382 INTRODUCTION TO CYTOLOGY 



sex-linked character, precisely like white eyes in Drosophila. It occurs 

 in a woman only if both of her X-chromosomes bear factors for it, 

 which means that such a factor must have been received from each 

 parent ; whereas one such factor is sufficient to produce colorblindness in 

 a man, because his F-chromosome carries no factors which might domi- 

 nate it. Furthermore, since the X-chromosome of the male is always 

 derived from the mother, a man can inherit colorblindness from his 

 mother but not from his father. From these facts it follows that a 

 colorblind woman transmits the defect to all of her sons and to half of 

 her grandsons and granddaughters; whereas a colorblind man transmits 

 the defect to none of his children and only to one-half of his grandsons. 1 



The first sex-linked character known in plants was that of narrow 

 rosette leaves in the red campion, Lychnis dioica (Shull 1910, 1911), a 

 plant which appears to have the XY type of sex inheritance, but in which 

 no distinguishable sex-chromosomes have been identified. 



Non-disjunction. — The chromosome interpretation of sex and of 

 sex-linkage has received an interesting confirmation in a phenomenon 

 discovered by Bridges (1913). In a certain strain of Drosophila the 

 white-eyed females were observed to give rise to a certain proportion of 

 " unexpected " forms. Most of their offspring (92 per cent) were red-eyed 

 females and white-eyed males, as expected in such an experiment, but 

 some of them (8 per cent) were white-eyed females and red-eyed males. 

 A long series of crosses showed that these results could be accounted for 

 if it were assumed that in the original white-eyed female both of the X- 

 chromosomes passed together to one pole in the reduction division instead 

 of separating as they should. This was termed non-disjunction (Fig. 147). 

 As a result the eggs, instead of having the normal single X-chromosome, 

 would have either two or none, and the distribution of the sexes and the 

 sex-linked characters would be altered in the manner observed. In a 

 cytological examination of the flies in which these abnormal phenomena 

 appear Bridges showed that this non-disjunction of the X-chromosomes 

 does occur occasionally in the female (Fig. 148). The chromosome 

 theory thus received confirmation. "An abnormal distribution of the 

 sex-chromosomes goes hand in hand with an abnormal distribution of all 

 sex-linked factors" (Morgan). Additional genetic and cytological data 

 have since been furnished by Bridges (1916) and Safir (1920). 



Linkage Groups. — An extensive series of studies on linkage relations 

 in various plants and animals has brought out the fact that the Mendel ian 

 characters of a given organism fall into a certain number of "linkage 

 groups," the members of each group being linked to one another in 

 various degrees but showing no linkage with the members of other groups. 

 It appears further, when the relationships of enough characters have 

 been worked out, that the number of linkage groups is the same as that 



1 This case is fully explained by Babcock and Clausen (1918, p. 197). 



