Heredity involves the transmission of characteristics from parents to their offspring. Genetics, the study of heredity, explores the patterns by which these traits are passed down through generations. These inherited traits determine many aspects of an organism’s structure, physiological processes, and behaviors.
What Incomplete Dominance Means
Incomplete dominance describes a form of inheritance where the heterozygous genotype results in an intermediate phenotype. This means the observable trait in the offspring is a “blend” or mix of the traits seen in the two homozygous parents. For instance, if one parent contributes an allele for a red trait and the other for a white trait, their offspring might display a pink trait. This phenomenon is also referred to as partial dominance or intermediate inheritance. The alleles in incomplete dominance are not expressed as purely dominant or recessive; instead, the dominant allele’s expression is reduced, leading to this intermediate outcome.
The Genetic Mechanism
The underlying mechanism of incomplete dominance involves the way different versions of a gene, known as alleles, produce their effects. One common explanation is that one allele might produce a functional protein or enzyme, while the other allele produces a non-functional or only partially functional version of that same product.
For example, a gene might be responsible for producing a certain amount of pigment. If an individual inherits two alleles for high pigment production, they will have a dark color. If they inherit two alleles for no pigment production, they will be white.
However, a heterozygous individual, having one allele for high pigment and one for no pigment, will produce an intermediate amount of the pigment. This occurs because the single functional allele only generates about half the amount of protein or enzyme compared to an individual with two functional alleles. This reduced quantity of gene product leads to an intermediate observable trait.
Common Examples in Nature
Incomplete dominance is observed across various organisms, illustrating how traits can blend. A classic example is the flower color in snapdragons (Antirrhinum majus). When a red-flowered snapdragon is crossed with a white-flowered snapdragon, the offspring produce pink flowers. This pink color is a direct blend of the red and white parental phenotypes, showing that neither the red nor the white allele is fully expressed over the other.
Another instance can be seen in the feather color of Andalusian chickens. If a black-feathered chicken is mated with a white-feathered chicken, their offspring will have blue-tinged feathers. This blue color represents an intermediate phenotype between black and white, resulting from a gene that dilutes melanin pigment intensity.
In humans, hair texture also provides an example. When an individual with curly hair and an individual with straight hair have offspring, their child often has wavy hair, which is an intermediate texture between the two parental traits.
How It Differs from Other Patterns
Incomplete dominance is distinct from other patterns of inheritance, specifically complete dominance and co-dominance. In complete dominance, one allele completely masks the effect of another allele in the heterozygous state. For example, in Gregor Mendel’s pea plants, if a pea plant inherited an allele for purple flowers and an allele for white flowers, the flowers would be entirely purple, as the purple allele fully masks the white allele.
Co-dominance, conversely, occurs when both alleles in a heterozygous individual are expressed equally and distinctly, without blending. A common example is the human ABO blood group system, where individuals with type AB blood express both A and B antigens on their red blood cells. Another illustration is the roan coat color in certain cattle, where both red and white hairs are present simultaneously, creating a mottled appearance rather than a blended color like pink.