The optimum design of genetic controls is considered with respect to several factors, such as a fixed total expenditure, differing costs for males and females, different plans for retaining parents and choosing replacements, the value of keeping spares and the effects of mortality and infertility among individuals in the control.

The effects of mortality and infertility are important for these will tend to reduce the annual effective size and lower the optimum age for replacement of parents. Keeping individuals to a fixed age, without spares, will usually be preferable in practice to the lifetime retention of parents with spares, since spares will be at the expense of other breeding animals. It is shown that with the same total expenditure, the annual effective size of a control may be substantially increased by the appropriate choices among the various factors studied and methods are given for finding the optimum design.