IOWA ACADEMY OF SCIENCE 
197 
For a study of inheritance of fat production, as shown by the relation 
of the production of dams to that of their offspring, 3,700 pairs of 
variates were taken from the 1910-1911 Official Year book of the Ad- 
vanced Registry of the Holstein-Friesian Association. The mean fat 
production of the offspring was 16.952 ± .039, while that of the dams was 
15.971=^.034. The. standard deviation and co-efficient of variability of the 
offspring were also greater than those of the dams, showing the tendency 
of the individuals of the F t generation to reach the extremes of the 
parental generations. The correlation coefficient of .29 would, according 
to the statistical method of study of Biparental Inheritance, show evi- 
dence of prepotency on the part of the dams as opposed to the sires. 
This fact may indicate a sex-linkage of the factors controlling inheri- 
tance of fat production. 
A rearrangement of the data, used in the work just discussed, in 
classes representing three generations, shows the following coefficients of 
variability,' — parenta] generation, 21.686, F x generation 18.737, and F 2 
generation 21.824. This is typically Mendelian, although the fact that 
there is an artilcial selection which leaves the poorer producers out of the 
Advanced Registry and also out of the breeding herd, lowers the coeffi- 
cient. With as large numbers as are under consideration here the effect 
is probably equal in each generation. 
Any attempt to distinguish the unit of inheritance is somewhat futile, 
when one depends entirely on written records. An attempt was made 
to find such a unit however, and a dividing point, that separated into 
two classes was readily recognized. The breeding records of the grand- 
dams. classified into different groups with the pound as the unit, were 
tabulated and compared. For example the granddams producing 12 lbs., 
(the lowest production allowed in Advanced Registry for a mature ani- 
mal) were grouped together, their daughters forming the relative class, 
and their granddaughters the subject class. All the granddams of dif- 
ferent productions w T ere grouped in the same way. By inspection of the 
result, it was found that the granddams producing up to 21 lbs., bred 
qualitatively the same. At this point appeared a sharp line of demar- 
kation, above which the production in the granddaughters averaged about 
21 lbs., wffiile below the production was 17 lbs. The figures for these 
groups are as follows: 
