EFFECTS OF INBREEDING AND CROSSBREEDING. 37 



n. DIFFERENTIATION AMONG INBRED FAMILIES. 



The first part of this bulletin gives a description of an inbreeding 

 experiment with guinea pigs begun in 1906 by the Bureau of Animal 

 Industry. An account is given of the origin of 23 families, descended 

 from 23 females and 9 males by matings exclusively of brother with 

 sister. It is shown that the inbred stock as a whole suffered a decline 

 in vigor in all characteristics studied, but most markedly in fertility. 

 This decline was not, however, a rapid one, the relatively high vigor 

 after more than a dozen generations of the closest inbreeding being, 

 perhaps, as noteworthy a result as the fact of a slow average decline. 

 In this part of the bulletin the different families will be considered 

 separately, in order to determine how far differentiation has taken 

 place among them and how far inbreeding has affected them alike. 



DIFFERENTIATION IN COLOR. 



A differentiation among the families in color has been obvious. 

 The original stock contained both intense and dilute agoutis, blacks 

 with red, yellow or cream spotting, reds, and albinos. All grades of 

 the piebald and tortoise-shell patterns were found. Each of the fami- 

 lies produced a variety of colors in the early generations. As time 

 went on, however, different colors automatically became fixed in 

 different lines, and as the families became more homogeneous through 

 the elimination of early branches from the main line of descent most 

 of them came to be characterized by a particular color. The mode 

 of inheritance of the main color varieties is thoroughly understood, 

 and as their automatic fixation through inbreeding is a well-known 

 consequence of their mode of inheritance, it will not be necessary to 

 go into the rather complex details. 



Of greater interest is the fixation of those color variations and 

 patterns whose heredity has not yet been analyzed as Mendehan and 

 which appear to follow a blending mode of inheritance. Among 

 these characters are the minor variations in intensity of color, and 

 the minor variations in the extent and localization of the piebald and 

 tortoise-shell patterns. Ajnong the minor differences in intensity 

 come interesting contrasts between Families 38 and 32 and between 

 Families 18 and 35. All four of these families have red or yellow 

 spotting. The red in Family 32 is always a remarkably intense 

 mahogany color, like that of no other guinea pigs which the writer 

 has seen. Family 38 has pale red or yellow, impossible to confound 

 with the red of Family 32, but rather similar to the pale red or yellow 

 t)f Family 18. The young of Family 18 are usually classified as light 

 red when born, appearing shghtly more intense than the young in 

 Family 38, but changing to a typical yeUow when older. Family 35 

 shows a typical yellow at all ages. When these 4 families are crossed 



