The two fundamental methods for genetical improvement of livestock are selection on die basis of performance and mating according to relationship. The most familiar forms of the latter are crossbreeding and inbreeding according as mates are chosen which are less closely or more closely related than the average of the breed.

The possibilities of inbreeding, followed by crossing, were exploited in the production of ‘ hybrid corn’ in U.S.A. and the success achieved led to the hope that similar methods might be successful in livestock breeding. Accordingly the Regional Swinebreeding Laboratory organised a programme of producing inbred lines of pigs and then crossing them. During the 12 years of this programme, about 90 inbred lines have been started and about 50 still survive. Inbreeding degeneration is greatest in those characters, such as fertility and viability, which have been subject to intense natural or artificial selection. The inbreeding decline in growth rate and economy of food utilisation is considerably less. When the lines are crossed in pairs the average performance of the first cross pigs is the same as that of the initial population.