MENDEL'S FIRST LAW 



35 



Marshall and Muller kept flies heterozygous for three re- 

 cessive mutant factor for about seventy-five generations, 

 and at the end of that time found that these factors had 

 not been weakened in any way as a result of juxtaposition 



Fig. 13. — Notch wings in the vinegar fly, extreme condition, a; average condition, 6; 



nearly normal condition, c. 



with their normal dominant allelomorphs. I have kept 

 a stock of notch-winged flies under selection for twenty- 

 five generations. Notch (Fig. 13) is a character varying 

 in the direction of normal wings (Fig. 13, c) ; in eveiy 

 generation of notch, many notch flies have normal wings. 

 The character is dominant, and exists only in heterozy- 



