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: 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 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.
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. Incomplete genetic dominance is similar to but different from co-dominance. Whereas incomplete dominance is a blending of traits, in co-dominance an additional phenotype is produced and both alleles are expressed completely. The best example of co-dominance is AB blood type inheritance. Blood type is determined by multiple alleles recognized as A, B, or O and in blood type AB, both phenotypes are fully expressed. 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. Bettmann Archive / Getty ImagesLike 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. 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).
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. Polygenic traits, such as height, weight, eye color, and skin color, are determined by more than one gene and by interactions among several alleles. The genes contributing to these traits equally influence the phenotype and the alleles for these genes are found on different chromosomes. The alleles have an additive effect on the phenotype resulting in varying degrees of phenotypic expression. Individuals may express varying degrees of a dominant phenotype, recessive phenotype, or intermediate phenotype.
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