The fundamental unit of heredity is the gene, a segment of DNA providing instructions for a specific protein or function. Humans typically inherit two copies of every gene, one from each biological parent. These different versions of a gene are known as alleles, which determine variations in a trait, such as eye color or blood type. Alleles are classified as either dominant or recessive, describing the way the associated trait is passed down and expressed. Inheritance is the process by which these alleles are transmitted from parents to offspring.
The Principle of Dominance
Dominance refers to the genetic rule where one allele can mask or override the effect of a different allele for the same gene. Only one copy of the dominant allele is needed for the trait to be expressed. An individual who inherits one dominant and one recessive allele is considered heterozygous for that gene. The dominant allele’s presence dictates the genetic output, effectively silencing the recessive counterpart.
At the molecular level, the dominant allele typically contains the correct instructions to produce a fully functional protein. The recessive allele often carries a mutation that results in a non-functional product. In a heterozygous individual, the single functional copy is usually enough to produce sufficient protein quantity to carry out the necessary cellular role. This sufficiency is the core reason why the dominant trait appears.
Observable Expression of the Trait
When an offspring inherits a dominant allele, the resulting biological consequence is the expression of the corresponding trait, known as the phenotype. This observable outcome happens regardless of whether the second inherited allele is also dominant or recessive. The individual’s genetic makeup, or genotype, may be either homozygous dominant (inheriting two dominant alleles) or heterozygous (inheriting one dominant and one recessive allele).
In both cases, the single dominant allele acts as a sufficient genetic blueprint, resulting in an identical physical or functional manifestation of the trait. If the dominant allele codes for a functional enzyme, inheriting one copy is enough to produce the enzyme and complete the associated biochemical pathway. The presence of the recessive allele does not diminish the function provided by the dominant allele, leading to the full expression of the dominant phenotype.
Examples of Dominant Inheritance
Many observable human characteristics and several genetic conditions follow this pattern of dominant inheritance. A common physical example is the presence of a widow’s peak, a distinctive V-shape at the hairline. Only one dominant allele is required for this trait to appear, while a straight hairline requires two recessive alleles. The ability to roll one’s tongue is also often cited as being determined by a single dominant allele.
In the context of health, many genetic disorders are inherited in an autosomal dominant fashion, meaning a single altered copy of the gene is enough to cause the condition. Huntington’s disease is a severe neurological disorder that illustrates this pattern, where inheriting just one copy of the mutated gene is sufficient for the disease to develop. Marfan syndrome, a connective tissue disorder, similarly requires only one dominant allele for its manifestation.