The principles of genetics

Abstract Some species of organism have melanic forms, while other morphologically similar and closely related species lack them. Furthermore, within a species, some populations contain no melanics, others are all melanic, and some have a mix of both melanics and non-melanics. If we are to understand the reasons for the pres ence or absence of melanic forms within a species, or why the frequencies of melanic forms vary in populations in either time or space, we must understand how melanic forms arise, how they increase or decrease temporally or spatially, and why in some species the increase of a melanic form stops before it completely replaces non-melanic forms. This is a long-winded way of saying that we have to understand the evolution of melanism, and to do that we need a knowledge of inheritance. How are traits passed from one generation to the next? Why do progeny have the characteristics of one or other of their parents in some features, appear to be a mix of their parents in other features, and are unlike either of their parents in still others? The study of these questions of inheritance is the field of genetics.