authors' abstract of this paper issmoD 
BY THE BIBLIOGRAPHIC SERVICE, DECEMBER 6 
STUDIES ON CROSSING OVER 
I. THE EFFECT OF SELECTION ON CROSSOVER VALUES^ 
J. A DETLEFSEN AND E. ROBERTS^ 
TWO TEXT FIGURES 
CONTENTS 
Introduction 333 
Materials and methods 335 
The data 338 
Series A; low selection 338 
Series A'; low selection; derived from series A 341 
Series B ; low selection 343 
Series C ; high selection 348 
Discussion and summary 349 
Literature cited 354 
INTRODUCTION 
The experiments described in this paper were undertaken in 
an effort to answer the question: can the percentage of crossing 
over be modified by selection. The significance of an answer 
(either affirmative or negative) and its relation to our present 
concepts on crossing over, locus, chromosome mapping, etc., 
were apparent and seemed to justify the time and attention 
necessary to carry these investigations over so long a period. 
They were planned and begun in February, 1916, and have 
involved the classification of over 300,000 individuals. Had 
the results been negative, the experiment would have been 
1 Paper no. 12, from the Laboratory of Genetics, Illinois Agricultural Experi- 
ment Station. 
2 We wish to give credit to the following graduate students for aid in these 
investigations: Mr. A. T. Fishman carried series A for three generations; Mr. L. 
E. Thorne carried series C for seven generations. The war called both men 
from their work. The late Prof. B. O. Severson carried series B from the begin- 
ning to the Fj4 generation. In the death of Professor Severson both genetics 
and scientific agriculture have lost a capable and enthusiastic student and 
investigator. 
333 
