470 GENETICS 



acters. When Fi hybrids are crossed, four kinds of offspring result 

 in the F2 generation, in the following ratio : 9 black-haired, rough- 

 coated: 3 white-haired, rough-coated: 3 black-haired, smooth- 

 coated: 1 white-haired, smooth-coated. Mendel's original work 

 on di-hybrids consisted in crossing wrinkled, green peas with 

 smooth, yellov/ peas. The offspring of such a hybridization are 

 smooth, yellow peas, which indicate the dominant characteristics. 

 In the F2 generation, smooth, yellow peas; smooth, green peas; 

 wrinkled, yellow peas; and wrinkled, green peas occur, in the 

 ratio of 9 : 3 : 3 : 1. 



Tri-hyhridization is the crossing of individuals with three differ- 

 ing characteristics. A third character that can be used in guinea 

 pigs is length of hair, short hair being dommant to long hair. 

 When a black, short-haired, smooth-coated guinea pig is crossed 

 with a white, long-haired, rough-coated guinea pig, ail the offspring 

 of the Fi generation are black, short-haired, and rough-coated 

 (Fig. 248). Eight different kinds of individuals are produced in 

 the F2 generation, in the ratio indicated: 27 black, short-haired, 

 rough-coated: 9 black, short-haired, smooth-coated: 9 white, 

 short-haired, rough-coated: 9 black, long-haired, rough-coated: 

 3 white, short-haired, smooth-coated: 3 black, long-haired, 

 smooth-coated: 3 white, long-haired, rough-coated: 1 white, long- 

 haired, smooth-coated. 



In the crosses previously considered, the question of the sex of the 

 parent having a particular characteristic has been of no impor- 

 tance. There are, however, cases of sex-linked inheritance in which 

 the sex of the parent that possesses a certain character modifies its 

 distribution in the offspring. Much of the most important experi- 

 mental breeding has been carried on by Professor Morgan (Fig. 

 249) and his students at Columbia University, with the fruit-fly, 

 Drosophila. An example of sex-Unked inheritance may be selected 

 from the abundant data concerning heredity in this small insect. 

 Red eye-color is dominant to white eye-color in Drosophila (Fig. 

 250). When a red-eyed female is crossed with a white-eyed male, 

 the offspring, both male and female, of the Fi generation are red- 

 eyed. If such red-eyed individuals are intercrossed, all of the 

 females and one-half of the males of the F2 generation will have red 

 eyes, while one-half of the males will have white eyes. The recip- 

 rocal cross between a white-eyed female and a red-eyed male gives 

 very different results (Fig. 251). The males of the Fi generation 



