Genetic Factors 431 



duce types with one or with two Y chromosomes combined with two, 

 three, or four X's, the number of autosomes being kept the same in each 

 combination. The Y was thus the only variable. They also combined one, 

 two, three, and four X chromosomes with one Y, again with the same 

 number of autosomes, so that the number of X chromosomes was the 

 variable. The results as to male, female, and hermaphrodite flowers led 

 to the conclusion that the autosomes have little sexual tendency ( as they 

 do have in Drosophila), that the Y contains strong male-determining 

 genes, and the X, weaker female-determining ones. The ratio of X to Y 

 chromosomes determines what the sex will be. It can also be shown 

 that there are at least three genes for maleness in the Y chromosome. 



Experiments like these support the hypothesis that genes with tenden- 

 cies to produce male structures or female ones are carried by the X, the 

 Y, or the autosomes and that the balance between them determines the 

 particular expression of sex. These genes may be present in chromosomes 

 where there is no morphological difference between members of the pair, 

 and such difference is obviously not significant for sex determination. The 

 sharp distinctions between maleness and femaleness in animals and the 

 relative scarcity of intersexual forms among them make genetic analysis 

 of sex much less difficult there than in plants. 



Aside from the determination of sex in individuals as a whole, the de- 

 gree of sex expression within the individual has also been shown to be 

 under gene control. Typical illustrations of this are the following: In 

 several cucurbits the monoecious condition is dominant over the andro- 

 monoecious (where some flowers are hermaphroditic and some stami- 

 nate), and a single gene is involved (Rosa, 1928). Poole and Grimball 

 (1939) extended this for Cucumis by demonstrating a two-factor dif- 

 ference between hermaphroditic and monoecious, since a cross between 

 these types gave in F 2 nine monoecious: three gynomonoecious: three 

 andromonoecious: one hermaphroditic. In Carica, Hofmeyr (1938) re- 

 ports that three alleles, M l5 M 2 , and m, are responsible for maleness, 

 hermaphroditism, and femaleness, respectively (M^m is staminate; M<jn, 

 hermaphroditic; and mm, pistillate). Homozygous Mi or M> are lethal. 

 Janick and Stevenson (1955) find that the monoecious character in 

 spinach is allelic to the XY pair of genes concerned in sex determination. 



A notable example of the effect of genes on sex determination which 

 could lead directly to the production of a dioecious condition from a 

 monoecious one was demonstrated by Jones (1934) with maize. In this 

 plant there are several recessive genes for tassel seed (ts) on chromo- 

 some 1, in which the staminate florets are replaced by pistillate ones 

 and the plant is thus essentially female. The recessive silkless gene (sk) 

 in chromosome 2 sterilizes the female flowers and thus produces a plant 

 essentially male. The silkless gene has no effect in the presence of tassel 



