No. 592] THE MECHANISM OF CROSSING-OVER 195 



same eggs as a particular sex-linked factor. But in 

 Bridges 's cases of non-disjunction, the maturation divi- 

 sions are often abnormal, so that some eggs are found to 

 have retained both X's; and in accordance with this it is 

 found that some of the offspring of such females have like- 

 wise received two sets of maternal sex-linked factors. 

 These cases, therefore, show that in the female also the 

 sex-linked factors "follow" the X-chromosome (1, 2). 



Morgan next studied the relation of different sex-linked 

 factors to each other in inheritance, and then another re- 

 markable fact came to light. Theoretically, the dihybrid 

 females resulting from a cross of a red-eyed fly having 

 rudimentary wings by a white-eyed fly having long wings 

 (both of these pairs of characters are sex-linked) should 

 have contained in one of their two X-chromosomes the 

 factors " red " and ' ' rudimentary, ' ' and in the homologous 

 X-chromosome the factors "white" and "long"; the ma- 

 ture eggs should retain either one X or the other and 

 should therefore have contained either red and rudimen- 

 tary or white and long. In other words, red and rudi- 

 mentary should be completely linked in their inheritance, 

 and similarly white and long. But the results showed that 

 these factors sometimes separate in heredity, for not only 

 the above types of offspring are produced, but also some 

 red longs and white rudimentaries (7) ; m fact, about 4- 

 per cent, of the offspring belong to one or the other of the 

 two latter classes. If we admit that white and long were 

 originally present in the same chromosome, the onin \\a> 

 to account for this separation of the factors is to suppose 

 that in some of the cells of the hybrid female the VHm>- 

 mosomes interchanged parts before being distributed to 

 the eggs. For if the factor for "long" of the chromosome 

 containing "white" and "long" should somehow change 

 Places with the "rudimentary" of the homologous chro- 

 mosome, then when homologous chromosomes are sep- 

 arated at the maturation division, the egg may come to 

 contain either an X-chromosome with white and rudimen- 

 tary or an X with red and long. 



