THE EFFECT OF LONG-CONTINUED HETEROZYGOSIS 



ON A VARIABLE CHARACTER IN 



DROSOPHILA 



WALTER W. MARSHALL AND HERMANN J. MULLER 



IHce Institute 



T"WO FIGUftES 



The belief that a factor may sometimes be contaminated by 

 its allelomorph when the two meet in the hybrid has been 

 upheld by Castle and by some geneticists of the non-Men- 

 dehan camp. On a priori grounds there is no reason why this 

 might not occur, but there is no evidence for arriving at such a 

 conclusion. The bulk of Mendelian inheritance seems to show 

 that factors are not affected by their allelomorphs. 



Bateson supposes that some cases of multiple factors are due 

 to fractionation and that the products of this 'quantitative disin- 

 tegration' segregate independently of one another. He explains 

 these supposed fractional degradations as due to irregularities 

 in the segregation of the factors in the germ cells, during cell 

 divisions in which he imagines the qualities to be sorted out 

 each to its place. In such a case a character might become 

 weaker and weaker as a result of continued 'crossing to other 

 stocks even though it originally differed from these stocks in 

 only a single factor. The aim of the experiment considered in 

 this paper was to contribute evidence in regard to the constancy 

 of factors in a state of heterozygosis; it was believed that the 

 apparent fluctuation in factors, which is thought by certain 

 workers to be contamination or some sort of quantitative disin- 

 tegration, can be accounted for on other and more satisfactory 

 grounds. 



The subject of the present investigation is a variable wing 

 character in Drosophila ampelophila which is called balloon, 

 and the factor for which lies in the second chromosome. 



457 



