Genetics is the study of heredity, examining how traits are passed from parents to offspring. The fundamental unit of heredity is the gene, a segment of DNA that provides instructions for a specific characteristic. Genes exist in variant forms called alleles, which are responsible for the differences we observe in a single trait. Complete dominance is a pattern of inheritance where the presence of a single version of one allele is enough to entirely determine the observable trait. This means the effect of the other, alternative allele is completely hidden in the physical appearance of the organism.
The Allelic Relationship in Complete Dominance
The concept of complete dominance is rooted in the distinct relationship between an organism’s genetic makeup, or genotype, and its physical expression, or phenotype. Every individual carries two alleles for a specific gene, one inherited from each parent. These two alleles constitute the individual’s genotype for that trait.
One allele is designated as dominant and is typically represented by a capital letter, such as ‘A’. The other allele is recessive and is represented by the same letter in lowercase, such as ‘a’. The dominant allele governs the trait’s expression, while the recessive allele is only expressed when the dominant one is absent.
The two alleles an individual possesses can result in three possible genotypes. An individual can be homozygous dominant (AA), meaning they carry two copies of the dominant allele. They can also be homozygous recessive (aa), carrying two copies of the recessive allele.
The third possibility is the heterozygous state (Aa), where the individual carries one dominant and one recessive allele. In complete dominance, the phenotype of the heterozygous individual (Aa) is identical to the phenotype of the homozygous dominant individual (AA). The dominant allele completely masks the presence of the recessive allele, ensuring the recessive trait is not expressed at all.
The masking effect occurs because the dominant allele often codes for a functional protein or enzyme. Having one copy (in the heterozygote) produces enough product to display the trait fully. The recessive allele may code for a non-functional product, but the functional product from the dominant allele overrides it. Therefore, only the homozygous recessive genotype (aa), lacking any dominant allele, results in the expression of the recessive phenotype.
Geneticists use a tool called a Punnett square to systematically predict the probabilities of these genotypic and phenotypic outcomes in the offspring of a cross. For example, a cross between two heterozygous parents (Aa x Aa) will produce a 3:1 phenotypic ratio, where three out of four offspring display the dominant trait and only one displays the recessive trait, illustrating the complete nature of the dominant masking.
How Complete Dominance Differs from Other Patterns
Complete dominance represents one specific way that alleles can interact, but other patterns of inheritance demonstrate different relationships between alleles. Understanding these alternatives helps clarify the unique nature of complete dominance, where one allele’s effect is entirely obscured.
Incomplete dominance is a pattern where the heterozygous phenotype is a blend or intermediate of the two homozygous phenotypes. For instance, if a plant with red flowers is crossed with a plant with white flowers, and the inheritance is incompletely dominant, the resulting offspring will have pink flowers. Neither the red nor the white allele is fully dominant, leading to a visible mixing of the traits.
Co-dominance is another distinct pattern where both alleles are fully and equally expressed in the heterozygous individual. Instead of blending, both traits appear simultaneously. A classic example is the coat color of certain cattle, known as roan, where a cross between red and white cattle results in offspring that have individual red and white hairs scattered across their coat.
In both incomplete dominance and co-dominance, the heterozygous genotype produces a unique phenotype different from either homozygous parent. This directly contrasts with complete dominance, where the heterozygote (Aa) cannot be distinguished from the homozygous dominant (AA) individual based on appearance alone. Complete dominance is characterized by a clear, binary expression—the trait is either fully dominant or fully recessive.
Illustrative Examples of Complete Dominance
Complete dominance is a common pattern observed in both human and plant traits. One of the best-studied examples comes from Gregor Mendel’s work with pea plants, where the allele for tall stems is dominant over the allele for short stems. A pea plant with one tall allele and one short allele will grow to be just as tall as a plant with two tall alleles, demonstrating the complete masking of the short trait.
Similarly, the allele for yellow seed color is dominant over the allele for green seed color in peas. A plant grown from a seed with both a yellow allele and a green allele will produce seeds that are entirely yellow, with the green trait completely suppressed. This established the foundational principle that a single dominant instruction is sufficient to produce the full trait.
In humans, brown eye color is a well-known example of a dominant trait. The presence of at least one dominant allele for the production of melanin pigment results in brown eyes, effectively masking the presence of a recessive allele for blue eyes. Only when both alleles are recessive, and the functional pigment production is absent, do blue eyes appear.
Other human traits often cited as examples of complete dominance include the presence of a Widow’s peak hairline and unattached earlobes. For these traits, the presence of a single dominant allele is enough to produce the characteristic phenotype.