Dominance (genetics)

In the study of genetics, dominance describes how different versions of a gene interact. These versions are called alleles. Every individual carries two alleles for a specific gene, one on each copy of a chromosome. Dominance occurs when one allele masks or overrides the effect of the other. The allele that performs the masking is called the dominant allele. The allele that is hidden is called the recessive allele. This interaction determines the phenotype, which is the observable physical trait of the organism.

To understand this mechanism, we can look at how alleles combine. When an organism has two identical alleles, it is called a homozygote. If it has two different alleles, it is a heterozygote. In complete dominance, a heterozygote will look exactly like a homozygote that carries only the dominant allele. For example, in pea plants, a round seed allele (R) is dominant over a wrinkled seed allele (r). A plant with the genotype RR is round. A plant with the genotype Rr is also round because the R allele masks the r allele. Only the rr genotype results in wrinkled peas.

There are different ways these patterns manifest in nature. Complete dominance is the most famous type, following the rules established by Gregor Mendel. However, other patterns exist, such as incomplete dominance. In this case, the heterozygote shows a phenotype that is an intermediate blend of the two parents. A classic example is the snapdragon flower. If you cross a red snapdragon with a white one, the offspring are pink. The red trait is not completely dominant; it is expressed less strongly.

Another distinct pattern is co-dominance. In co-dominance, neither allele masks the other. Instead, both alleles are fully visible in the phenotype. This is different from incomplete dominance because the traits do not blend into a middle ground. Instead, they coexist. For instance, if a red cow and a white cow mate, their offspring might show both red and white hairs. This creates a spotted or patterned appearance.

Gregor Johann Mendel discovered these principles in the 1860s while studying garden peas. He observed that certain traits, like tall versus short plants, appeared in predictable patterns. When he crossed different plant lines, one trait would often disappear in the first generation. However, that trait would reappear in the next generation in a specific 3:1 ratio. Mendel used capital letters for dominant alleles and lowercase letters for recessive ones. This notation is still the standard in science today.

Scientists have continued to refine these ideas for over a century. In 1928, Ronald Fisher proposed that dominance might be influenced by natural selection through modifier genes. In 1929, Sewall Wright suggested that dominance is a physiological result of metabolic pathways. We also study how multiple genes work together. This includes dihybrid inheritance, where two different pairs of genes are tracked at once. In a dihybrid cross between heterozygotes, the offspring show a phenotypic ratio of 9:3:3:1.

Human biology provides clear evidence of these complex rules. The ABO blood group system is a perfect example of co-dominance. There are three alleles involved: IA, IB, and i. The IA and IB alleles are both dominant over the recessive i allele. However, IA and IB are co-dominant with each other. An individual with both IA and IB alleles will have type AB blood. This happens because both alleles produce different chemical modifications on the surface of the blood cells.

Dominance is a relative concept rather than a permanent trait. An allele might be dominant over one partner but recessive to another. It is also possible for one allele to be dominant for one specific trait but not for others. It is important to distinguish dominance from epistasis. Epistasis is a different phenomenon where one gene masks the effect of a completely different gene. Understanding these layers helps scientists predict how traits move through entire populations.