What is incomplete dominance, you ask? It is a particular expression of gene interactions. Therefore, before we discuss the category of incomplete dominance, we need to touch on genetics and how it works. All living things, from humans to trees to bacteria, contain genes. These are tiny bits of information that make grass green or your skin freckled. They determine everything from the shape of your eyes to what allergies you may have. These genes are in the DNA which makes up chromosomes. This structure is vital to understanding incomplete dominance.
Alleles are genes that occur in pairs and make up the chromosomes of all living things. When both members of the pair are identical, they are homogeneous. If they are different, they are heterogeneous. When heterogeneous pairs occur, one may be dominant over the other, and mask the expression of the recessive allele. Variations of this relationship will be covered later in the article.
Scientists often mention genotype and phenotype when discussing genetics. What exactly do they mean? Genotype is the genetic material which makes up an individual. A phenotype is the physical expression of a gene. Quite simply, what it looks like and how it behaves. Various factors, including the environment, can influence the phenotype's expression of the genetic structure.
With incomplete dominance, both traits influence the phenotype, resulting in a fusion of their characteristics. In co-dominance, both traits have equal weight and are thus both expressed. The universal example of co-dominance is blood type. It is likely that when you were in high school biology, you pricked your finger and typed your blood. As with all genes, you received one marker from each parent. The alleles express themselves as A, B or O. People with AB blood inherited the A from one parent and the B from the other. Both are equally expressed.
Gregor Mendel (1822-1884), often called the Father of Genetics, made many discoveries while breeding pea plants. Before his experiments, it was believed that if a purple flower was crossed with a white, the offspring would have lavender flowers. He obtained a different result. The offspring always had true purple flowers or white ones, no lavender flowers resulted. Mendel's traits were only expressed if there was a dominant allele present or two recessives.
Carl Correns (1864-1933) built on Mendel's work. When researching with four o'clock plants, he noticed color blends on his flower petals. From this, he realized that genotypes were not so simple after all. There must, therefore, be more alleles asserting their influence. This led to the understanding of incomplete dominance.
Just as Chemistry has a shorthand, genetics has its own version. Don't worry; it is much simpler. Take for instance a gene for flower color where the allele for purple is dominant and white is recessive. Since purple is the one that asserts itself, we call that gene (P). To keep it from being confused with another gene, the same letter is used for white, but it is put in lowercase (p). Therefore, a plant that gets two dominant alleles is (PP), two recessives is (pp), and if it has one of each, it is written as (Pp).
Do you know what color the flowers are in the example above? (PP) is going to be a purple flower and (pp) will be white. So, what color would (Pp) be? If you said purple, you are absolutely correct. Think of it this way: The uppercase P is larger and overpowers the poor little lowercase p, just as the dominant gene will overpower the recessive.
Here's where it gets trickier. Not all traits function as simple dominants. Sometimes they affect each other, and you end up with a new result. This is the simplest form of incomplete dominance. Unlike Mendel's pea plants, Snapdragon color is expressed by monohybrid crosses. In this case, red (R), is changed by the presence of white (r). The resulting offspring (Rr) is pink!
Here we come to the really fun part. Phenotype becomes more complicated than the simple monohybrid cross. Polygenic traits, another form of incomplete dominance, are characteristics that are determined by multiple genes influencing each other. These traits are not simple is/is not of (Pp) or even red and white makes pink (Rr). Here we find:
The phenotypes mentioned above are caused by several genes and interactions between multiple alleles. These genes have equal influence over the final appearance and can be found on multiple chromosomes. It is difficult to predict what the ultimate expression will be since so many factors are at play. However, it is true that the more dominant alleles that are present, the greater their expression will be. The more recessives that are present, the stronger they will be represented. This is why humans have such a broad range of skin tones and beautiful shades of eyes.