CHAPTER V 



THE ORIGIN OF MUTANT CHARACTERS 



THE modern study of heredity has been intimately 

 bound up with the origin of new characters. In 

 fact, the study of Mendelian inheritance is possible 

 only when there are pairs of contrasted characters that 

 can be followed. Mendel found such contrasted characters 

 in the commercial stocks that he used, tall and short, yel- 

 low and green, round and wrinkled peas. Later work has 

 also extensively used such material, but some of the best 

 material is supplied by new types whose origin, in pedi- 

 gree cultures, is better known. 



These new characters arise for the most part suddenly, 

 fully equipped, and maintain their constancy in the same 

 way as do the characters in the original type from which 

 they arose. For example, the white-eyed mutant of Droso- 

 phila appeared in a culture as a single male. When mated 

 to a common red-eyed female, all the offspring had red 

 eyes (Fig. 38). These were inbred and produced in the 

 next generation red-eyed and white-eyed individuals. All 

 the white-eved individuals were males. 



These white-eved males were then mated to different 

 red-eyed females of the same generation. Some of the 

 pairs produced equal numbers of white-eyed and red- 

 eyed offspring, both males and females. When the white- 

 eyed individuals were bred together they gave rise to 

 pure white-eyed stock. 



We explain these results in accordance with Mendel's 

 first law, which postulates a red-producing and a white- 

 producing element (or gene) in the germinal material. 



