By now you most likely have discerned that human biologists focus much of their research on variation. Their studies have investigated humans at multiple levels of organization and interaction, from within cells to between large populations. The primary subject of this chapter will be genes, and our aims are to define what genes are and what they do, how they become variable, how they are transmitted between generations, and how they undergo evolutionary processing. More formally, the areas to be addressed are Mendelian genetics, human genetics, molecular genetics, and population genetics. In addition, there will be some discussion of newly developing research areas of interest to human biologists, for instance, epigenetics. Along the way we will point out where certain topics covered here are addressed, and often more fully presented, in other chapters of the volume. We will begin with a brief look back into history when earlier notions of hereditary transmission began to be transformed into increasingly more accurate foundations that eventually led to our current understanding of the nature of genes.

PARTICULATE THEORY OF INHERITANCE

A prevailing notion up through the nineteenth century was that parents passed on to their offspring equal portions of their traits, such as stature or skin color, that blended into an inseparable mixture. Thus, for example, a mating between a tall and short parent would result in children of intermediate height, who themselves would then go on to produce children of intermediate height.