Fig. 26-6. Male and female fruit fly, Drosophila 

 melanogaster. (After Morgan. From Guyer, Animal 

 Biology. Permission of Harper and Row.) 



diminutive animal, Drosophila melanogaster, 

 commonly called the fruit fly (Fig. 26-6). 

 Morgan and his students found that Droso- 

 phila is ideally suited for experimental breed- 

 ing. At the age of 12 days these little flies 

 begin to breed; and by the end of three 

 weeks a single female can produce 300 off- 

 spring. Accordingly, within 3 years it is pos- 

 sible to study more than 60 generations of 

 Drosophila — or about as many generations 

 as have accrued to mankind during all the 

 Christian era. Furthermore, many specimens 

 of Drosophila can be housed in a single milk 

 bottle; and the food of Drosophila, which is 

 mainly yeasts, growing on fermenting ba- 



Heredity - 481 



nanas, is a negligible laboratory expense. Be- 

 cause of these advantages, experiments on 

 Drosophila have pioneered practically all 

 advances in modern genetics. However, the 

 laws of heredity, as worked out in Drosophila 

 and other lower forms, have proved to be 

 generally applicable to man and all other 

 higher organisms. 



Mendelian Inheritance: Segregation. The 

 law of segregation deals with the transmis- 

 sion of a single hereditary difference from 

 parent to offspring in successive generations. 

 Mendel worked out the law of segregation 

 by studying the inheritance of flower color; 

 but our first example will be chosen from a 

 special breed of chicken, the Andalusian foivl 

 (Fig. 26-7). 



Two stocks of Andalusians have been cul- 

 tivated for many years: one white and the 

 other black. If these stocks are inbred, white 

 to white, or black to black, no change of 

 feather color ever appears: white crossings 

 never (barring mutation) yield anything but 

 white offspring, and black crossings yield 

 only blacks. 



If a white hen (or rooster) is crossed to a 

 black mate, all the offspring display an inter- 

 mediate pigmentation — a sort of gray-blue. 

 But if any two of these offspring are inbred, 

 25 percent of the offspring are pure white, 

 like the one grandparent; 50 percent are blue, 

 like the immediate parents; and 25 percent 

 are pure black, like the other grandparent. 

 Or, to state these results more technically, 

 when the Y x (first parental) generation of 

 Andalusians are pure white and black re- 

 spectively, all individuals of the Fj (first 

 filial) generation will be blue; but the F 2 

 (second filial) generation, obtained by in- 

 breeding the Fj, consists of white, blue, and 

 black fowl in a ratio ef 1:2:1 (Fig. 26-7). 

 Furthermore, by inbreeding the different 

 kinds of F 2 individuals the same results are 

 obtained: whites always give only whites; 

 blacks give only blacks; a white and black 

 yield all blues; and a blue crossed with a blue 

 gives a 1:2:1 ratio of whites, blues, and 

 blacks. 



