13. The primary breeders maintain and develop the quality of the genetic stock within the population by selection of the most suitable individuals. They keep a large number of pure grandparent genetic lines which are crossed and inter-crossed and the parents of the commercial growing stock are selected from their progeny. Primary breeders sell parent stock, either as hatching eggs or poults, to breeders. Male and female lines of turkeys are to some extent selected for different genetic characteristics. Selection pressure within the male line is usually concentrated on growth and conformation, whilst selection pressure within the female line is usually concentrated on fecundity. General characteristics that are important to the health and welfare of the birds are taken into account in breeding. An example is the ability of the birds to walk without difficulty.