EFFECTS OF INBREEDING AND CROSSBREEDING. 5 



Record was made of the coat color patterns as a means of identification. This record 

 has later served as a means of noting certain family characteristics, which, in many 

 cases, were transmitted to succeeding generations of inbred stock with quite uniform 

 regularity. All animals of the foundation stock were between 5 and 6 months old 

 when mated. 



Twenty-four females of uniform size and conformation were selected and num- 

 bered consecutively from 1 to 24. The males were selected in a like manner and 

 numbered in another series beginning with 1. The number of each of these females 

 of the foundation stock was given to the family of guinea pigs which descended from 

 her. Generation 1 is the progeny of the foundation stock, and is, therefore, not 

 inbred. In order to follow the closest line of inbreeding, brother and sister of the 

 same litter were mated . In all cases the best individuals in the litter were selected 

 for breeders. Their progeny were selected and mated in a like manner. This method 

 is being continued, and at present (1913) individuals in a few of the families have 

 been inbred for 13 generations. All breeders in each of the families have been inbred 

 through 4 generations. 



In Family 4, parents were bred to their progeny: that is, sires were mated with 

 their daughters, granddaughters, etc., of each succeeding generation during the breed- 

 ing period of their lives. In a similar manner dams were mated with their sons, 

 grandsons, etc., of succeeding generations. 



In Test 2 of this experiment the animals of the foundation stock were unrelated to 

 each other. Some of the breeders were selected from the same general stock at the 

 Bureau of Animal Industry Experiment Station as were those in Test 1, but in all 

 cases they were mated with stock which was obtained from a different source. The 

 alien stock was apparently as healthy and vigorous as the other. The foundation 

 females used in this test were numbered consecutively from 31 to 42, inclusive. The 

 families were numbered in the same manner as those in Test 1. All animals were 

 housed, fed, and cared for in the same manner. Practically no change has been made 

 in the method of rearing our breeding stock since the experiment was started. The 

 methods which we use are those which have been followed successfully for the past 

 14 years by the Bureau of Animal Industry Experiment Station, where thousands of 

 guinea pigs are raised annually for laboratory purposes. 



It was found that more satisfactory results could be obtained by having one male 

 and only one female occupy each breeding cage, because frequently two females gave 

 birth to young at about. the same time, making it impossible to tell to which female 

 the young belonged. In all such cases these mixed litters were eliminated from the 

 experiment. Inafew instances young females became pregnant by their sires before 

 they were weaned., which was at the age of 33 days. Young from these matings were 

 also eliminated from the experiment. 



According to this report, 35 families were started, 24 wholly from 

 the line-bred stock of the Bureau of Animal Industry and 1 1 from a 

 cross between this stock and guinea pigs obtained from a local 

 dealer. All were carefully selected, for vigor, from large stocks. 

 In all but one family, matings were made exclusively between full 

 brothers and sisters. The data from Family 4, in which matings 

 were made between parent and offspring, have not yet been analyzed. 

 Seven of the remaining 23 families in Test 1 (from the line-bred 

 stock) went out of existence before the second generation was pro- 

 duced, for various reasons, such as the early death of the female or 

 the failure to produce living young of both sexes. Four of the 12 

 families in Test 2 failed for similar reasons, and another was disposed 



