238 



The Journal of Heredity 



those with 250 to 699 units. These 

 divisions were not wholly arbitrary 

 since the division points appeared to 

 be regions of somewhat lower fre- 

 quency in the whole distribution. We 

 then listed the results of all matings 

 within and between these groups, and 

 have arranged these data in Table 2. 



The cross of light by light was 

 found to give practically all light. The 

 two medium animals recorded were in 

 one case a very light medium (250 

 units of black) which might easily 

 have been placed in the "light" group 

 and in another case a medium (450 

 units) whose dam was a cow with 

 broken spotted areas distributed over 

 the whole surface of the coat. The 

 actual amount of black on this cow 

 was 200 units, left and 250 units right. 

 Because of the extended distribution 

 of spotting she might have been 

 placed in the medium class. We 

 should make no great misstatement in 

 saying- that the lighter grades of spot- 

 ting breed true to this condition. 



The cross of light and dark animals 

 produced principally animals of me- 

 dium grade with a few light and a few 

 dark offspring. On the hypothesis of 

 the partial dominance of dark the ap- 

 pearance of light animals would indi- 

 cate that some of the darks, including 

 the dark bull, were heterozygous. The 

 appearance of some dark offspring in- 

 dicates that the dominance of dark may 

 be nearly complete. Medium animals 

 bred together produced principally me- 

 dium calves with some evidence of the 

 segregation of light and dark animals. 

 Medium females bred to light males 

 produced a preponderance of light 

 calves and some mediums but no 

 darks, while the medium male bred 

 to light cows produced in addition two 

 dark calves. Medium by dark pro- 

 duced principally darks and mediums 

 with distinct evidence of segregation 

 of some lights. Dark by dark pro- 

 duced few calves of which four were 

 dark and one was mefliuni. A general 



survey of the individual matings 

 showed that with few exceptions the 

 calves produced were generally no 

 darker than the darker of the parents, 

 making the case somewhat analogous 

 to the inheritance of shades of eye 

 color in man. The exceptions were 

 the dark calves produced by the me- 

 dium bull when bred to both light and 

 medium cows. It is possible that the 

 medium bull was of a dark genotype, 

 although he was phenotypically me- 

 dium. Certainly he bred more like a 

 dark than a medium bull. In general 

 then it appears that the light animals 

 used were fairly pure for this condi- 

 tion, while both the medium and dark 

 bulls were heterozygous. The results 

 of reciprocal crosses were not always 

 similar, there being usually an excess 

 of dark animals in the offspring of the 

 dark and medium bulls ; the males ap- 

 ])eared more "prepotent" in transmit- 

 ting darkness. But with this small 

 sample of animals tested no conclu- 

 sions from this are warranted. The 

 chief result of the evidence consid- 

 ered in this way is the demonstration 

 of the segregation of the lighter from 

 the darker grades, without the reverse 

 phenomenon except in the case of the 

 phenotypically medium bull. 



Non-Genetic Variation 



We have no good evidence on the 

 amount of non-genetic variation in 

 these spotting patterns, but it probably 

 occurs to a degree. The best measure 

 of it would be the amount of variation 

 existing in identical twins ; and we 

 have been unable so far to assemble 

 indisputable cases of identical twinning. 



The recent data of Gowen^ and of 

 Lillie" indicate quite conclusively that 

 identical twinning in cattle is extreme- 

 ly rare, if it does occur. Lillie finds 

 in his Keller's and Tandler's cases 

 (p. 52) only one certain case of iden- 

 tical twinning out of 126 cases of 

 monosexual twins in which the num- 

 ber of corpora lutea in both ovaries 



*GowEN, J. W. Biological Bulletin 41: 1-6. 1922. 

 "Lillie, F. R. Ibid., 44:47-78. 1923. 



