A population of identical homozygotes constitutes a pure line. Pure lines "breed true"; that is, they maintain their phenotype indefinitely if genetic crosses are restricted to members of the pure line (see True Breeding).
The concept of pure line was introduced by Johannsen (1). Pure lines are obtained through repeated self-fertilization of heterozygous individuals or consanguineous crosses in heterozygous populations. Under Mendelian inheritance, the offspring of a single self-fertilization will be heterozygous for one-half as many genes as the parent. Repeated self-breeding approaches full homozygosity asymptotically. Several generations of self-fertilization are needed to obtain an acceptable pure line (97% of the genes that were originally heterozygous would have turned homozygous after five generations). A population of an autogamous organism is a mixture of pure lines; and, if started from a single organism, the whole population is a single pure line.
Repeated crosses between close relatives also approach full homozygosity, but more slowly than self-fertilization. Thus, about 20 generations of crosses between a brother and a sister from each generation may be carried out to produce a pure line of mammals.
Populations of allogamic organisms, in which crosses occur at random or even with preference for distantly related partners (outbreeding), maintain a high level of heterozygosity. In these populations, rare recessive alleles that are deleterious when homozygous have little effect on the fitness of the population, because most of them are maintained in heterozygotes. The total number of such alleles may be high, because they may occur at many genes. The overall deleterious effect on the population (the genetic load of the population) may be considerable. Thus, human populations suffer on average a genetic load that is equivalent to that caused by the presence in each individual of four recessive lethal alleles (or a larger number of less disabling alleles). Inbreeding in allogamic organisms bring the deleterious recessive alleles to homozygosity; the immediate consequence is an increase in the frequency of defective offspring, or, in another words, an increase in the genetic load of the population. This phenomenon is called inbreeding depression or inbreeding degeneration. As inbreeding continues, the deleterious alleles are selected out and eventually disappear. The original heterozygous populations are often more fit than the resulting pure lines because they profit from heterosis and balanced polymorphisms; the main advantage of pure lines is the quick production of many individuals with the same well-adapted genotype, while the allogamy continuously generates new genotypes.