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THE AMERICAN NATURALIST [Vol. LI 



cross, together with the 3:1 ratio of the selfed ear, shows 

 that C is homozygous in plant (31). Third, the remain- 

 ing factor, R, should occur only in a heterozygous or in a 

 homozygous recessive condition to account for the 1 : 1 

 aleurone ratio on the Fj ear of the original back cross. 

 The fifth cross in Table VII proves that R is heterozygous 

 in plant (31), and the sixth cross shows that in plant (41) 

 this same factor is homozygous recessive. 



From this series of interrelations it is seen that the 

 hypothesis is verified in all cases and that it is the R factor 

 for aleurone that is linked with G. 



On the Question of Crossing Over in the Male and 

 Female 



An interesting observation regarding the question of 

 crossing over in plants can be derived from some of the 

 data presented. It has been shown that crossing over 

 occurs in gametogenesis of the female (Table I). Does 

 it take place in the formation of the male gametes as well? 

 In certain animals, crossing over seems to be limited to 

 one sex. It occurs only in the female of Drosophila (Mor- 

 gan, 1915) and only in the male of the silkworm (Tanaka, 

 1914). Castle (1916) has noted that the phenomenon 

 occurs in both sexes of the rat. Among plants, the studies 

 with sweet peas and Primula indicate that crossing over 

 is not restricted to one sex. Perhaps it is to be expected 

 that, in the case of most plants, where the pistillate and 

 staminate parts are borne on the same individual, there 

 should be no difference in the genetic behavior of the two 

 reproductive systems in this respect. Nevertheless, it is 

 interesting to note the condition in the monecious corn 

 plant. 



From the data in Table II, it is possible to demonstrate 

 that crossing over is found in both sexes of maize. In 

 order to do this, the observed frequencies can be com- 

 pared with the theoretical expectation when crossing over 

 occurs in both sexes and when it takes place only in the 

 female. Such a comparison is arranged in Table VIII, 

 which follows : > 



