198 



GENETICS IN RELATION TO AGRICULTURE 



mating, simplex woman (BX)(bX) by color-blind man (bX)Y, or by the 

 still less frequent mating of color-blind woman (bX)(bX) by color-blind 

 man (bX) Y, in which latter case all the offspring whether sons or daughters 

 are color-blind. A considerable list of other sex-linked factors demon- 

 strate beyond question that the inheritance of sex and the distribution of 

 sex-linked factors in man is strictly analogous to that which we have 

 found to obtain in Drosophila. 



Non-disjunction in Drosophila. Of particular interest from the 

 standpoint of the inheritance of sex and of the relation between factors 



and the chromosomes are the results which 

 Bridges has obtained from his extensive in- 

 vestigations of non-disjunction in Drosophila. 

 The investigations on non-disjunction had 

 their origin in certain "exceptions" which ap- 

 peared from time to time in cultures of 

 Drosophila. Ordinarily in the case of sex- 

 linked characters when a female with the 

 recessive character is mated to a male with 

 the dominant character all the females in F\ 

 exhibit the dominant sex-linked character 

 and all the males the recessive character. 

 The reason for this fact has been explained 

 already, but it will be clearly apparent from 



FIG. 89.-The relations of the a consideration of Fig. 89, which is a diagram 

 sex chromsomes to sex produc- of the results of crosses between vermilion 



tion and to the inheritance of f prna i p <, anr l rpr l m ,1po TVip vprmilinn fartnr 

 the recessive sex-linked char- ea males - J- ne vermilion iact< 



acter, vermilion eye color, in y is borne by the sex chromosomes, and since 



S!^J 1 3rSta: the males from crosses between vermilion 

 somes, and the crooked ones the females and red males receive their only 



Jr~omid2 S ' ^"^^ X-chromosome from the mother they should 



all be vermilion-eyed. The females from 



such a cross receive from the father an X-chromosome bearing the 

 dominant allelomorph of v, consequently they should all be red-eyed. 

 In the great majority of cases, this is the result actually obtained from 

 such matings, but occasionally, about once in 1700 individuals, an 

 exception, a vermilion female or a red male, is produced. The in- 

 vestigation of the "exceptional" females from such matings has pro- 

 vided unique evidence in support of the chromosome theory of heredity 

 and in regard to the relations existing between the sex chromosomes 

 and sex differentiation. 



The production of exceptional individuals from matings such as we 

 have considered above apparently results from occasional aberrant re- 

 duction divisions in the female such that the two ^-chromosomes fail 



