In genetic science, which is the branch of biology that studies heredity and variation in organisms, 'dominance' refers to the relationship between two alleles (i.e., the two variant forms of a single gene), which results in one allele masking the expression of the other in influencing a particular trait.
When the phenotype of the heterozygote (i.e., an organism which has two different alleles of a single gene owing to which it gives rise to varying offspring) is completely identical to that of the dominant homozygote (i.e., an organism having two identical alleles of a particular gene), it is known as 'complete dominance'. That, however, is not always the case. There are instances when the phenotypes of the heterozygote is an intermediate of the phenotypes of homozygote, and that is referred to as incomplete dominance.
Gregor Mendel's Principle of Dominance
In genetic inheritance, two different alleles are inherited―one each from either parent. One of the two alleles is 'dominant' and thus, is seen in the offspring, while the other is 'recessive' and therefore, remains hidden. The principle of dominance was put forth by Gregor Mendel, an Augustinian monk and botanist who carried out hybridization experiments with species of pea plant. He showed that when two parents with different versions of a said trait are hybridized, one of these versions disappears in the offspring (hybrid), but reappears in the third generation without any change in it.
Of the two versions, the one which is visible in hybrid is the dominant allele and the one which disappears from the hybrid (to reappear in the third generation) is the recessive allele. Some examples of dominant and recessive traits in humans include natural curly hair, cleft chin, dimples, allergies, etc.
Incomplete Dominance in Genetics
Incomplete dominance is a type of inheritance, wherein one allele for a specific trait in not completely dominant over its counterpart. In this type of inheritance, the end result is mostly a combined phenotype. Simply put, the traits of neither parent are dominant or recessive in this case. Genetic traits are also expressed by co-dominance, wherein both the traits appear in the offspring, but in different parts of the body.
Due to a great deal of resemblance between the two concepts, they are often confused with each other. The fact though, remains that both are different from each other to a great extent. If two alleles are involved in an incomplete dominance relationship, the phenotype of the heterozygote will be the intermediate of the phenotypes of the two homozygotes. Even though the alleles themselves do not blend, the entire process is referred to as blending as it appears as if both have blended together.
Examples of Incomplete Dominance
One of the best examples is snapdragon flower, which is basically red or white in color. If a red homozygous snapdragon flower is paired with a white homozygous snapdragon flower, the hybrid will be pink in color. While that was an example in plants, the same in human genetics is not very rare as such. For instance, if a person inherits the gene of curly hair from one parent and straight hair from the other, the end result will be a blend of the two in the form of wavy hair.
Other than these two expressions of dominance, there exists yet another expression, which is referred to as pseudo-dominance. In this case, the relationship between two alleles appears to be that of complete dominance, which is not actually the case.