sa OF 
om .5 DEPARTMENT OF AGRICULTURE 
in? 
Washington, D. C. PROFESSIONAL PAPER December, 1922 
THE EFFECTS OF INBREEDING AND CROSS- 
BREEDING ON GUINEA PIGS.’ 
Ill. CROSSES BETWEEN HIGHLY INBRED FAMILIES. 
By Sewatt Wricut, Senior Animal Husbandman in Animal Genetics, Animal Hus- 
bandry Division, Bureau of Animal Industry. 
CONTENTS. 
Page. Page. 
_) SLUDGE DSS a ee eee ih) Adal wept — sb ce ee. ee ee eee 24 
menecowstnwricel triste. 5 tS a FS 2 ty #requency of litters?22% 272 AL) 2 ee 25 
Crossbreeding experiments. ............:.... Dil ASCO mMmAtuviy . 25255226 25-55 588. 72 42-beee 27 
EE CW in he Sei OOMIDICLE SECRINEN A 5 ee pee eee eee eee 28 
Allowance for seasonal fluctuations.......... a 12s OPER 6 oe eee 255 ee. I oe 28 
Allowance for heredity - -.......:............ 102}, Petal fertility oss: 2: seperes - -ceg3 ceeb es 30 
Allowance for age of dam.................... 1 A SCOB PONG 5 cine so oeeet Be oes See 32 
(oT EP go ae ee a SS hh ae 12 | Resistance to tuberculosis.................-. 32 
Significance of differences...................- 13 | Early views on inbreeding................... 33 
0 See ee ae meee 15 | Pre-Mendelian experiments.................. 34 
The percentage born alive. ..:............... 16 | Mendelian heredity and the problem of in- 
The percentage raised among the young born renting. <2 os2s2e, 2b 22 ereeee ict. 35 
0 LS aS ee ey ee 18 | Mathematical considerations................. 39 
The percentage raised of all young born......- 20 | Comparison of results with theory........... 46 
(US SO seth ah ee See 21 | Summary and general conclusions........... 48 
0 ST ee Se se ee eS 22 Vaberatiite G00 5 og. 25s s Face ntsc nest aasece 59 
Weight at weaning.......................... 23 | 
INTRODUCTION. 
It has been shown in Part I of this series of papers? that con- 
tinuous mating of brothers with sisters in a stock of guinea pigs has 
been accompanied by a decline in all of the elements of vigor which 
have been studied. These include the percentage born alive, the 
percentage raised to weaning (33 days), birth weight, rate of gain to 
weaning, adult weight, size of litter, and number of litters produced 
per year. Just how much of the decline in these respects was due to 
inbreeding and how much to environmental conditions was not 
wholly certain. It was shown, however, that the inbred stock had 
1 The two preceding parts of these studies have been published as Bulletin 1090, U. S. Department of 
Agriculture. 
6448—22—Bull. 11211 1 
