Why Are Genes Dominant and Recessive?

Dominant and recessive are terms used to refer to the manner in which versions of genes influence physical characteristics. Individuals receive two versions, also known as alleles, of each gene from their parents. Generally, each can be either dominant or recessive (more complex categories do exist). For example, the gene BEY2, which is known by many other names, helps to determine whether an individual’s eye color can be brown or blue (there are other genes that work with this one and result in other eye colors). The allele for brown eyes is dominant, while the allele for blue eyes is recessive. If an individual receives a dominant allele from each parent, then they will have the dominant trait (brown eyes). If an individual receives a recessive allele from each parent, then they will have the recessive trait (blue eyes). However, if an individual receives one recessive allele from one parent and also receives a dominant allele from the other parent, then they will have the dominant trait (brown eyes). This is due to the fact that dominant alleles are always expressed over recessive alleles. These are simple examples as the genetics of eye color are far more complex.

What Makes an Allele Dominant?

Some alleles for a gene may produce products that go on to result in a specific trait. The same gene may also have alleles that have mutations that result in fewer products, that result in faulty products, or that prevent the gene from producing anything at all. Alleles without these mutations are typically dominant and will produce sufficient amounts of their particular products to make up for any recessive allele that an individual may receive from the other parent. This explains why the allele for brown eyes is dominant, as it results in high production of melanin.

What Makes an Allele Recessive?

As mentioned before, some alleles of a gene have a mutation that leads to little production, no production, or faulty production. These alleles are recessive. They create such small amounts of functional product that their influence can be easily masked by a dominant allele for the same gene. This is why the allele for blue eyes is recessive. It produces so little of the melanin pigment that any impact that it has on eye color can be overridden by higher melanin production from the dominant (brown eye) allele. Therefore, alleles that make more of a product or that make a more functional product will be dominant over alleles that make less of a product, no product, or a less functional product.