SPECIAL CASES OF SEX-INHERITANCE 223 



mosome that each red male contains is one of the two 

 chromosomes present in the original red-eyed grand- 

 mother. If this chromosome contains a factor which 

 if present causes the death of the male that contains it, 

 and this factor is closely linked to the red factor, 

 the results are explained. All the females escape the 

 fatality, because all females contain two sex chromo- 

 somes. If a female should contain the fatal factor, 

 her life is saved by the other, normal, sex chromosome. 

 The hypothesis has been tested in numerous ways and 

 has been verified. We keep this stock going by mat- 

 ing the red females to white males. This gives con- 

 tinually the 2 : 1 ratio. The white sisters, on the other 

 hand, are normal and give normal sex ratios. 



(3) Another aberrant result, discovered by Mr. 

 Bridges, is shown by a different race of these same fruit 

 flies. It will be recalled that when an ordinary white- 

 eyed female is bred to a red-eyed male all the sons have 

 white eyes. But in the race in question a different re- 

 sult follows, as shown by the diagram. From 90 to 

 95 per cent of the offspring are regular, but 5 per cent 

 of the females and 5 per cent of the males are uncon- 

 formable, yet persistently appear in this stock. 



The results can be explained if we suppose that the 

 two sex chromosomes in the egg sometimes stick to- 

 gether (Fig. 107). They will then either pass out into 

 one of the polar bodies, in whii^h case the red-eyed males 

 will develop if the egg is fertilized by a female-producing 

 sperm; or the two sex chromosomes will both stay in 

 the egg, and give a kind of female with three sex chro- 

 mosomes. 



Here also numerous tests can be made. They verify 



