Exchanges of sister chromatids (mitotic crossing-over). Exchanges in cells of Allium cepa. The Harlequin-like look of the chromosomes is caused by treatment of the cells with bromine desoxyuridine (BrdU) during one period of replication. BrdU causes a drastic enhancement of exchanges. After this treatment, the chromatids have been exposed to a dye that stains BrdU-free chromatids strongly while BrdU substituted chromatids are only weakly stained. Every change in the colour-intensity of a chromatid corresponds to a crossing-over. (see also the analogous evidence of semiconservative DNA). (J. B. SCHVARTZMAN, F. CORTES, A. GONZALES-FERNANDEZ, C. GUTIERREZ and J. F. L”PEZ - SAEZ, 1979).