inheritance where a heterozygous individual shows a phenotype that is an intermediate blend of the two homozygous phenotypes. Unlike complete dominance, neither allele in the gene pair completely masks the other. For instance, if a red flower (RR) is crossed with a white flower (rr), the heterozygous offspring (Rr) will be pink, a phenotype that is in-between red and white.

2. How does incomplete dominance differ from codominance?

The key difference lies in how the alleles are expressed in the heterozygote.

In incomplete dominance, the phenotypes blend together to create a new, intermediate phenotype. A classic example is the pink snapdragon flower resulting from red and white parents.
In codominance, both alleles are fully and distinctly expressed at the same time. For example, in human AB blood type, both A and B antigens are present on the surface of red blood cells.

3. What are some classic examples of incomplete dominance in plants and animals?

Several well-known examples illustrate the concept of incomplete dominance:

Snapdragon Plants (Antirrhinum majus): The cross between a pure-red flowered plant (RR) and a pure-white flowered plant (rr) produces F1 offspring with pink flowers (Rr).
Four O'Clock Plants (Mirabilis jalapa): Similar to snapdragons, these plants also show a 1:2:1 ratio of red, pink, and white flowers in the F2 generation.
Andalusian Chickens: When a black chicken (BB) is crossed with a white chicken (WW), the heterozygous offspring (BW) are blue-grey, an intermediate feather colour.
Rabbit Fur Length: The mating of a rabbit with long fur and one with short fur can result in offspring with medium-length fur.

4. Why is incomplete dominance considered a deviation from Mendel's Law of Dominance?

Incomplete dominance deviates from Mendel's Law of Dominance because this law states that in a heterozygote, one allele (the dominant one) will completely conceal the presence of the other allele (the recessive one). However, in incomplete dominance, the heterozygous phenotype is not identical to the dominant parent's phenotype. Instead, it is a new, intermediate phenotype, which shows that neither allele is truly dominant over the other, thus challenging the simple dominant-recessive relationship described by Mendel.

5. What is the molecular basis that explains incomplete dominance?

The molecular reason for incomplete dominance often relates to gene dosage or the amount of protein produced. For example, one allele (R) might code for an enzyme that produces red pigment, while the other allele (r) might code for a defective, non-functional enzyme. In a homozygous RR individual, two doses of the enzyme produce a lot of red pigment. In a heterozygous Rr individual, only one dose of the functional enzyme is produced, leading to about half the amount of pigment and resulting in a lighter, pink colour.

6. Are there any examples of incomplete dominance found in humans?

Yes, certain human traits and genetic disorders exhibit incomplete dominance. A prominent example is Familial Hypercholesterolemia (FH).

Individuals with two normal alleles have normal cholesterol levels.
Individuals with two mutated alleles (homozygous recessive) have extremely high cholesterol levels and severe cardiovascular disease from a young age.
Heterozygous individuals have one of each allele and exhibit an intermediate phenotype with cholesterol levels higher than normal but lower than those with the severe form.

Another example is Tay-Sachs disease, where heterozygotes produce about half the amount of the functional enzyme compared to non-carriers, showing an intermediate biochemical phenotype.

7. How do the F2 generation's genotypic and phenotypic ratios in an incomplete dominance cross compare to a Mendelian cross?

This is a critical point of comparison. In a monohybrid cross:

For Mendelian (complete) dominance, the F2 generation shows a genotypic ratio of 1:2:1 (e.g., 1 TT : 2 Tt : 1 tt) but a phenotypic ratio of 3:1 (e.g., 3 tall : 1 dwarf), because both TT and Tt express the dominant tall phenotype.
For incomplete dominance, the F2 generation's genotypic ratio is also 1:2:1 (e.g., 1 RR : 2 Rr : 1 rr). However, because the heterozygote (Rr) has its own unique intermediate phenotype (pink), the phenotypic ratio is also 1:2:1 (e.g., 1 red : 2 pink : 1 white). In this case, the genotypic and phenotypic ratios are identical.