Genetics is the study of heredity, exploring how traits are passed from parents to offspring. Within this field, patterns of dominance are fundamental rules that dictate how characteristics are physically expressed. Complete dominance describes the most straightforward relationship between different versions of a gene, known as alleles. This pattern determines that the presence of just one specific allele is sufficient to completely mask the effect of another allele in the pair.
Defining Complete Dominance
A gene is a segment of DNA that provides instructions for building a specific product, usually a protein, and alleles are the variant forms of that gene. Every individual inherits two alleles for each gene, one from each parent, which together form the genotype. Complete dominance is defined by the outcome when an individual possesses two different alleles, a condition called heterozygous. In this case, the observable trait, or phenotype, is identical to the one seen when two copies of the dominant allele are present, the homozygous dominant state.
The physical mechanism is rooted in molecular function. The dominant allele typically codes for a functional protein or enzyme necessary to produce a certain characteristic, while the recessive allele often codes for a non-functional version.
In a heterozygous individual, the single functional copy from the dominant allele produces enough product to carry out the full biological function. Therefore, the heterozygous genotype results in the same phenotype as the homozygous dominant genotype. The recessive trait only appears when an individual inherits two copies of the non-functional, homozygous recessive allele.
Classic Examples in Genetics
Complete dominance is a characteristic of many traits first studied in classical genetics, such as those found in Gregor Mendel’s pea plants. For instance, the allele for yellow pea color is dominant over the allele for green pea color. A plant with two yellow alleles, or one yellow and one green allele, will produce yellow peas, demonstrating a clear, distinct difference between the dominant and recessive phenotypes with no blending.
In humans, certain traits also follow this simple Mendelian pattern. The presence of a widow’s peak hairline, a V-shaped point in the center of the forehead, is a dominant trait. An individual needs only one allele for a widow’s peak to display the hairline, while a straight hairline is the recessive phenotype. Normal skin pigmentation is similarly dominant over the recessive trait of albinism, which results from the inability to produce melanin pigment.
These patterns are often mapped using a Punnett square, which predicts the possible genotypes of offspring from two parents. When two heterozygous parents reproduce, the offspring will exhibit the dominant phenotype approximately three-quarters of the time. This predictable 3:1 ratio of dominant to recessive phenotypes is a classic indicator of complete dominance.
How Complete Dominance Differs from Other Patterns
Complete dominance is one of several ways alleles can interact, and it is distinct from incomplete dominance and codominance. The distinction lies in how the heterozygous genotype is expressed in the phenotype. In complete dominance, the heterozygote looks exactly like the homozygous dominant individual.
In contrast, incomplete dominance results in a blended or intermediate phenotype in the heterozygote. A classic example is the flower color of snapdragons, where a cross between a red-flowered plant and a white-flowered plant produces pink-flowered offspring. Here, the single red allele does not produce enough pigment to create the full red color, resulting in the intermediate pink shade.
Codominance presents a third scenario where both alleles are fully and simultaneously expressed in the heterozygote. Neither allele masks the other, and both traits are distinctly visible. The human ABO blood group system is an example: an individual with the AB blood type has alleles for both A and B antigens. Both antigens are present on the surface of their red blood cells, demonstrating the full expression of both inherited alleles.