ORIGIN OF MUTANT CHARACTERS 63 



spring showed the same character. A change in a gene in 

 one of the second chromosomes must have taken place, 

 either in the mother or father of the mutant. The germ- 

 cell containing this gene met a cell containing a normal 

 gene at the time of fertilization, and the first mutant 

 arose. The first indi^ddual was, therefore, hybrid or 



Fig. 41. 



The mutant character Curly of D. melanogaster. The wings curl 



up at the ends and are held somewhat apart. 



heterozygous, and, as stated above, produced, when mated 

 to a normal individual, both lobe^ and normal offspring 

 in equal numbers. From these heterozygous forms some 

 pure lobe^ flies were produced by mating two lobe- indi- 

 viduals. The pure type (homozygous for lobe^) resembles 

 the heterozygous type, but the eyes are often smaller, and 

 one or both may be absent. 



It is a curious fact that many dominant mutants are 

 lethal in homozygous conditions. Thus curly wing (Fig. 

 41), a dominant character, nearly always dies when homo- 



