GENETICS — STERN 273 



Sweden. Success has been slow in coming, for reasons which might 

 have been foreseen. In evolution, establishment of new genetic types 

 results not directly from the occurrence of a new, mutated gene variety 

 within the genie assemblage of an individual. Rather, it requires a 

 process of selecting some of the varied genie assemblages of the species, 

 namely, those which happen to complement in a harmonious way the 

 newness of the mutation. Similarly, in artificial breeding the produc- 

 tion of a useful mutant requires a selection of suitable genetic back- 

 ground. This adjustment between the new and the suitable elements 

 of the old takes time. By now, improved strains with superior stiff- 

 ness of the straw in barley, and an improved strain with superior seed 

 and oil yield in white mustard, have been created on the basis of 

 X-ray-induced mutations. 



They were pure geneticists who discovered the peculiar mode of 

 inheritance of some special traits which are called "sex-linked." In 

 suitable crosses with such traits all daughters resemble their fathers 

 and all sons their mothers. It was another pure geneticist who pro- 

 posed to use this criss-cross inheritance for the sexing of chicks. Male 

 and female chicks are hard to distinguish until they are several weeks 

 old. But when you cross barred Plymouth Rock hens and not-barred 

 Rhode Island Red roosters, the female chicks are not barred and the 

 male chicks are barred. This trick is now being used each year for 

 the commercial production of several million chicks in the United 

 States alone. Obviously, only females can be used for the all-impor- 

 tant egg production, and it is more economical to destroy most of the 

 newly hatched now easily recognized males than to feed them until 

 their normal development betrays their relative uselessness. 



One more example from agriculture. Cattle and horse breeders 

 as well as breeders of other animals have always been troubled by the 

 birth of abnormal types of young, often doomed to early death. 

 When genetics arose it showed the frequent occurrence of lethal genes 

 in mice and in fruitflies, genes that lead to abnormal, destructive 

 development of their bearers. Soon it was recognized that many 

 of the stillbirths which had troubled the animal breeder were due to 

 specific lethal genes, often brought into a herd by some famous sire 

 who carried the lethal gene in a harmless combination with a normal 

 one. Among his numerous descendants many would be carriers again, 

 and breeding the carriers with one another would result in 25 percent 

 of the pregnancies ending in disaster. This insight into the stock 

 breeders' troubles carried with it means of avoiding them. It was 

 easy, on the basis of Mendel's first law, to devise trial matings through 

 which bulls or stallions could be recognized as carriers of dangerous 

 lethal genes and, if found genetically unsound, be excluded as sires 

 of large numbers of offspring. The savings accomplished in this 



