Our bodies are made of trillions of cells, and within each cell’s nucleus lies our genetic instruction manual: DNA. This DNA is organized into structures called chromosomes, and along these chromosomes are segments known as genes. Genes contain the blueprints for building and maintaining our bodies, influencing everything from hair color to disease susceptibility. Different versions of these genes, called alleles, account for the variations we observe in traits among individuals. An individual inherits two alleles for each gene, one from each biological parent, and their specific combination determines how a particular trait will manifest.
Defining Dominant Alleles
A dominant allele is a version of a gene that expresses its associated trait, meaning it will be observable even when only one copy of that allele is present. If an individual inherits a dominant allele from one parent and a different allele for the same trait from the other, the dominant allele’s trait will be observable. It effectively masks or overrides the effect of the other allele. This observable expression is called the phenotype.
For example, consider a gene that determines flower color where red is dominant over white. If a plant inherits one allele for red flowers and one for white, it will produce red flowers because the red allele is dominant.
Understanding Recessive Alleles
In contrast to dominant alleles, a recessive allele only expresses its associated trait when two copies are present in an individual. If a dominant allele is also present, the effect of the recessive allele is completely masked.
For instance, if we consider the flower color example again, a plant would only have white flowers if it inherited two copies of the recessive allele for white flowers. If it inherited one white allele and one red (dominant) allele, the red color would be expressed, and the white trait would remain hidden. Recessive traits can appear to skip generations because carriers, who possess one dominant and one recessive allele, do not display the recessive trait themselves but can still pass the recessive allele to their offspring.
How Traits Are Inherited
Traits are passed from parents to offspring through the transmission of genes, which provide instructions for specific characteristics. The combination of alleles an individual receives forms their genotype, which is their specific genetic makeup for a particular trait. This genotype then determines the observable trait, or phenotype.
In simple Mendelian inheritance, named after Gregor Mendel’s pioneering work with pea plants, distinct patterns emerge. For example, if an offspring inherits at least one dominant allele, the dominant trait will be expressed. The recessive trait will only appear if the offspring inherits two recessive alleles. These principles explain why certain traits are more common or appear predictably across generations, as seen in the reappearance of recessive traits in later generations even if they were not visible in the immediate parental generation.
Common Examples
Numerous human traits illustrate the concept of dominant alleles exerting their effects. For example, dark hair is dominant over blonde or red hair. Curly hair is a dominant trait compared to straight hair. The presence of a widow’s peak, a V-shaped hairline, is also determined by a dominant allele.
Other observable dominant traits include freckles, dimples, and a cleft chin. The ability to roll one’s tongue is another well-known dominant trait, while the inability to do so is recessive. Detached earlobes are also dominant over attached earlobes.