The transmission of traits from parents to offspring is a fundamental process in biology, governed by principles of heredity. Each individual inherits genetic information, organized into units called genes, from their biological parents. Genes exist in different versions known as alleles, and these alleles determine specific characteristics, such as eye color or flower color. In basic Mendelian genetics, traits often appear in a dominant-recessive pattern, where one allele’s effect can mask that of another. However, not all genetic traits follow this simple dominant-recessive relationship, leading to more varied outcomes in offspring.
Incomplete Dominance Explained
Incomplete dominance describes an inheritance pattern where the heterozygous phenotype is an intermediate blend of the two homozygous phenotypes. When present together, the alleles produce a new phenotype that appears as a mixture of the parental traits, as neither allele is completely dominant.
A classic example of incomplete dominance is observed in snapdragon flowers. If a true-breeding red-flowered snapdragon is crossed with a true-breeding white-flowered snapdragon, the offspring will all have pink flowers. The pink color arises because the allele for red pigment production is not fully dominant over the allele for white. In heterozygous pink flowers, the amount of red pigment produced is less than in homozygous red flowers, leading to a diluted, intermediate coloration. When two pink snapdragons are crossed, their offspring can exhibit red, pink, and white flowers in a predictable ratio, illustrating the blending nature of this inheritance.
Codominance Explained
Codominance is an inheritance pattern where both alleles in a heterozygote are fully and distinctly expressed, without any blending. Both parental traits are simultaneously and equally visible in the phenotype, as neither allele masks the other.
The human ABO blood group system provides a clear illustration of codominance. The alleles for A and B antigens are codominant. An individual inheriting both the A and B alleles will have AB blood type, meaning their red blood cells express both A and B antigens distinctly. Another example is seen in roan cattle, which have a coat composed of both red and white hairs. A cross between a red cow and a white cow can produce roan offspring, where individual red hairs and individual white hairs are present, rather than a blended pink or light red color.
Distinguishing the Two: Key Differences
The primary distinction between incomplete dominance and codominance lies in the appearance of the heterozygous phenotype. In incomplete dominance, the heterozygous individual exhibits a blended or intermediate phenotype, like pink snapdragons from red and white parents. This often involves a reduction in the product of one allele, leading to less intense expression.
Conversely, codominance shows both parental traits fully and distinctly in the heterozygote, with no blending. For example, individuals with AB blood type express both A and B antigens, and roan cattle have both red and white hairs. Thus, incomplete dominance creates a novel, intermediate phenotype, while codominance ensures both original phenotypes are expressed side-by-side.