CHROMOSOMES AND GENES 



47 



somes, especially when suddenly an artificial situation is 

 created by increasing or decreasing the normal comple- 

 ment of chromosomes to which the machinery of develop- 

 ment is already adjusted. 



Norma 



Hapio- nZ" 



Fig. 29. 

 Normal and haplo-IV flies of Drosophila melanogaster. Their 

 respective chromosome groups are sho^vn above and to the right 

 of each. 



Probably the most complete and convincing evidence 

 concerning the significance of the chromosomes in hered- 

 ity has come from the recent genetic results that have to 

 do with the specific effects of changes in the number of 

 the chromosomes where each one carries genetic factors 

 that enable us to identify its presence. 



Recent evidence of this kind comes from the loss or 

 from the addition of one of the small fourth chromosomes 

 of Drosophila (chromosome-IV). It has been shown both 

 by genetic and cytological methods that chromosome-IV 

 is sometimes lost from one of the germ-cells — egg or 

 sperm. If an egg lacking this chromosome is fertilized by 



