The passing of traits from parents to offspring is a fundamental concept in biology, governed by genetic inheritance. While some traits follow a straightforward pattern where one gene variant (allele) completely masks another, many others exhibit more intricate interactions. Genetic expression is not always a simple case of dominance and recessiveness; instead, genes can interact in diverse ways, leading to a wider range of observable characteristics.
Incomplete Dominance Explained
Incomplete dominance describes a genetic scenario where one allele does not completely conceal the effect of another allele in a heterozygous individual. This interaction results in a blended or intermediate physical characteristic. Neither allele is fully dominant, leading to a new, distinct phenotype.
A classic illustration of incomplete dominance is seen in the snapdragon flower (Antirrhinum majus). When a pure-breeding red-flowered snapdragon is crossed with a pure-breeding white-flowered snapdragon, the resulting offspring are all pink. This pink coloration is an intermediate phenotype, not truly red or white, because the allele for red pigment does not produce enough pigment to fully express red when only one copy is present. Another example involves human hair texture, where a cross between an individual with curly hair and one with straight hair can produce offspring with wavy hair, representing an intermediate blend.
Codominance Explained
Codominance is an inheritance pattern where two different alleles for a gene are both fully and distinctly expressed in a heterozygous individual. In this case, there is no blending of traits; instead, both characteristics appear simultaneously. Each allele contributes equally to the observable phenotype.
A prime example of codominance in humans is the ABO blood group system. Individuals with type AB blood possess both the A and B alleles, and both are expressed on the surface of their red blood cells. This means that both A and B antigens are present, not a blend or intermediate form. Another common example is seen in roan cattle, which have both red and white hairs present in their coat, creating a speckled appearance rather than a uniform color. Similarly, some chickens can have both black and white feathers, appearing speckled, if they inherit alleles for both feather colors.
Key Distinctions
The fundamental difference between incomplete dominance and codominance lies in the phenotypic expression of the heterozygous individual. In incomplete dominance, the alleles blend to create a new, intermediate phenotype that is distinct from either parent. This blending suggests a quantitative effect where the single dominant allele produces insufficient product for full expression.
Codominance, conversely, involves the simultaneous and distinct expression of both alleles in the heterozygote. There is no blending; instead, both parental traits are fully visible at the same time. This indicates that both alleles produce functional products that are expressed independently, contributing equally to the overall phenotype.