MENDEL'S FIRST LAW 



29 



food and moisture become less and less, the bands become 

 more and more regular until at last the flies are indistin- 

 guishable from normal flies. If a cross is made between 

 a female with abnormal bands and a wild male, the off- 

 spring that first hatch under favorable conditions are all 

 very abnormal. Here abnormal completely dominates 

 normal bands. But as the culture dries up, the hybrid 

 offspring become more and more normal, until finally they 

 are all normal. At this time it might be said that normal 

 dominates abnormal. Both statements are correct, if we 

 add that in one environment abnormal banding dominates, 



Fig. 5. — Normal and abnormal abdomen of D. melanogaster. 



in another environment normal banding dominates. The 

 genetic behavior of the pairs of genes is the same here 

 as in all other cases of Mendelian behavior, but this is 

 revealed onlv when the environment is one in which the 

 abnormal gene produces one effect, the normal a different 

 one. That the gene is not itself affected by the environ- 

 ment can be shown very simply. If a female from the 

 abnormal stock be picked out, at a time when the stock 

 has onlv normal bands, and crossed to a wild male, the 

 offspring will all be as ' ' abnormal ' ' as when the mother 

 herself is abnormal, provided the food and moisture 

 conditions are of the right kind. The late hatched normal 

 flies of abnormal stock may be bred from for several 



