Inherited traits are characteristics passed from parents to offspring. Gregor Mendel’s pioneering work in the mid-19th century established key principles of heredity, forming the basis of modern genetics.
The Core Principle of Dominance
The Law of Dominance, a foundational concept in genetics, describes how certain versions of genes can mask the expression of others. Each inheritable characteristic is determined by a pair of factors, now known as alleles, with one allele inherited from each parent. When an individual possesses two different alleles for a specific trait, one allele, termed the “dominant allele,” expresses its associated trait, while the effect of the “recessive allele” is not observed in the organism’s physical appearance. The recessive trait only appears when an individual inherits two copies of the recessive allele.
The genetic makeup of an organism, including its specific combination of alleles, is called its genotype. In contrast, the observable physical characteristics or traits that result from this genetic makeup are referred to as the phenotype. For example, if an organism inherits two identical alleles for a trait, it is “homozygous” for that gene. If an organism inherits two different alleles for a trait, one dominant and one recessive, it is “heterozygous”; in this case, the dominant allele dictates the phenotype, masking the recessive allele.
Witnessing Dominance in Action
Gregor Mendel’s experiments with pea plants provided clear demonstrations of the Law of Dominance. He studied various traits, such as plant height and flower color. When Mendel crossed purebred tall pea plants with purebred short pea plants, all the offspring in the first generation (F1) were tall, showing that the allele for tallness was dominant. Similarly, when purebred pea plants with purple flowers were crossed with purebred plants having white flowers, all the F1 generation produced purple flowers, indicating the purple flower allele was dominant. The white flower trait, though not visible in the F1 generation, could reappear in subsequent generations if the plants were allowed to self-fertilize, demonstrating the recessive allele was still present.
In humans, free earlobes are a dominant trait. Huntington’s disease, a neurodegenerative disorder, is another example of an autosomal dominant condition, meaning only one copy of the altered gene is sufficient for the disease to develop.
Beyond Simple Mendelian Dominance
While the Law of Dominance explains many inheritance patterns, biological reality often presents more complex scenarios. Two notable variations are incomplete dominance and co-dominance. In incomplete dominance, the heterozygous phenotype is an intermediate blend of the two homozygous phenotypes. For example, crossing a red snapdragon flower with a white snapdragon flower can produce offspring with pink flowers.
Co-dominance occurs when both alleles in a heterozygous individual are fully and simultaneously expressed, without any blending. A prime example in humans is the ABO blood group system. An individual with both A and B alleles (genotype IAIB) will have AB blood type, meaning both A and B antigens are present on their red blood cells. These examples highlight that while dominance is a fundamental principle, it is part of a broader spectrum of genetic interactions.