The study of heredity reveals that the transmission of traits from parents to offspring often follows predictable patterns based on gene interactions. Genes determine an organism’s characteristics, such as flower color or hair texture, by coding for specific proteins. While early models of genetics suggested a simple system where one version of a gene always masks another, the reality of inheritance is far more nuanced. Many traits are determined through interactions that do not result in a straightforward “on or off” expression.
What Incomplete Dominance Is
Incomplete dominance is a pattern of inheritance where a cross between two different purebred parents results in an offspring with a third, entirely new phenotype. This new trait is expressed as a blend of the two parental characteristics. The phenomenon occurs because neither of the two gene versions, known as alleles, is powerful enough to completely mask the effect of the other. Consequently, the heterozygous individual expresses an intermediate trait that appears as a combination of the two homozygous traits.
A classic example is the flower color in the snapdragon plant. When a plant that is purebred for red flowers is crossed with a plant that is purebred for white flowers, the resulting offspring do not express red or white. Instead, all the resulting flowers exhibit a uniform pink color. The pink hue is the observable, intermediate phenotype of the two parent colors.
How Alleles Create a Blended Trait
The mechanism behind this blending involves the specific alleles and the resulting genotype of the offspring. Geneticists often use specialized notation, such as superscripts on a single base letter, to denote the different alleles without implying absolute dominance. For instance, the allele for red might be designated as $C^R$ and the allele for white as $C^W$. When a red parent ($C^R C^R$) is crossed with a white parent ($C^W C^W$), the resulting first generation ($F_1$) offspring are all heterozygotes with the genotype $C^R C^W$.
This heterozygous genotype, $C^R C^W$, produces the pink phenotype because the $C^R$ allele does not produce enough red pigment protein to fully color the flower, and the $C^W$ allele produces no pigment. This reduced amount of red pigment creates the intermediate pink color. When these pink $F_1$ heterozygotes are crossed, the next generation ($F_2$) shows a distinct pattern in their genetic makeup.
The $F_2$ generation exhibits a predictable genotypic ratio of 1:2:1, corresponding to $C^R C^R$, $C^R C^W$, and $C^W C^W$. In incomplete dominance, the phenotypic ratio is identical to the genotypic ratio, 1:2:1. This means that one-quarter of the flowers are red ($C^R C^R$), two-quarters are pink ($C^R C^W$), and one-quarter are white ($C^W C^W$). The emergence of the original red and white traits in the $F_2$ generation confirms that the alleles did not permanently merge.
The Key Differences from Other Inheritance
Incomplete dominance is often contrasted with two other major inheritance patterns: complete dominance and codominance. In complete dominance, the presence of just one dominant allele completely hides the expression of the recessive allele. For example, a pea plant with one allele for purple flowers and one for white flowers will express only the purple phenotype, as the dominant allele fully masks the recessive one.
Codominance, in contrast, involves a heterozygous individual expressing both parental traits simultaneously. A classic example is the roan coat color in cattle, where an animal with alleles for both red hair and white hair will have a coat that features distinct patches of red hairs and white hairs. Both traits are fully expressed side-by-side.
The primary distinction is in the phenotype of the heterozygote. In complete dominance, the heterozygote looks exactly like one of the purebred parents. In incomplete dominance, the heterozygote looks like a blend of the two parents. In codominance, the heterozygote shows both parental traits at the same time. While all three patterns involve the interaction of two alleles, only incomplete dominance produces a new, intermediate trait that sits halfway between the two parental phenotypes.