A model relating gene replicas and gene repression to phenotypic expression and variability.

It is suggested that the development of a dominant character in higher organisms starts with the activation of a major gene. This gene replicates; the number of replicas is controlled by a second gene or set of genes, which also ensure that the most suitable allele, if more than one is present, makes the replicas. The major gene replicas then make messenger RNA and are reinforced in this by minor genes. The joint product of major gene replicas and minor genes activates a fourth gene or set of genes which produce a repressor that damps down the activity of the major gene replicas. The evidence for this scheme, which comes from studies of selection experiments in populations, is summarized and the evidence from studies of DNA and RNA metabolism is referred to in this paper.