Basic Principles of Inheritance
Genetic inheritance describes how characteristics pass from parents to offspring across generations. Genes, which are segments of DNA, carry the instructions for these traits. Each gene has different versions called alleles, and an individual inherits two alleles for each gene, one from each parent. In many cases, one allele can mask the presence of another, a pattern known as simple Mendelian inheritance.
A dominant allele expresses its trait even if only one copy is present. Conversely, a recessive allele only expresses its trait when two copies are present, meaning the dominant allele is absent. For instance, if an allele for a dominant trait is inherited alongside an allele for a recessive trait, only the dominant trait will be observable in the organism.
Incomplete Dominance Explained
Incomplete dominance represents a pattern of inheritance where the heterozygous phenotype appears as an intermediate blend of the two homozygous phenotypes. This means neither allele fully masks the other. Instead, they combine to produce a new, distinct phenotype that falls somewhere between the expressions of the two pure traits. The resulting trait is a mixture, not a full expression of either parental characteristic.
A classic example of incomplete dominance is seen in the snapdragon flower color. If a homozygous plant with red flowers (RR) is crossed with a homozygous plant with white flowers (WW), their offspring, the heterozygotes (RW), will produce pink flowers. The red and white pigments do not fully express, leading to a diluted, blended color. This blending illustrates that the alleles contribute to the phenotype in an additive manner.
The pink phenotype in snapdragons is not merely a less intense red or white; it is a novel color resulting from the partial expression of both alleles. The heterozygous individual produces about half the amount of pigment compared to the homozygous dominant parent, leading to the intermediate color.
Codominance Explained
Codominance is another distinct pattern of inheritance where both alleles in a heterozygote are fully and simultaneously expressed. Unlike incomplete dominance, there is no blending or intermediate phenotype. Instead, both parental traits are distinctly visible in the offspring.
A well-known example of codominance in humans is the ABO blood group system. Individuals with type AB blood inherit an A allele and a B allele. Both the A and B antigens are present on the surface of their red blood cells, demonstrating that both alleles are fully expressed without one masking the other.
Another illustration of codominance can be observed in roan cattle. When a homozygous red cow is bred with a homozygous white bull, their offspring exhibit a roan coat. This coat is not pink, but rather a mixture of individual red hairs and individual white hairs, both distinctly visible.
Distinguishing Incomplete Dominance and Codominance
The primary distinction between incomplete dominance and codominance lies in the phenotypic outcome observed in heterozygous individuals. In incomplete dominance, the heterozygote displays a blended or intermediate phenotype that falls between the two homozygous traits. This means the individual does not fully express either parental characteristic, but rather a diluted or mixed version.
Conversely, codominance results in a heterozygous phenotype where both alleles are fully and distinctly expressed simultaneously. There is no blending of traits; instead, both characteristics are visible and identifiable in the organism. The presence of both original traits side-by-side, without any dilution or mixture, is a defining feature of codominance.
For instance, consider flower color inheritance. If red and white flowers produce pink offspring, that is incomplete dominance because the colors have blended. However, if red and white flowers produce offspring with distinct red and white patches, that would be codominance, as both colors are expressed fully and separately.