176 THE THEORY OF THE GENE 



When two triplo-IV flies (obtained in the way de- 

 scribed above) that have two ordinary IV-chromosomes 

 and another IV-chromosome carrying eyeless, are mated, 

 they give approximately 26 full-eyed flies to one eyeless. 

 From this cross some flies might be expected that con- 

 tained fonr chromosome-IV's, since half of the eggs and 

 half of the sperm-cells carry two of these chromosomes. 

 If such tetra-typic flies developed, the expected ratio 

 would be 35 full-eyed to one eyeless. The ratio found (26 

 to 1) instead of the expected ratio (on the assumption 

 that the tetra-typic flies come through) is due to the death 

 of the tetra-types. In fact, no flies of this composition 

 have been detected, which means that, despite the small- 

 ness of these chromosomes, the presence of four of them 

 upsets the balance of the genes to such an extent that 

 such an individual does not develop into an adult. 



In contrast to these triplo-types of Drosophila there is 

 another heteroploid type, the haplo-IV type (Fig. 29), in 

 which one of the small chromosomes is absent. This type 

 has appeared very often, which is interpreted to mean 

 that one of these small chromosomes is sometimes lost in 

 the germ track — possibly as a result of two passing to 

 one pole at the reduction division. The haplo-IV has a 

 paler body color but a more marked trident on the thorax, 

 rather large eyes with a rough surface, slender bristles, 

 and somewhat shortened wings, and the aristae are re- 

 duced or even absent. In all these respects its characters 

 are the opposite of those of the triplo-type. This is not at 

 all surprising if the IV-chromosome contains genes that 

 affect many parts of the body in conjunction with other 

 genes. These effects are increased by the presence of an 

 additional chromosome and diminished when one is ab- 

 sent. The haplo-IV 's emerge four or five days later than 

 the normals; they are often sterile and generally poor 

 producers; their mortality is very high. There is abun- 



