Inheritance is a fundamental process in biology, explaining how characteristics are passed from one generation to the next. This transfer occurs through genetic material, primarily DNA, from parents to offspring. While Gregor Mendel’s foundational work in genetics established principles of dominant and recessive traits, not all inheritance patterns fit neatly into these categories. Some traits exhibit more intricate relationships between different versions of a gene, known as alleles, leading to diverse observable characteristics.
Understanding Codominance
Codominance describes a genetic scenario where two different alleles for a trait are both fully and equally expressed in a heterozygous individual. This means that neither allele masks the other; both traits are distinctly visible. The offspring displays characteristics from both parents without any blending.
A well-known example of codominance in humans is the ABO blood group system. An individual inheriting both the A allele and the B allele will have AB blood type, where both A and B antigens are present on the surface of their red blood cells. Another illustration of codominance is seen in roan cattle. These animals possess both red and white hairs, resulting in a speckled appearance rather than a blended color like pink. Each hair is distinctly red or white.
Understanding Incomplete Dominance
In contrast, incomplete dominance occurs when the heterozygous phenotype is an intermediate blend of the two homozygous phenotypes. Here, the dominant allele does not completely mask the effect of the recessive allele, leading to a new, combined trait.
A classic example of incomplete dominance is flower color in snapdragons. When a red-flowered snapdragon is crossed with a white-flowered snapdragon, the offspring produce pink flowers. The pink coloration is an intermediate phenotype, demonstrating a blending of the red and white pigments. Similarly, human hair texture can exhibit incomplete dominance. Individuals with one allele for curly hair and one for straight hair often have wavy hair, which is an intermediate texture between very curly and straight.
Key Differences Between Codominance and Incomplete Dominance
The fundamental distinction between codominance and incomplete dominance lies in how the alleles are expressed in a heterozygous individual. In codominance, both alleles are fully and distinctly expressed. For instance, in roan cattle, both red and white hairs are present.
Conversely, incomplete dominance involves a blending of traits, creating an intermediate phenotype. The red and white snapdragon flowers producing pink offspring exemplify this blending. While both patterns deviate from simple dominant-recessive inheritance, codominance results in the distinct appearance of both traits, whereas incomplete dominance yields a new, combined trait.