Authors worked out a breeding scheme in order to utilize the yearly genetic progress in milk and beef production as well as "hybrid vigor" and "type heterosis". Latter term is proposed to be introduced into the terminology of animal production. This scheme which integrates milk and beef production, keeps space with the international genetic progress and attains a heterosis effect over 6 0 per cent in milk production and at the same time assures a heterosis effect (hybrid vigor as well as type

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... terosis) of i oo per cent in the single suckler cow stock as well as in the population for direct fattening. In this scheme pure breeding is practiced only in herds meant for producing sires of high genetic quality whereas in milk and beef production only crossbred populations are involved. The scheme is already in practice within a large scale experiment of 6 ooo cows in criss-cross breeding by using the best genetic material of USA-Canadian and Danish f e y sey breeds and of three types of beef breeds. In this conception breeds are no more the objects of production but the building bricks of the production systems. The gene for halothane sensitivity (Hal 8 ) is favoured in the selection for muscle development which has occurred in some modern breeds of pig (e.g. Piet y ain). Such a selection tends to increase the frequency of another gene (PHI B ) at the closely linked locus for PHI, by virtue of a so-called "hitch-hiking effect", the PHI locus itself being selectively neutral. Another effect of selection at the Hal locus is to generate a linkage disequilibrium between the two loci, Hal and PHl, the evolution of this disequilibrium depending on the type of selection applied at the Hal locus. This point is demonstrated by the observation of two lines of pigs selected differently for 4 generations, and thus confirms a recent theoretical work (TxoNtsorr, 1977 ) showing that a possible source of linkage disequilibrium may be the hitch-hiking effect of a selected locus on another closely linked neutral locus.