Heredity, the transmission of characteristics from parents to offspring, is a fundamental process in all living organisms. It explains why offspring resemble their parents but also exhibit individual differences. Understanding how traits are passed down through generations provides insight into the vast diversity of life observed around us.
Genetic Building Blocks and Dominance
Genes are units of heredity containing instructions for building and operating an organism. They reside on chromosomes within the cells. For most traits, an individual inherits two versions of each gene, called alleles, one from each parent.
An individual’s genotype is the specific combination of alleles for a particular gene. This genetic makeup translates into an observable characteristic, or phenotype. Alleles do not always interact equally; some can mask the effect of others. This interaction is termed dominance, where one allele’s influence is apparent in the phenotype, while the other’s is concealed.
Unpacking Complete Dominance
Complete dominance describes a genetic interaction where one allele fully conceals the effect of another in a heterozygous individual. The expressed allele is the dominant allele, and the hidden allele is the recessive allele. Even with only one copy of the dominant allele, its associated trait is observed in the phenotype.
The recessive phenotype only manifests when an individual inherits two copies of the recessive allele (homozygous recessive). For instance, if a plant has a dominant allele for purple flowers and a recessive allele for white flowers, it will produce purple flowers because the purple allele completely masks the white.
Seeing Complete Dominance in Action
Complete dominance is widely observed in nature, providing clear examples of trait inheritance. Gregor Mendel’s work with pea plants demonstrated this principle. He observed that crossing tall and short pea plants resulted in all tall offspring in the first generation, showing tallness was dominant over shortness.
Similarly, Mendel found that purple flower color was dominant over white flower color, and round seed shape was dominant over wrinkled seed shape. In these cases, the dominant trait appeared in the first generation, while the recessive trait only reappeared in subsequent generations when specific allele combinations were present. In humans, several traits also follow complete dominance. For example, brown eye color is dominant over blue, meaning individuals with at least one brown eye allele will have brown eyes. Dimples and the genetic condition of dwarfism (achondroplasia) are also dominant traits.