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In this paper, McClintock and Creighton conclusively demonstrated the occurrence of genetic and cytological crossingover, a process assumed but not proven since 1910. In it they took two genetic markers and two cytological markers and demonstrated that when crossing-over occurred, they traveled together.
Periodical:
Proceedings of the National Academy of Sciences of the United States of America
Periodical. Creighton, Harriet B., and Barbara McClintock. "A Correlation of Cytological and Genetical Crossing-Over in Zea Mays." Proceedings of the National Academy of Sciences of the United States of America 17, (1931): 492-497. Article. 6 Images.. Proceedings of the National Academy of Sciences of the United States of America