220 Maine Agricultural Experiment Station. 1920. 



It is of interest to examine the results of these experiments 

 on butter-fat percentage in the light of those for milk yield. It 

 will be remembered that in the Fi crossbreds milk yield was 

 intermediate between that of the high and low parents but ap- 

 proached most nearly that of the high parent. In the genetics 

 of many economic characters as yield of grain, size of the ani- 

 mal etc. the explanation used to account for such a phenomena 

 is the heterozygous nature of the factors contained in the F! 

 animal as compared to the homozygous nature of the factors in 

 the parental breeds or strains. Without question there may be 

 something to this hypothesis for certain crosses. The results 

 for milk yield and butter- fat percentage do present a paradoxical 

 position when this hypothesis is applied to them. Thus milk 

 yield is increased over what the true intermediate should be. 

 This follows the expectation generally agreed upon and ac- 

 counted for by heterosis. But on these identically same animals 

 the butter-fat percentage is decreased below the intermediate. 

 This is not the expectation generally considered as due to hetero- 

 sis although it is by no means impossible to assume that in- 

 creased vigor may reduce rather than increase a character. The 

 double nature of such a position does not appeal to the author, 

 however, as furnishing more than a verbal explanation of the 

 results having little parallel in the rest of genetics. The explan- 

 ation which really seems most likely is that we have in these 

 two cases the resultant of partially dominant factors. Numer- 

 ous similar cases can be cited in genetics literature. Perhaps the 

 best known case is that of black in Drosophila where the factor 

 for this is normally classified as a recessive but where if occasion 

 demands it may be used as a dominant ; such a factor differs 

 quite distinctly from another like speck which is consistently 

 recessive. Such a parallel will explain the inheritance for but- 

 ter-fat percentage by considering that the factors for low butter- 

 fat percentage display more dominance in their expression than 

 do the factors for high butter-fat percentage. 



The inheritance of butter- fat percentage has occupied a 

 prominent place in the discussions of breeding operations by 

 practical dairymen. These men have held the following views 

 as to the mode of this inheritance. The first has claimed that 

 the tendency for high or low butter-fat percentage is trans- 

 mitted by the sire to his offspring ; the second that the dam 



