HETEROPLOIDS 175 



stability should be attained, the more important factor 

 of gene balancing may make it improbable that a per- 

 manent increase in the chromosome pairs could be estab- 

 lished in this way. When a large chromosome number is 

 present the initial stages of unbalancing might be slight 

 as compared with forms having fewer chromosomes, be- 

 cause in the former the ratio would be less disturbed. 



In Drosophila Bridges found a triplo-type for the 

 small IV-chromosome, and since three genetic factors 

 are present in this small chromosome it has also been 

 possible to study not only the characters that are affected 

 by the presence of an additional IV-chromosome, but the 

 bearing of this condition on genetic questions in general. 

 On the other hand, it has been found that an individual 

 with three X-chromosomes usually dies, and that indi- 

 viduals with either chromosome-II or -III in triplicate do 

 not live. 



The triplo-IV Drosophila is not strikingly different 

 from the normal, and the two can be distinguished only 

 with difficulty. The general color of the body is a little 

 darker and the trident marking on the thorax is absent 

 (Fig. 32) ; the eyes are somewhat smaller and have a 

 smooth surface ; the wings are narrower and more 

 pointed than those of the mid type. That these slight 

 effects are due to the presence of an additional small 

 chromosome was shown both by a cytological demonstra- 

 tion of its presence (Fig. 32) and by genetic tests. When 

 a triplo-IV is crossed to eyeless (eyeless is a IV-chromo- 

 some recessive mutant type) some of the offspring (FJ 

 can be distinguished by the characters given above as 

 triplo-IV flies. If these are back-crossed to eyeless (Fig. 

 33), flies with full eyes and flies with "eyeless eyes" are 

 produced approximately in the ratio of 5 to 1. As sho^vn 

 in Fig. 33 this result agrees with expectation provided 

 that one normal gene is dominant to two eyeless genes. 



