CHAPTER VII 



Chaeacteeistics of Pedigeees in the Diffeeent Beeeds 



inbeeeding 



The distinction between inbreeding and line breeding is so hazily 

 drawn that it is futile to consider them separately. The system 

 of measuring inbreeding which is presented is that of Pearl. Chapter 

 IV of Dr. Raymond Pearl's book on Modes of Research in Genetics 

 should be consulted for a discussion of the method of measuring 

 inbreeding before the student commences his own work. 



Inbreeding is noticed in any pedigree if within that pedigree there 

 are two or more ancestors which are identical. In other words the 

 different ancestors of an inbred animal are less in number than if 

 that animal were not inbred. A purely objective measure of in- 

 breeding is, then, the ratio between the number of different ancestors 

 in one generation to the total possible number of ancestors. This 

 is the measure used in this work. The pedigree of Rioter's Jersey 

 Lad illustrating its calculation is given on page 45. 



The animals marked with the solid circles are repeated previously 

 in the pedigree. Those marked with an open circle containing a 

 cross are ancestors of repeated animals. In this . pedigree, the 

 repeated animals are Ida's Rioter of St. Lambert, Bachelor of St. 

 Lambert, Ida of St. Lambert and Ida's Stoke Pogis. The genera- 

 tions are numbered Ai, A 2 , A 3 , A 4 . The coefficient of inbreeding 

 is represented by Z. There are always two ancestors in Ai, conse- 

 quently there is no inbreeding or Z = 0. In the second generation, 

 A 2 , there are four possible ancestors and all four are different so 

 there is no inbreeding, Zi = 0. In the third generation, A 3 , Ida's 

 Rioter of St. Lambert reappears, so there is inbreeding. The 

 amount of this inbreeding is equal to 1, the number of repeated 

 ancestors, divided by 8 the possible number of ancestors, or 

 12.5 per cent of the inbreeding. In the fourth generation, there are 

 5 animals repeated, namely, Bachelor of St. Lambert (twice) 



43 



