Introduction: Understanding Incomplete Dominance in Genetics

Incomplete dominance is a fascinating genetic concept where the phenotype of heterozygotes is an intermediate between the phenotypes of the two homozygous parents. Unlike complete dominance, where one allele completely masks the effect of the other, in incomplete dominance, both alleles contribute to the phenotype in a more blended way. This results in offspring showing a phenotype that is a mix of both parental traits.

What is Incomplete Dominance?

In a typical inheritance scenario, one allele can be dominant over another. However, in incomplete dominance, neither allele is completely dominant or recessive. Instead, the heterozygous offspring express a phenotype that is somewhere between the two homozygous phenotypes.

For example, consider a case where a red flower allele (A) is incompletely dominant over a white flower allele (a). When heterozygous, the offspring may exhibit an intermediate phenotype, such as pink flowers, rather than the full red or full white expression.

Genotypes and Phenotypes of Incomplete Dominance

When two heterozygous parents (Aa x Aa) are crossed, the possible genotypes of the offspring are:

• AA (homozygous dominant): Red flowers

• Aa (heterozygous): Pink flowers (intermediate phenotype)

• aa (homozygous recessive): White flowers

Expected Phenotypic and Genotypic Ratios

Since the alleles exhibit incomplete dominance, the phenotypic ratio and genotypic ratio in the progeny will differ from those in classic Mendelian inheritance.

• Genotypic ratio: The genotypic ratio in the offspring will be 1:2:1 (1 AA, 2 Aa, 1 aa).

• Phenotypic ratio: The phenotypic ratio will also be 1:2:1, with one group having red flowers (AA), two groups having pink flowers (Aa), and one group having white flowers (aa).

Thus, the progeny of a cross between two heterozygous parents will show three distinct phenotypes: red, pink, and white flowers.

Why is the Phenotypic Ratio 1:2:1?

In a cross between two heterozygous parents (Aa x Aa), the following combinations are possible for the offspring:

• AA: The offspring with this genotype will show the phenotype of the homozygous dominant parent (e.g., red flowers).

• Aa: The offspring with this genotype will show an intermediate phenotype, which is the result of incomplete dominance (e.g., pink flowers).

• aa: The offspring with this genotype will show the phenotype of the homozygous recessive parent (e.g., white flowers).

This results in a 1:2:1 phenotypic ratio — one-third of the offspring will resemble one homozygous parent (red), one-third will have the intermediate phenotype (pink), and one-third will resemble the other homozygous parent (white).

The Correct Answer

For a cross between two heterozygous parents where the allele A is incompletely dominant over allele a, the expected phenotypic ratio is:

(3) 3:1 phenotypic ratio

This is because the three phenotypes will be red, pink, and white, where the intermediate phenotype (pink) represents the incomplete dominance in the heterozygotes.

Conclusion: Incomplete Dominance and Its Impact on Inheritance Patterns

Incomplete dominance results in an intermediate phenotype in heterozygous organisms, and the progeny of two heterozygous parents will show a 1:2:1 genotypic ratio and a 3:1 phenotypic ratio. Understanding incomplete dominance provides insights into how traits can be inherited in more subtle ways than previously thought. This concept is crucial for understanding various genetic disorders and plant breeding practices.