Incomplete dominance is a form of intermediate inheritance in which one allele for a specific trait is not completely expressed over its paired allele. This results in a third phenotype in which the expressed physical trait is a combination of the phenotypes of both alleles. Unlike complete dominance inheritance, one allele does not dominate or mask the other.
Incomplete dominance occurs in the polygenic inheritance of traits such as eye color and skin color. It is a cornerstone in the study of non-Mendelian genetics.
Incomplete dominance is a form of intermediate inheritance in which one allele for a specific trait is not completely expressed over its paired allele.
Who discovered the Incomplete Dominance?
Scientists have noted the blending of traits back into ancient times, although, until Mendel, no one used the words “incomplete dominance.” In fact, Genetics was not a scientific discipline until the 1800s when Viennese scientist and friar Gregor Mendel (1822–1884) began his studies.
Like many others, Mendel focused on plants and, in particular, the pea plant. He helped define genetic dominance when he noticed that the plants had either purple or white flowers. No peas had lavender colors as one might suspect.
Up to that time, scientists believed that physical traits in a child would always be a blend of the traits of the parents. Mendel proved that in some cases, the offspring can inherit different traits separately. In his pea plants, traits were visible only if an allele was dominant or if both alleles were recessive.
Mendel described a genotype ratio of 1:2:1 and a phenotype ratio of 3:1. Both would be consequential for further research.
While Mendel’s work laid the foundation, it was German botanist Carl Correns (1864–1933) who is credited with the actual discovery of incomplete dominance. In the early 1900s, Correns conducted similar research on four o’clock plants.
In his work, Correns observed a blend of colors in flower petals. This led him to the conclusion that the 1:2:1 genotype ratio prevailed and that each genotype had its own phenotype. In turn, this allowed the heterozygotes to display both alleles rather than a dominant one, as Mendel had found.
Examples of Incomplete Dominance in plant:
As an example, incomplete dominance is seen in cross-pollination experiments between red and white snapdragon plants. In this monohybrid cross, the allele that produces the red color (R) is not completely expressed over the allele that produces the white color (r). The resulting offspring are all pink.
The genotypes are: Red (RR) X White (rr) = Pink (Rr).
When the first filial (F1) generation consisting of all pink plants is allowed to cross-pollinate, the resulting plants (F2 generation) consist of all three phenotypes [1/4 Red (RR): 1/2 Pink (Rr): 1/4 White (rr)]. The phenotypic ratio is 1:2:1.
When the F1 generation is allowed to cross-pollinate with true-breeding red plants, the resulting F2 plants consist of red and pink phenotypes [1/2 Red (RR): 1/2 Pink (Rr)]. The phenotypic ratio is 1:1.
When the F1 generation is allowed to cross-pollinate with true-breeding white plants, the resulting F2 plants consist of white and pink phenotypes [1/2 White (rr): 1/2 Pink (Rr)]. The phenotypic ratio is 1:1.
In incomplete dominance, the intermediate trait is the heterozygous genotype. In the case of snapdragon plants, plants with pink flowers are heterozygous with the (Rr) genotype. The red and white flowering plants are both homozygous for plant color with genotypes of (RR) red and (rr) white.
Other examples in Plant:
1) 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.
2) 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.
3) 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.
4) 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!
Examples of Incomplete Dominance in Animals:
Another example of incomplete dominance is the inheritance of straight, wavy, and curly hair in dogs. The KRT71 gene is used to synthesize the keratin 71 protein. Genes in the KRT family provide instructions for making proteins called keratins.
Keratins are a group of tough, fibrous proteins that form the structural framework of epithelial cells, which are cells that line the surfaces and cavities of the body. Epithelial cells make up tissues such as the hair, skin, and nails. These cells also line the internal organs and are an important part of many glands.
Keratins are best known for providing strength and resilience to cells that form the hair, skin, and nails. The mutation which causes curly hair in dogs, such as the labradoodle seen in the figure, is in exon 2 of the gene and is predicted to substantially disrupt the structure of the keratin 71 protein. This change in protein shape prevents the keratin proteins from interacting together correctly within the hair, altering the structure of the hair and resulting in a curly coat.
When a dog has two curly alleles (KCKC), it has a very curly coat. A dog with two straight alleles (K+K+) has a straight coat. Dogs that are heterozygous (K+KC) have an intermediate or wavy coat like the labradoodle as shown in the figure below.
1) 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.
2) 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.
3) 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.
4) 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.
Examples of Incomplete dominance in Humans:
There are 29 recognized blood groups, most involving only one gene. Variations (polymorphisms) within the genes that determine blood group give rise to the different antigens for a particular blood group protein.
For example, changes in a few DNA building blocks (nucleotides) in the ABO gene give rise to the A, B, and O blood types of the ABO blood group. The changes that occur in the genes that determine blood group typically affect only blood type and are not associated with adverse health conditions, although exceptions do occur.
The A and B alleles are codominant, which is similar to incomplete dominance in that heterozygotes have an intermediate phenotype. If both the A and B alleles are present, both will be seen in the phenotype. The O allele is recessive to both A and B.
1) 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.
2) 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.
3) When one parent with straight hair and one with curly hair have a child with wavy hair, that’s an example of incomplete dominance.
4) 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 the incomplete dominance of multiple genes related to green and brown eye color.