I lO 



MENDELISM 



Black ^ X 9 Yellow 



is confined to the male sex. If we assume that the 

 heterozygous female is-tortoiseshell instead of yellow, 

 the results shown in Fig. 34 are closely comparable 

 to those given in Fig. 27. 

 Moreover, the results of the 

 I reciprocal cross are such as 



Yellow^ X ?Tort. might be expected on the 

 theory. The Fj generation 

 from black female crossed with 

 a yellow ^ consists of tortoise- 

 shell females and black males. 

 Such animals bred together 



I 1 — ^ 1 



^ (^ ? 9 



Black Yellow Tort. Yellow 



Fig. 34. 



Scheme illustrating the result of should glVC eOUal nUmbcrS of 

 crossing a black torn with a o i 



yellow female. Compare with the hetei'OZVffOUS and of the 



Fig. 27, p. 99. . , . , , 



recessive forms in both sexes, 

 and this has been found to be the case (cf Fig. 35). 

 To this interpretation a difficulty is offered by the 

 existence of tortoiseshell toms. Though they are 

 very rare, there is no question 

 of their occurrence, but at 

 present too little is known 

 about their progeny to enable 

 us to form any clear idea as 

 to their nature. It may not, 

 however, be without signifi- 

 cance that in one case, at any 

 rate, an attempt to breed from 

 such an animal proved him 

 to be sterile. 



The other species of verte- 

 brates that offers examples of what may be termed 

 the Drosophila type of sex-limited inheritance is man 

 himself It is a matter of common, experience that 



Yellow cJ X f Black 



I 1 



Black ^ X ? '^'"■'• 



1 — n — — I 



Yellow Black Tort. Black 



Fig. 35. 



Scheme illustrating the result of 

 crossing a yellow torn with ablack 

 female. Compare with Fig. 28, 

 p. 100. 



