198 GENETICS IN RELATION TO AGRICULTURE 
mating, simplex woman (BX) (bX) by color-blind man (6X)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 PF; 
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 
Fia. 89.—The relations of the 
sex chromsomes to sex produc- 
tion and to the inheritance of 
the recessive sex-linked char- 
acter, vermilion eye color, in 
Drosophila. The straight chro- 
mosomes are the X-chromo- 
somes, and the crooked ones the 
Y-chromosomes. (Adapted 
from Bridges.) 
a consideration of Fig. 89, which is a diagram 
of the results of crosses between vermilion 
females and red males. The vermilion factor 
v is borne by the sex chromosomes, and since 
the males from crosses between vermilion 
females and red males receive their only 
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 X-chromosomes fail 
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