CHAPTER 3 



SEX-LINKAGE. NON-DISJUNCTION 

 GYNANDROMORPHISM 



Sex-Linkage. When describing the transmission of a char- 

 acter from parent to offspring it is not necessary in the great 

 majority of instances to make any reference to the sex of the 

 parent or of the offspring; it is enough to state that, for 

 example, the dominant character exhibited by one of the 

 parents is displayed by all or by 50 per cent, of the offspring, 

 or that the recessive character exhibited by one of the 

 grandparents reappears in approximately 25 per cent, of the 

 grandchildren. In certain instances, however, like that of the 

 grossulariata and lacticolor characters of Abraxas, a correct 

 description involves reference not only to the distribution 

 of these characters among the individuals of the different 

 generations but also to the sex of the individuals that display 

 these characters. 



Although the exact chromosome constitution of Abraxas 

 is unknown, the results obtained by Doncaster can be most 

 easily explained as follows. Assume that Abraxas has the 

 Lygaeus type of sex-chromosome constitution, that the 

 female is the heterogametic sex, that the genes for the char- 

 acters grossulariata and lacticolor are resident in the 

 X-chromosome (in any one X there being either that for 

 grossulariata or else that for lacticolor), and that there are 

 no genes in the Y that in any way interfere with the action of 

 these X-borne genes. 



I. lact. ? 



(gX)Y 



(gX) Y 



(GX)(gX) 



heterozygous 



gross, c? 



(GX) (gX) 



gross. S 



(GX)(GX) 



(GX) 



(GX)Y 



gross. $ 

 (GX) Y 



P.I 



gametes 

 F.I 



gametes 



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