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A locus is a spot or “address” on a chromosome at which a gene for a particular trait is located in all members of a species. It can also refer to the location of a mutation or other genetic marker. A given locus can be found on any pair of homologous chromosomes (Brown, 2009).
In 1909, the botanist Wilhelm Johannsen coined the term “gene” to describe the heritable characters discussed by Mendel. Between 1907 and 1919, the geneticist Thomas Hunt Morgan (a Nobel Laureate in Medicine) and his colleagues conducted an elegant series of experiments using the fruit fly Drosophila melanogaster. Fruit flies have marked advantages for use in transmission genetics research: they are abundant in nature, and they are very easily fed and accommodated. In addition, they are prodigious reproducers. They have a generation time of 10 days, and females produce around 300 eggs (of which half are female). This mathematics means that in the space of a single month, i.e., just...