448 Genetics of Sex Determination 



many features of general genetics, like dominance, maternal inherit- 

 ance, heredity within the Y-chromosome, and crossing over in one or 

 both sexes show specific features from case to case. But the underlying 

 principle, the F/M balance, is always the same. 



BALANCE AND 

 MODIFICATION 



It is obvious that a genetic system of the F/M balance type must be 

 rather easily modifiable by environmental and genetic actions, since 

 time relations and similar features of the kinetics of the genetic 

 systems and also threshold conditions are involved. If the balance is 

 eflFected by a 2X-1X system, it is to be expected that it is more easily 

 upset by modifying action in the homogametic sex. If we use arbitrary 

 measures for the genetic actions, for examples, for Lynwntria 

 F(Y) = 10, M(X) = 6, the female imbalance is in favor of F 10/6; 

 the male one, in favor of M 12/10. That is, in the homogametic sex, 

 maleness outbalances femaleness by only one-fifth; in the hetero- 

 gametic sex femaleness outbalances maleness by two-thirds. Modi- 

 fiers acting upon the thresholds will therefore more easily affect the 

 homogametic sex, especially in intersexes with already impaired 

 balance. However, this does not mean that the homogametic sex more 

 easily becomes intersexual. Haldane's rule that the opposite is true 

 applies to many cases, for a special reason. If we take Lymantria as 

 an example, with the balance F(Y)/M(X) = $ , the facts prove that 

 an increase in the potency of M ( as found in different races ) produces 

 female intersexes in Fi of a cross of a weak race with the male of a 

 strong one. But for the males F(Y)/M(X)M(X) the reciprocal cross 

 transmits only one weak M to the Fi males, which then become inter- 



