An orthology is a trait in two or more species that was inherited from a common ancestor. It can be contrasted with an evolutionary analogy, which is a similar trait that evolved independently. Genes passed from parents to offspring can remain intact, which will often result in the manifestation of similar physical traits. If genes mutate many times, however, they can end up producing similar outward traits through natural selection. Orthology in different species refers to similar traits whose genes have an inherited, rather than mutated, origin.
Different species exhibit different characteristics, but some of these traits may be very similar. For example, many animals have hands with five fingers, though these fingers may vary significantly in size or shape. Orthology concerns the origin of genes that are responsible for physical traits such as fingers. These genes can evolve in two different ways—either independently through mutation or hereditarily through orthology.
Nature is replete with examples of traits that evolved independently. For instance, both sharks and dolphins have streamlined bodies with a vertical fin on their backs. The internal anatomy of sharks and dolphins, however, is very different; in fact, they are quite distant evolutionarily. Their closest common ancestor lived a long time in the past. Rather than being inherited, the genes responsible for many of their similar traits evolved independently due to their similar environment.
Closely-related species, on the other hand, inherit many of the same traits from a common ancestor. When this happens, patterns in a deoxyribonucleic acid (DNA) molecule get transmitted intact to offspring down many generations. Without any mutations to alter genes, they will be inherited by offspring and will often result in the same outward traits as in ancestor organisms. If heredity is the cause of a particular gene, it is said to be orthologous.
When an orthology occurs, the gene in question may or may not have the same function in the life and reproduction of its host. In many cases, the gene will perform the same role. Fingers in primates, for example, typically have similar functions— gathering food, moving through trees and fighting off enemies. Orthologous genes will occasionally perform different roles, though. Humans have five toes, but they can not grasp branches like the toes of a gorilla.
Identifying orthology is critical in comparing gene function between organisms. It is also useful in the classification of life on Earth, which is called taxonomy. Knowing the origins of particular genes can help scientists understand why some animals, like sharks and dolphins, look similar but behave very differently.