Genetic instructions, or genes, determine how traits are passed from one generation to the next through various inheritance patterns. This exploration will focus on codominance, a specific mode of inheritance where both parental traits are simultaneously and fully expressed.
Defining Codominance
Codominance is an inheritance pattern where two different versions, or alleles, of the same gene are both fully and separately expressed in an individual. This occurs in heterozygous organisms, meaning they possess two distinct alleles for a particular trait. Neither allele is dominant over the other, nor is one recessive, allowing both traits to appear simultaneously. The resulting observable characteristic, or phenotype, distinctly displays features associated with each allele without any blending.
The Genetic Mechanism of Codominance
Codominance occurs because both alleles in a heterozygous individual are active. Each allele produces its respective gene product, such as a specific protein or enzyme. These functional products then contribute to the observable phenotype. This differs from other inheritance patterns where one allele might prevent the expression of another. The mechanism ensures that the traits governed by both alleles are visible and distinct in the organism.
Real-World Examples of Codominance
A prominent example of codominance in humans is the ABO blood group system. This system involves three alleles: I$^A$, I$^B$, and i. While I$^A$ and I$^B$ are codominant to each other, both are dominant over the i allele. An individual inheriting both the I$^A$ and I$^B$ alleles will have AB blood type, meaning both A and B antigens are present on the surface of their red blood cells.
Another clear illustration of codominance is observed in roan cattle. In these animals, the alleles for red hair (R) and white hair (W) are codominant. When a red cow (RR) is crossed with a white cow (WW), their offspring, the F1 hybrids, exhibit a roan coat (RW). This roan coloration is not a blend of red and white, but rather a mixture of individual red and white hairs visibly present across the animal’s coat.
Codominance Versus Other Inheritance Patterns
Codominance is often contrasted with other common inheritance patterns, such as complete dominance and incomplete dominance. In complete dominance, one allele completely masks the expression of another allele in a heterozygous individual. For instance, in Mendel’s pea plants, the allele for purple flowers is completely dominant over the allele for white flowers, so a heterozygous plant would only show purple flowers.
In contrast, incomplete dominance results in a blended or intermediate phenotype in heterozygotes. If a red flower and a white flower exhibit incomplete dominance, their offspring might produce pink flowers, representing a mix of the two parental colors. The dominant allele does not fully mask the recessive allele, leading to this intermediate expression.
Codominance stands apart because both alleles are fully and distinctly expressed, rather than one masking the other or blending to form an intermediate. In codominance, the traits associated with each allele are simultaneously displayed, as seen with both A and B antigens on AB blood cells or the distinct red and white hairs in roan cattle. This means the heterozygous phenotype differs significantly from either homozygous genotype.