what is incomplete dominance in genetics

2 min read 20-03-2025

what is incomplete dominance in genetics

Incomplete dominance, also known as partial dominance, is a type of inheritance where neither allele for a specific trait is completely dominant over the other. This results in a heterozygous phenotype that is a blend or intermediate between the two homozygous phenotypes. Unlike complete dominance, where one allele completely masks the other, incomplete dominance shows a third, mixed phenotype. This article will delve into the concept of incomplete dominance, providing examples and clarifying its differences from other inheritance patterns.

Understanding Alleles and Phenotypes

Before diving into incomplete dominance, let's review some fundamental genetic terms. An allele is a variant form of a gene. Genes determine traits, like flower color or hair color. Most genes have two alleles, one inherited from each parent. The phenotype refers to the observable characteristics of an organism, while the genotype describes the genetic makeup.

How Incomplete Dominance Works

In incomplete dominance, a heterozygote displays a phenotype that is a mixture of the two homozygous phenotypes. Let's illustrate this with a classic example: snapdragons.

Example: Snapdragon Flower Color

Suppose you have a red-flowered snapdragon (RR) and a white-flowered snapdragon (rr). If these plants are crossed, you would expect the offspring (F1 generation) to have a pink phenotype (Rr). This is because neither the red (R) nor the white (r) allele is completely dominant. The heterozygotes show a blending of the two parental phenotypes, resulting in pink flowers.

Image: Include an image here showing a Punnett square for a snapdragon cross (RR x rr), resulting in Rr offspring with pink flowers. Use alt text like: "Punnett square showing incomplete dominance in snapdragons."

The F2 Generation

If you cross two pink snapdragons (Rr x Rr), the resulting F2 generation will show a phenotypic ratio of 1:2:1 - one red (RR), two pink (Rr), and one white (rr). This demonstrates the blending inheritance characteristic of incomplete dominance.

Incomplete Dominance vs. Complete Dominance

It's crucial to distinguish incomplete dominance from complete dominance. In complete dominance, one allele completely masks the expression of the other. For example, if you crossed a homozygous dominant tall pea plant (TT) with a homozygous recessive short pea plant (tt), all the offspring (Tt) would be tall, as the tall allele (T) is completely dominant over the short allele (t).

Incomplete Dominance vs. Codominance

Incomplete dominance is also different from codominance. In codominance, both alleles are fully expressed in the heterozygote. A classic example is ABO blood type. Individuals with the AB blood type express both A and B antigens equally on their red blood cells, not a blend of the two.

Other Examples of Incomplete Dominance

Incomplete dominance isn't limited to flower color. It's seen in other traits across various species, including:

Human hair: In some populations, curly hair (CC) and straight hair (cc) can result in wavy hair (Cc) in heterozygotes.
Animal coat color: Certain breeds of animals show incomplete dominance in their coat color genes.
Fruit and vegetable color: Examples can be found in various fruits and vegetables.

Summary: Understanding Incomplete Dominance

Incomplete dominance is a fascinating aspect of genetics, showcasing the complex interplay of alleles. It demonstrates that gene expression isn't always a simple case of one allele completely dominating another. Understanding incomplete dominance helps to better interpret and predict inheritance patterns in a variety of organisms. Remember that this type of inheritance shows a blended phenotype, different from the complete masking in complete dominance or the equal expression in codominance.