NON-DISJUNCTION 23 



The science of genetics owes more to the fortuitous selec- 

 tion by T. H. Morgan and his colleagues at Columbia 

 University in the early days of this century of Drosophila 

 melanogaster as an experimental material than to any other 

 event that has occurred during its development. It so hap- 

 pened that this small fly possessed every possible attribute 

 that the geneticist could wish to find. It thrives under 

 laboratory conditions, it multiplies rapidly and it con- 

 tinually threw up mutant forms at a time when the geneticist 

 was seeking new phenotypes to explore. When the chromo- 

 some complex came to be recognized as the mechanism 

 involved in the transmission of the hereditary char- 

 acters, it was found that in respect of chromosome 

 number Drosophila possessed but four pairs, easily dis- 

 tinguished one from the other and that the X and Y of the 

 male differed markedly, so that cytologically the fly was as 

 excellent a material as it had proved to be genetically. Then 

 when cytological inquiry came to overtop the purely 

 genetical in importance it was found that in the fly's salivary 

 glands the chromosomes existed in a giant form. To a very 

 large extent the choice of Drosophila at that time deter- 

 mined the rate and the direction of the development of 

 genetics thereafter; it also affected profoundly the develop- 

 ment of the whole range of the biological sciences to give to 

 the Theory of the Gene an importance not less than that of 

 Evolution Theory itself. 



An early mutation in Drosophila was the recessive sex- 

 linked white-eye character. It was quickly explained as 

 indicated below: 



Wild type X Mutant 



red eyed o white eyed ? 



(WX)Y (wX)(wX) P.I 



(WX) Y : (wX) gametes 



(WX)(wX) (wX)Y F.I 



red-eyed white-eyed 



daughters sons 



But to this rule there were exceptions, a number of 

 white-eyed daughters and of red-eyed sons making their 



