Genes are fundamental units of heredity, segments of DNA that carry instructions for an organism. These instructions determine specific features, known as traits, ranging from observable characteristics like eye color to less visible attributes such as blood type. Each gene can have different versions, called alleles, which account for variations in these traits within a population. How these alleles interact dictates trait expression.
How Co-dominance Works
Co-dominance is a pattern of inheritance where both alleles for a gene are fully and simultaneously expressed in a heterozygous individual. This means the phenotypes associated with each allele are distinctly visible, rather than one masking the other or blending.
Both alleles produce functional products that contribute to the observed trait. Neither allele is dominant or recessive over the other; they exert their effects equally. This leads to a phenotype that is a combination of both parental traits, without any intermediate blending.
Real-World Examples
A widely recognized example of co-dominance in humans is the ABO blood group system. The gene for blood type has three common alleles: IA, IB, and i. IA and IB are co-dominant, while ‘i’ is recessive to both.
Individuals inheriting both the IA and IB alleles will have AB blood type. Their red blood cells express both A antigens and B antigens distinctly on their surface. This simultaneous expression demonstrates co-dominance.
Another illustration of co-dominance is the roan coat color of cattle and horses. A roan animal results from a cross between a red-coated parent and a white-coated parent. Instead of a blended color like pink, the offspring exhibits a mixture of individual red and white hairs.
Each hair on a roan animal is either entirely red or entirely white, reflecting the full expression of both the red and white coat color alleles. This distinct presence of both colors, rather than an intermediate shade, exemplifies co-dominance.
Distinguishing Co-dominance
Understanding co-dominance is clearer when contrasted with other inheritance patterns, such as complete dominance and incomplete dominance. In complete dominance, one allele completely masks the other. For instance, in pea plants, the allele for purple flowers is completely dominant over the allele for white, so a plant with one of each will have purple flowers.
Incomplete dominance occurs when alleles blend to create an intermediate phenotype in heterozygotes. A classic example is the snapdragon flower, where a cross between a red and white-flowered plant produces pink offspring. The pink color is a blend, unlike the distinct display seen in co-dominance.
The fundamental distinction lies in the outcome for heterozygotes: co-dominance results in both traits being expressed separately and simultaneously, whereas incomplete dominance leads to a combined or intermediate trait. Complete dominance, on the other hand, shows only the trait of the dominant allele.