Incomplete dominance is an important concept in the study of genetics. It refers to a circumstance in which the two copies of a gene for a particular trait, or alleles, combine so that neither dominates the other. This creates a new phenotype or set of observable characteristics caused by the interaction of genetics and environment. In short, incomplete dominance is when neither gene is fully dominant, and the result is a brand new trait.
Codominance is a similar yet different phenomenon. While in incomplete dominance, neither allele is dominant, in codominance, both alleles are. As a result, codominant traits are expressed equally in the phenotype; that is, the result is a combination of the two traits. That differs from incomplete dominance, in which the result is something new, as the examples of incomplete dominance below demonstrate.
It's important to note that most observable traits in any living thing are caused by more than one gene. Incomplete dominance is specific to traits that occur on just one gene. However, there are many such traits, and incomplete dominance occurs in every sort of organism that has genes, including plants, animals and even human beings.
Incomplete dominance in animals is most widely studied in domestic animals since it's important for their health, appearance, and value. Here are several examples of the effects of incomplete dominance in animals:
- Chickens with blue feathers are an example of incomplete dominance. When a black and a white chicken reproduce and neither allele is completely dominant, the result is a blue-feathered bird.
- When a long-furred Angora rabbit and a short-furred Rex rabbit reproduce, the result can be a rabbit with fur longer than a Rex, but shorter than an Angora. That's a classic example of incomplete dominance producing a trait different from either of the parents.
- Tail length in dogs is often determined by incomplete dominance. Pups of long-tailed and short-tailed parents often split the difference and have medium-length tails.
- On the subject of dogs, lots of labradoodles have wavy hair. Just like humans, that comes from having straight-haired and curly-haired parents. The result is an intermediate inheritance: the wavy-haired labradoodle.
- The cream gene in horses is a classic incomplete dominant. When paired with a red allele, the cream allele produces horses with golden coats such as palominos and buckskins.
The science of genetics began with plants. People have been interbreeding plants for particular traits since we first started farming more than 11,000 years ago. Gregor Mendel, one of the founders of genetic science, began his studies by recording the ways he planted his garden. Whether for food, other uses, or simple beauty, humans have employed genetic selection of plants, including incomplete dominance, throughout our history.
- Incomplete dominance was first recorded in plants. The German scientist Josef Kolreuter bred red and white carnations, expecting to get offspring with the dominant red coloration. Instead, many came up pink! Kolreuter found that neither allele was fully dominant in his flowers and identified the concept of incomplete dominance.
- Four-o-clocks are flowering plants that get their funny name from their inclination to bloom in the late afternoon. Wild four-o-clocks tend to have red flowers, while "pure" four-o-clocks with no coloration genes are white. Mixing the two results in pink flowers, just like Dr. Kolreuter's carnations. Those pink flowers are a result of incomplete dominance. However, mixing the pink flowers results in ¼ red, ¼ white and ½ pink. That 1:2:1 ratio - a quarter like one parent, a quarter like the other, and the remaining half different from either - is common in cases of incomplete dominance.
- Pink snapdragons are a result of incomplete dominance. Cross-pollination between red snapdragons and white snapdragons result in pink when neither the white or the red alleles are dominant.
- The fruit color of eggplants is another example of incomplete dominance. Combining deep purple eggplants with white eggplants results in eggplants of a light violet color.
- Incomplete dominance is a key element of improving crops such as corn. Corn with multiple incompletely dominant traits is generally healthier and provides greater yields than "purer" strains with fewer such traits. Just compare the original plant, teosinte, with a modern ear of corn to see the genetic difference!
Incomplete dominance is rare in humans; we're genetically complex and most of our traits come from multiple genes. However, there are a few examples. Incomplete dominance is just part of what makes our species so complicated and interesting.
- The disease familial hypercholesterolemia (FH) is an example of incomplete dominance. One allele causes liver cells to be generated without cholesterol receptors, while another causes them to be generated normally. The incomplete dominance causes the generation of cells that do not have enough receptors to remove all dangerous cholesterol from the bloodstream.
- Tay-Sachs Disease is an example of incomplete dominance in humans. This neurological disease is caused by an enzyme imbalance and is autosomal recessive; that is, people who actually suffer from the disease have two recessive genes that cause it. However, one or both of their parents may have been carriers who had incompletely dominant genes, causing them to produce one half of the necessary enzyme, which is enough for a normal life.
- When one parent with straight hair and one with curly hair have a child with wavy hair, that's an example of incomplete dominance.
Eye color is often cited as an example of incomplete dominance. In fact, it's a little more complicated than that, but hazel eyes are partially caused by incomplete dominance of multiple genes related to green and brown eye color.
Genetics can be an intimidating subject. Many traits come down to complex interactions between lots of different genes. But by learning the basic mechanisms involved, such as incomplete dominance, it becomes far easier to understand.
For more of the fundamental concepts of genetics, let our article on examples of genotypes and phenotypes help you distinguish between those two confusing concepts.