300 Maine Agricultural Experiment Station. 1919. 



The relative merits of conformation as a guide to the milk 

 producing capacity of a cow and a short time milk record are 

 considered. The results show that a seven day test has a cor- 

 relation coefficient with the year milk yield of the cow of ap- 

 proximately 2^2 times that of the conformation or any part of 

 the conformation. The short test consequently is superior to 

 the conformation as a guide to milk production. 



A BIOMETRICAL STUDY OF CROSSING OVER. ON 



THE MECHANISM OF CROSSING OVER IN THE 



THIRD CHROMOSOME OF DROSOPHILA 



MELANOGASTER* 



This paper presents an analysis of the normal fluctuating 

 variations in crossing over from the viewpoint of the theories 

 advanced to account for crossing over. The means, standard 

 deviations and coefficients of variation show crossing over to 

 be one of the most highly variable phenomena known. Conse- 

 quently the mechanism behind crossing over works with less 

 precision than the mechanism behind most other physiological 

 phenomena. Resolution of the single crossing over ratios into 

 their component elements shows that there is a significant cor- 

 relation between crossing over of the different regions into 

 which the chromosome is divided by the factors used. The 

 correlation between the single and double crossing over ratios 

 shows that a crossing over in one region is more likely to be 

 accompanied by another simultaneous crossing over in a region 

 25 to 35 units away than it is to be accompanied by a simultane- 

 ous crossing over in any other region. Thus the left hand region 

 (1) correlated with double crossing overs including one, gave 

 correlations, proceeding from left to right, +0.30541+1.0395, 

 -j-0.5170i.0319 and - T -0- 2 997±-°396\ This rise and fall, to- 

 gether with a definite mode, is held to mean that there is a 

 model interval between two successive crossing overs. Thus 

 the two finely spun out chromosomes, when they come together 

 prior to crossing over, apparently twist about each other loosely 

 and generally have the points of contact where breaking may 

 take place about 25 to 30 units apart. 



*This is an abstract from a paper by John W. Gowen having the 

 same title and published in Genetics, Vol. 4, No. 3, pp. 205-250. 



