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The genetical interpretation of the continuous variation (or indeed any variation) shown by a population, family or group of families requires the use of specifications of two distinct kinds. Firstly, it is necessary to specify the genetical structure of the population, family or families. In principle, this requires the specification in suitable terms of the relative frequencies of the various alleles of the genes involved, the distribution of the alleles at a locus between the various possible homozygotes and heterozygotes and the distribution of the alleles of different genes in respect of one another. These specifications will depend on the ancestry of the material, the mating system which has been in force, the selection which has been practised (if any), and the linkage or other relation of the genes in transmission from parent to offspring. Specification of the genotype of every individual, or indeed of any individual, is not essential for most biometrical purposes so long as the relative frequencies of the different possible genotypes can be given, and indeed it is sufficient for many purposes to specify only the average, taken over all genes, of the allele frequencies, homozygosis, linkage relations and so on.
Hayman et al. (Tue,) studied this question.