574 GENETICS IN RELATION TO AGRICULTURE 



truly a remarkable demonstration of the relation of prepotency to the 

 development of speed in the American Standard bred. But even more 

 remarkable is the record of the ten greatest producers of speed up to that 

 time. They are given in Table LXIV. Of the ten great sires here listed, 

 one is by Hambletonian 10, eight have Hambletonian 10 for grandsire, 

 and finally Nutwood, the greatest in the list, is by Belmont, by Adallah, 

 by Hambletonian 10. Every one of the ten premier stallions of the breed, 

 therefore, belongs to the great Hambletonian family. 



Further evidence as to the existence of prepotency has been given 

 from time to time for many other breeds and for different characters. 

 A typical investigation of this kind has been conducted by Hover for 

 butter-fat production of pure-bred Guernsey cattle. From the advanced 

 registry records for this breed up to December, 1915, Hover found that 

 only thirty-two sires had produced three or more daughters having records 

 equivalent to 600 pounds of butter fat at maturity. Of these thirty- 

 two sires only three had produced more than ten such daughters, and 

 all of these belong to the May Rose family. This same family contains 

 six more of the thirty-two superior sires. Of the rest the Masher family 

 contains seven; the Governor of the Chene family, five; the Glen wood 

 family, five; and the Sheet Anchor family, six. Some of the sires of 

 course belonged to two or more of these families. The results are in no 

 particular different from those which might be obtained with any other 

 dairy breed. 



A demonstration of the existence of prepotency, however, is far from 

 a scientific treatment of the subject. While many geneticists admit that 

 prepotency is as yet an unsolved problem, they have not failed to point 

 out several ways in which prepotency might operate. These suggestions 

 have pointed to the relations of dominance and recessiveness, to variations 

 in the potency of factors themselves, and to interrelations within the 

 hereditary complex as providing firm bases for the existence and in- 

 terpretation of prepotency. We shall discuss each of these briefly below. 



The Mendelian Interpretation. That interpretation of prepotency 

 which refers it solely to the particular characters and the relations of 

 dominance and recessiveness within them may be called the Mendelian 

 interpretation. The simplest expression of this interpretation is found 

 in the relation of homozygous dominants to those which are heterozygous. 

 These two classes are often indistinguishable phenotypically, but the 

 homozygous dominant when mated to recessives impresses its characters 

 on all the offspring, whereas the heterozygous dominant only impresses its 

 characters on half the offspring. The practical bearing of prepotency 

 of this kind may be seen by reference to Pearl's analysis of the inheritance 

 of fecundity in domestic fowls. Here a Barred Plymouth Rock male 

 of the genetic constitution (ZM~)(ZM)LL will transmit high laying 



