430 Morphogenetic Factors 



In others, like the dioecious species of the strawberry, Fragaria, it is the 

 female that is the heterozygous sex and all male gametes are alike. Other 

 difficulties appear. In Rumex, for example, the female has two X chromo- 

 somes and the male one, but the male has two different Y chromosomes. 

 In Humulus lupulus, hops, the female apparently has two pairs of dif- 

 ferent X chromosomes and the male has one of each of these plus two 

 different Y chromosomes. 



The situation is so complex and the results reported often so conflict- 

 ing that some botanists, among them Schaffner and Yampolsky, have 

 entirely repudiated the chromosome theory of sex, particularly since in 

 some cases, notably in Cannabis, sex can readily be reversed by various 

 environmental factors, as Schaffner was able to do by altering the 



Fig. 19-8. Chromosomes of Sphaerocarpos. From female gametophyte, above, showing 

 X chromosome, and from male, below, showing Y. ( From C. E. Allen. ) 



photoperiod (p. 317). McPhee (1924) obtained similar results. He showed 

 that this does not invalidate the genetic basis for sex but simply demon- 

 strates that the range of expression for the genotype in hemp in response 

 to the environment is very wide. 



The early ideas that two X chromosomes produce a female and one X 

 a male are clearly too simple. The modern view of sex determination 

 conceives of a balance between several, probably many, genes of which 

 some are in the so-called sex chromosomes and others may be in the 

 autosomes. This theory of balance is well shown by the results of several 

 workers (Warmke and Blakeslee, 1940; Westergaard, 1940; and Warmke, 

 1946) with a dioecious race of Melandrium which had been made tetra- 

 ploid by colchicine treatment. Here the female had four X chromosomes 

 and four sets of autosomes, 4A + XXXX, and the male 4A -f- XXYY. By 

 crosses among these and with diploids, the investigators were able to pro- 



