Genetic inheritance explains how traits pass from parents to offspring, forming the basis of individual characteristics. Organisms inherit genetic information from their parents, influencing aspects from physical appearance to certain predispositions. Understanding how these traits manifest is fundamental to genetics.
Understanding Complete Dominance
Complete dominance describes a genetic scenario where one version of a gene, known as an allele, entirely masks the effect of another allele for the same trait. If an organism inherits both a dominant and a recessive allele, only the trait associated with the dominant allele will be observable.
A gene is a segment of DNA that provides instructions for a specific trait, such as flower color or eye color. Alleles are different forms or variations of a single gene. For instance, a gene for flower color might have an allele for purple flowers and an allele for white flowers.
An individual’s genetic makeup for a particular trait is called its genotype, while the observable physical characteristic is its phenotype. Genotypes can be homozygous dominant (two copies of the dominant allele), homozygous recessive (two copies of the recessive allele), or heterozygous (one dominant and one recessive allele). In complete dominance, both a homozygous dominant genotype and a heterozygous genotype will result in the same dominant phenotype, while the recessive phenotype only appears when the genotype is homozygous recessive.
Real-World Examples of Complete Dominance
Classic examples of complete dominance are evident in Gregor Mendel’s experiments with pea plants. For instance, the allele for tall stems (T) is dominant over the allele for short stems (t). A pea plant with either two tall alleles (TT) or one tall and one short allele (Tt) will grow tall. Only plants inheriting two short alleles (tt) will exhibit a short stature.
Similarly, purple flower color (P) is dominant over white flower color (p). Thus, plants with PP or Pp genotypes will have purple flowers, while only pp plants will have white flowers.
Complete dominance also applies to human traits. Brown eye color, for example, exhibits complete dominance over blue eye color. An individual with at least one allele for brown eyes will have brown eyes. Only individuals with two alleles for blue eyes will express blue eyes. The presence of a widow’s peak (a V-shaped hairline) is another human trait dominant over a straight hairline.
Beyond Complete Dominance: Other Inheritance Patterns
While complete dominance explains many inherited traits, not all genetic interactions follow this simple pattern. Incomplete dominance is a pattern where the heterozygous genotype results in a phenotype that is an intermediate blend of the two homozygous phenotypes. For example, if a red snapdragon flower (RR) is crossed with a white snapdragon flower (WW), the offspring (RW) will have pink flowers, a blend of the parental colors.
Codominance is another pattern where both alleles in a heterozygous individual are fully and simultaneously expressed, rather than blending. A common example is the human ABO blood group system. An individual with alleles for both A and B blood types (genotype IAIB) will have AB blood type, meaning both A and B antigens are present on their red blood cells. This contrasts with complete dominance, where one allele would overshadow the other.