JOm£ OF AGRICDLTDRAL ISEMCH 



DEPARTMENT OF AGRICULTURE 



Vol. IV Washington, D. C, September 15, 1915 No. 6 



DEGREE OF RESEMBLANCE OF PARENTS AND OFF- 

 SPRING WITH RESPECT TO BIRTH AS TWINS FOR 

 REGISTERED SHROPSHIRE SHEEP 



By H. L. RiETZ, Professor of Mathematical Statistics and Statistician, and ElmER 

 Roberts, Instructor and First Assistant in Genetics, Agricultural Experiment Sta- 

 tion, University of Illinois 



INTRODUCTION 



In 1 906 Rommel and Phillips ^ reported an investigation into the 

 inheritance of the size of litter in the female line in Poland China sows 

 based on data of the American Poland China Record. In 1907 there 

 was published a statistical analysis of data from the records of Weldon ^ 

 on heredity in the size of litters in mice. 



The present investigation resembles somewhat the above investigations 

 in that it is concerned with the question of the likeness of animals and 

 their offspring with respect to being born as singles, twins, or triplets. 

 We are concerned with the question whether and to what extent the 

 offspring of parents born in twins and of grandparents born in twins are 

 more likely to be twins than if these ancestors are born as singles. 



Stated in another way, when we know that certain animals of a given 

 breed or class and certain of their ancestors are born as twins or triplets, 

 is there greater probability that the offspring will be twins or triplets than 

 if the animals were born as singles ? 



Our problem may be made clearer by asking a question that covers 

 only part of the problem: Does the subclass of sheep of a certain breed 

 which consists of those born as twins tend to beget twins in larger pro- 

 portion than the subclass that consists of those born as singles ? 



In the Journal of the Royal Agricultural Society of England ^ there 

 appears a statement of some of the possible causes that are favorable 

 to the production of twins. It is there stated that " there is some reason 

 to believe that twin lambs produce more twins than single lambs, and 

 that the influence of heredity is brought to bear." The main purpose 



' Rommel, G. M., and Phillips, E. F. Inheritance ia the female line of size of litter in Poland China 

 sows. In Biometrika, v. 5, pt. 1/2, p. 203-205. 1906. 

 2 [Weldon, W. F. R.] On heredity in mice from the records of the late W. F. R. Weldon. In Biometrika, 



V. 5. pt- 4, P- 436-449- 1907- 



' Heape, Walter. Abortion, barrenness, and fertility in sheep; an abstract of records obtained for the 

 year 1896-97. In Jour. Roy. Agr. Soc. England, s. 3, v. 10, pt. 2, p. 236. 1899. 



Journal of Agricultural Research. Vol. IV, No. 6 



Dept. of Agriculture, Washington, D. C. Sept. 15. 1915 



111.— 2 

 (479) 



