Abstract
1. The effect of genetic sampling, when this sampling is without replacement, on variation in gene frequency is studied, and equations describing the genetic drift are derived. The effective size turns out to be about one greater than under sampling with replacement.2. The relation between ‘spread of genes’ and genetic drift is worked out.3. The University of Queensland control poultry flock is analysed by these methods.4. The design of control populations is discussed with particular reference to the relative importance of genetic drift and phenotypic sampling.