I lO 



MENDELISM 



CHAP. 



YeUow (^ X ? Tort. 



I I \ 1 



rf (^ ? 9 



Black Yellow Tort. Yellow 



Fig. 34. 



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. 

 Black ^ X $YeUow Moreover, the results of the 

 _, reciprocal cross are such as 



might be expected on the 

 theory. The F^ generation 

 from black female crossed with 

 a yellow ^ consists of tortoise- 

 shell females and black males. 

 Such animals bred together 



Scheme illustrating the result of should givC CQUal nUmbcrS of 

 crossing a black torn with a o ^ 



yellow female. Compare with the hcterOZygOUS 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 (^ X ? Black 



I 1 



Black ^ X ? ^°'^- 



Yellow Black Tort. Black 

 Fig. 35. 



Scheme illustrating the result of 

 crossing a yellow torn with a black 

 female. Compare with Fig. 28, 

 p. 100. 



