302 



Cell as Basis of Organic Activity 



Those genes affecting a single character are located in the same 

 position or locus in homologous chromosomes and are known as alleles 

 or allelomorphs. The exact nature of the gene is still a subject of much 

 speculation and study by biologists. It is believed that the gene is a 

 complex protein molecule which is capable of retaining its identity and 

 duplicating itself during cell division. 



Drosophila as an Experimental Animal. — Much of the cytolog- 

 ical work and final confirmation of observed genetic results has been 

 done with the fruit fly, Drosophila melanogaster. 



Fig. 92. — The fruit fly, Drosophila melanogaster. 



In order to obtain accurate results from various crosses, a large 

 number of individuals must be counted and tabulated. For this reason, 

 large, slow-breeding animals or plants are hardly suitable. It was the 

 famous geneticist, T. H. Morgan, who, with his associates, first worked 

 with the now famous fruit fly. This small fly has many advantages for 

 these studies. First it is small and easy to raise. It produces large 

 numbers of progeny in a very short time. It would almost appear 

 that the fruit fly was in some way specifically designed to aid geneticists. 

 Not only does it have the already mentioned advantages, but in addition 

 it has but few chromosomes (four pairs) and many of its characters 

 are easily seen. (Fig. 103.) 



