﻿26 BULLETIN 1372, U. S. DEPAKTMENT OP AGRICULTURE 



better daughters on the whole are from the lower-producmg dams 

 and his lower-producmg daughters from the higher-producmg dams. 



In the list of a small number of daughters of a sire, the production 

 of the daughters may not follow very closely that of the dams, owing 

 perhaps to the sire's being more homozygous for the factors that 

 will govern high-producing ability than the dams with which he is 

 mated. But when a large number of daughters and dams are con- 

 sidered, the higher-producing daughters will as a rule be found to 

 have good dams. This is to be expected, for the high-producing 

 dam is certain to have at least a part, if not all, of her germinal 

 factors governing production, those that will determine high pro- 

 duction, and she will therefore transmit high production to a part 

 or all of her offspring. 



The evidence seems to point to both parents' contributing equally 

 to the inheritance governing the milk and butterfat producing 

 capacity of their daughters. But if one parent is homozygous or 

 pure for the hereditary factors determining high production and the 

 other parent is heterozygous in its inheritance, then the homozygous 

 parent will have the greater influence on the producing capacity of 

 the daughter; yet this daughter will transmit to a part of her progeny 

 the inheritance for low production that she may receive from her 

 heterozygous parent. From two heterozygous parents, it is to be 

 expected that the daughters will show a great range in producing 

 capacity, from very poor to very good. 



THE PERCENTAGE OF BUTTERFAT 



Koberts (3) found a significant negative correlation between the 

 percentage of butterfat and .the 3aeld of milk for Jerseys, Guernseys, 

 and Holsteins, but did not find so significant a correlation for Ayr- 

 shires. That is, as the yield of milk increased, the percentage of 

 butterfat in the milk decreased. Wilson (4) studying Ayrshire 

 records, concluded that the yield of milk and the percentage of 

 butterfat were independent of each other. Pearson {5) also found 

 a small but significant negative correlation between percentage of 

 butterfat and yield of milk. 



The material for the study by Roberts was made up largely of 

 advanced-register and register-of-merit records of the various breeds. 

 The animals in each breed were classed according to age, and the 

 correlation was between the milk yield and the percentage of butter- 

 fat for the group of animals in each class. The negative correlation 

 was significant for the Jerseys and Guernseys; but the coefficient 

 for the Holsteins, when judged by the probable error, was not sig- 

 nificant in any of the classes. 



Evidence on the question whether a buU which has the ability 

 to increase the milk yield of his daughters can also increase the per- 

 centage of butterfat in the milk, or whether if he increases the milk 

 yield he will decrease the percentage of butterfat (as is indicated in 

 the results obtained by Roberts with Jersey and Guernsey records), 

 is ofi^ered in Tables 14 and 15. These tables show that most of the 

 sires making the greatest increase in milk yield also increased the 

 average percentage of fat very materially, whereas' a few sires that 

 increased the average milk yield of their daughters decreased the 

 percentage of fat. Several sires whose daughters showed an average 

 decrease in milk yield also had a decrease in percentage of fat. In 

 those cases where the daughters showed the greatest decline in milk 



