HEREDITY AND VARIATION 



Trihybridization is the crossing of two individuals differing in three charac- 

 teristics. A third character that can be used in guinea pigs is length of hair, 

 short hair being dominant to long hair. When a black, short-haired, smooth- 

 coated guinea pig is crossed with a white, long-haired, rough-coated guinea 

 pig, all the offspring of the Fj generation are black, short-haired, and rough- 

 coated (Fig. 6.7). Eight different kinds of individuals (phenotypes) 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 sex of the parent having a particu- 

 lar characteristic has been of no significance. 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. Extensive work in experi- 

 mental breeding for the study of heredity and variation was first carried on 

 in this country by T. H. Morgan (Fig. 6.8) and his students, who used the 



27 black, short-haired, 

 rough -coated 



3 white, short-haired, 

 smooth-coated 



Black, short-haired, 

 smooth-coated 



White, long-haired, 

 rough-coated 



Fj : all black, short- 

 haired, rough-coated 



Interbred 



9 black, short-haired, 

 smooth-coated 



3 black, long-haired, 

 smooth -coated 



3 white, long-haired, 

 rough-coated 



1 white, long-haired, 

 smooth-coated 



Fig. 6.7. Results of trihybridization in guinea pigs which differ in color and length of hair 

 and quality of coat; black hair is dominant to white hair, short hair to long hair, and rough 

 coat to smooth coat. (Rearrangement of figures from W. E. Castle, Genetics and Eugenics, 

 copyright 1916 by Harvard University Press, printed by permission.) 



181 



