Heredity is the process through which characteristics are passed from parents to their offspring. This transmission of traits ensures continuity across generations, with genetic information guiding the development and functioning of every living organism.
The Basics of Complete Dominance
A gene is a segment of DNA that carries instructions for a specific trait. Different versions of the same gene are called alleles. For any given gene, an individual inherits two alleles, one from each parent. The combination of these alleles makes up an individual’s genotype, which is their unique genetic makeup for that trait. The observable expression of this genotype, such as hair color or height, is known as the phenotype.
In complete dominance, one allele, called the dominant allele, fully masks the presence of another allele, the recessive allele, in a heterozygous individual. The trait associated with the dominant allele will be expressed if at least one copy is present. The recessive trait only appears when an individual inherits two copies of the recessive allele.
When an individual has two identical alleles for a trait, their genotype is homozygous (either two dominant or two recessive alleles). A heterozygous genotype means the individual has one dominant and one recessive allele. In this case, the dominant trait is expressed.
Real-World Examples
Gregor Mendel’s work with pea plants provided foundational insights into complete dominance. He observed that when he crossed tall pea plants with short pea plants, all the first-generation offspring were tall. This demonstrated that the allele for tallness was dominant over the allele for shortness. The short trait only reappeared in subsequent generations when two recessive alleles were present.
In pea plants, purple flower color is also completely dominant over white. A plant inheriting one allele for purple and one for white will produce only purple flowers. White flowers appear only if the plant inherits two recessive alleles.
In humans, several traits also exhibit complete dominance. For instance, having a widow’s peak hairline is a dominant trait, while a straight hairline is recessive. Another example involves earlobes: unattached earlobes are dominant, whereas attached earlobes are recessive.
How Complete Dominance Differs
Not all genetic traits follow the pattern of complete dominance; other inheritance patterns exist. In incomplete dominance, a heterozygous individual displays a blended phenotype that is intermediate between the two homozygous phenotypes. For example, crossing a red snapdragon flower with a white snapdragon flower produces offspring with pink flowers, a blend of the parental colors.
Codominance is another pattern where both alleles are fully and simultaneously expressed in the heterozygous individual. An example in humans is the AB blood type, where both A and B antigens are present on the red blood cells. Neither allele masks the other; instead, both traits are observable.
Complete dominance differs from these patterns because the dominant allele entirely overshadows the recessive allele in a heterozygous individual. This results in only the dominant trait being expressed, unlike blending or simultaneous expression seen in other inheritance modes.