GENETICS 689 



this field. The regularity of the mitotic process appears to assure every 

 cell of the body the same number and kinds of genes as every other cell, 

 yet the tissues of the body have marked differences in their chemical, 

 physical and biologic properties. These differences apparently result 

 from the different metabolic effects of similar genes working in different 

 cytoplasmic environments. 



One of the clearest demonstrations that the same genes working in 

 dissimilar environments do have different effects was provided by experi- 

 ments with three races of frogs found naturally in Florida, Pennsylvania 

 and Vermont. Each of these races normally develops at a speed which 

 is adapted to the length of the spring and summer season in its normal 

 environment. Southern frogs develop slowly and Northern frogs develop 

 more rapidly. Eggs of Northern frogs raised under Southern conditions 

 are overaccelerated in development whereas eggs of Southern frogs raised 

 under Northern conditions are overretarded. By fertilizing an egg with 

 sperm of a different race, and then removing the egg nucleus before the 

 sperm nucleus unites with it, it is possible to establish a situation in 

 which Northern genes are operating in Southern cytoplasm or vice versa. 

 Northern genes for rapid development in Southern, slow developing 

 cytoplasm resulted in poorly regulated development; the animal's head 

 grew more rapidly than the posterior region and was disproportionately 

 large. When Southern genes were introduced into Northern cytoplasm 

 theie was poorly regulated development but the head, rather than the 

 posterior region, was retarded in development. Genes from the Pennsyl- 

 vania race acted as "Northern" with Florida cytoplasm, but as "South- 

 ern" with Vermont cytoplasm. Thus, exactly the same set of genes pro- 

 duced opposite morphologic effects when acting upon, and interacting 

 with, different cytoplasmic environments. 



290. Cytoplasmic Inheritance 



The gene theory of inheritance is well established; there is no doubt 

 that the genes within the chromosomes afford the physical basis for the 

 transmission of traits from one generation to the next. The question as 

 to whether the genes are the sole means of inheritance, or whether some 

 characters may be transmitted by other means, has been hotly debated. 

 No definite answer can be made at present, other than that if instances 

 of nongenic inheritance do occur they are quite rare. Some of the ex- 

 perimental evidence which at first was interpreted as proof of cytoplasmic 

 inheritance has since been shown to be explainable in terms of the usual 

 genie mechanism. 



The contributions of egg and sperm to the nucleus of the zygote 

 are equal, but their cytoplasmic contributions are not. If any trait were 

 inherited by factors located in the cytoplasm and independent of the 

 genes, the offspring would resemble the mother and not the father. ^Vith 

 almost all traits tested in a wide variety of plants and animals, the char- 

 acters of the offspring are the same whether the cross is made female 

 AA X male aa or female aa X male AA. This indicates that the con- 



