Population genetics considers how the frequencies of alternative states of genes in populations are maintained or changed from generation to generation. First, however, it is important that we understand the terms that are used; otherwise, it is easy for beginners to become confused. It is also important to know how the terms will be used in this book, because many of the terms are not used consistently in the wider literature.

Terminology

The following should clarify how the various terms introduced in this chapter are used throughout the book.

phenotype The morphological, physiological, behavioural or biochemical characteristic of an individual, or a group of individuals in a population. Typically, the term refers to a single characteristic, such as body colour or blood group type, but can also refer to more than one characteristic. Almost invariably, there is more than one phenotype for a given characteristic. For example, there may be both short and tall plants in a population.

genotype This is the genetic constitution of an individual, or a group of individuals in a population, which is related by simple Mendelian rules to the phenotype. The theory in this book mainly considers genes with just two different alleles in the population, e.g. A and B, so that there will be just three different genotypes, AA, AB and BB. These will result in three different phenotypes if there is no dominance, but only two if there is dominance. If genotypes AA and AB give rise to the same phenotype, A is considered to be dominant to B, and if AB and BB give the same phenotype, B is considered dominant to A. Theory relating to multiple genes and alleles is considered in Chapter 12.