GAMETIC AND OBSERVED RATIOS IN DROSOPHILA 



DR. CALYIN B. BRIDGES 



The populations and families with which the geneticist deals 

 are not the real objects of his investigation; for him, the distri- 

 bution of characters is only an index of the preceding distribu- 

 tion of genes in gametes. But the whole course of embryonic 

 development, with heavy mortality possible at every step, has 

 intervened between the individuals that he classifies and the 

 gametes from which they came. The observed classes correspond 

 accurately to the original gametic series only in case this mor- 

 tality is indiscriminate — that is, only if there is no differential 

 viability. 



In the breeding work with Drosophila there has been a con- 

 tinual effort to eliminate distortion in the ratios, which depends 

 largely upon : (1) the extent of the mortality involved, this being 

 characteristic in amount for each mutant type and character 

 combination, (2) the suitability of the culture media and condi- 

 tions, and (3) the competition when the number of developing 

 individuals is in excess of the optimum number for the available 

 food supply. 



The problem of over-crowding (3) is simplest of solution, 

 though over-crowding was the largest source of disturbance in 

 most of the early work, as well as in some of the later. The rem- 

 edy is, in the first place, to limit the number of eggs per culture 

 to the output of a single female. No mass-cultures should be 

 raised in experiments in which the ratios among the offspring are 

 of importance. In the second place, as the larvae grow larger and 

 also increase in number with each day's output of eggs, the com- 

 petition becomes intensified throughout the later stages of the 

 culture. To meet this increasing demand, there must be fresh 

 supplies of food, or enough food niust be proxided at the start 

 so that even at the end there is sufficient for free development of 

 all larvie. In point of economy it is better to concentrate on a 

 few cultures that are liberally supplied than to raise a greater 



