"It is agreed that most, if not all, new mutations are deleterious. In plants, much fewer than 1% of radiation-induced mutants are found to be useful. In insects, a few induced mutations confer some advantage, but only to completely inbred populations - a situation quite in contrast with the human condition (NEWCOMBE 1960). According to DOBZHANSKY (1957) : ‘if anything, radiation-induced mutants are more destructive than the spontaneous ones. As far as genetic effects are concerned, the only safe dose of high-energy radiation is no radiation’. As observed in mammals, mostly the mouse, the effects of radiation-induced mutations may be : (1) Dominant: lethals, seen as reduced litter size, or non-lethals, evidenced as offspring which are malformed or diminished in vitality or life-span; semi-sterility, resulting from translocation, is seen one generation later as a reduction of litters by one-half and is handed down to subsequent generations (PAULA HERTWIG 1940). (2) Recessive mutations are demonstrated by mating the offspring of exposed males to females known to be (heterozygous) carriers of certain traits (W. L. RUSSELL 1954; CARTER et coll. 1956). (3) Mutations involving sex-chromosomes may be reflected by a change in the sex ratio of the offspring (sex-linked lethals, see Fig. 1)"