The transmission of characteristics from parents to their offspring is a biological process known as heredity. This process explains why offspring often resemble their parents, carrying various traits and even predispositions to certain conditions. It involves the passing of genetic information encoded in DNA, where some instructions exert a stronger influence than others in determining observable traits.
Defining Dominant and Recessive Genes
Individuals inherit two copies of each gene, one from each parent. These different versions of a gene are called alleles. Alleles are categorized as either dominant or recessive based on how their associated traits are inherited and expressed. A dominant allele expresses its trait even when only one copy is present, meaning the dominant trait will be observed if inherited from just one parent. In contrast, a recessive allele only expresses its trait if an individual inherits two copies of that allele, one from each parent. If only one copy of a recessive allele is present alongside a dominant allele, the recessive trait remains unexpressed, and the individual is considered a “carrier.”
The Mechanism of Genetic Dominance
The mechanism by which a dominant gene influences or “masks” a recessive gene involves the production of functional proteins. Dominant alleles produce a functional protein or a sufficient quantity of a product that performs a specific cellular task. For instance, if a gene codes for an enzyme, a single dominant allele might produce enough functional enzyme to achieve the desired outcome.
Recessive alleles result in a non-functional or less-functional protein, or sometimes no protein at all. When a dominant allele is present, its functional product can compensate for the lack of a functional product from the recessive allele. This single functional copy is enough to express the trait, preventing the recessive allele’s influence from being observed.
Common Examples of Dominant Traits
Numerous human traits follow a simple dominant-recessive inheritance pattern. For example, dark hair is dominant over blonde or red hair, and curly hair is dominant over straight hair. The ability to roll one’s tongue into a tube shape is another common dominant trait, as is the presence of dimples on the cheeks when smiling. Having detached earlobes is dominant over attached earlobes. A single dominant allele is sufficient for these traits to manifest.
Variations in Gene Expression
While simple dominant-recessive inheritance is a common pattern, gene interactions can be more complex. Incomplete dominance occurs when the heterozygous phenotype is an intermediate blend of the two homozygous phenotypes. For example, crossing a red flower with a white flower might result in offspring with pink flowers, where neither red nor white is fully dominant. This blending effect shows that the influence of both alleles is partially expressed.
Another variation is codominance, where both alleles are fully and simultaneously expressed in the heterozygous individual. A classic example in humans is the ABO blood group system, where individuals with both A and B alleles express both A and B antigens on their red blood cells, resulting in AB blood type. In this scenario, neither allele masks the other; instead, both contribute to the observable phenotype. These variations highlight that gene expression can be more complex than simple dominance suggests.