Genetics often reveals inheritance patterns more intricate than simple dominant or recessive relationships. While some traits follow a clear “either/or” rule, others present a blend, creating a new, intermediate characteristic. This blending phenomenon is incomplete dominance, where genetic instructions from both parents contribute to a unique outcome in their offspring. It shows that inheritance is not always about one trait completely overshadowing another.
Defining Incomplete Dominance
Incomplete dominance describes a genetic situation where neither of two versions of a gene, called alleles, is entirely dominant over the other. When an individual inherits two different alleles for a specific trait, the resulting observable characteristic, or phenotype, is an intermediate blend of the traits associated with each allele. This means the heterozygous individual, possessing two different alleles, displays a phenotype that falls somewhere between the two homozygous phenotypes (where an individual has two identical alleles). For example, if one allele codes for a red pigment and another for no pigment (white), an incompletely dominant interaction might produce a pink outcome.
Distinguishing Incomplete Dominance
To fully grasp incomplete dominance, it helps to understand how it differs from other inheritance patterns. In complete dominance, one allele completely masks the effect of the other. For instance, if a gene for flower color shows complete dominance, a plant with one red allele and one white allele would only display red flowers, as the red allele completely conceals the white one.
In contrast, codominance involves both alleles being fully expressed simultaneously and separately, without blending. A classic example is human ABO blood types, where an individual with both A and B alleles will have AB blood, meaning both A and B antigens are present and distinct. In incomplete dominance, the alleles blend to create a new phenotype, rather than one masking the other or both appearing separately.
Real-World Examples
Incomplete dominance is observed in various organisms, providing clear illustrations of blended traits.
A classic example is the flower color in snapdragons ( Antirrhinum majus ). When a true-breeding red snapdragon is crossed with a true-breeding white snapdragon, their offspring are all pink. This pink color is an intermediate phenotype.
Another instance of incomplete dominance can be seen in human hair texture. Individuals with two alleles for curly hair have very curly hair, and those with two alleles for straight hair have straight hair. If a person inherits one allele for curly hair and one for straight hair, their hair texture will be wavy, an intermediate blend of the two extremes. This blending of traits, from flower colors to hair textures, highlights how incomplete dominance contributes to the diversity of observable characteristics in nature.