Crossing over and Gene Rearrangement in Flowering Plants.

K i n d s of gene rearrangement “ Crossing over” exists apparently in all those flowering plants which have been sufficiently investigated with regard to it. (By itself, it does not rearrange different genes, but only exchanges alleles.) Genetically i t consists in exchange of normally equal and homologous segments of a gene string (MORGAN 1911, STURTEVANT 1915, MULLER 1916). It has been proved, by the microscope, to consist in the exchange, a t meiosis, of normally equal portions of homologous chromonemas (CREIGHTON and MCCLINTOCK 1931, STERN 1931). It may thus be termed “homologous exchange”. Since the process is a normal one, the mechanism is apparently so perfect that no genes are lost in the exchange. “Reversed crossing over” consists in the exchange of different arms of homologous chromosomes, apparently at the fusal (spindle) constriction. It has been proved to occur at the origin of the secondary mutants of Datura, usually in trisomics, and probably a t meiosis. “ Reciprocal translocation” (heterologous interchange) is much rarer than crossing over. It consists in the mutual substitution of usually unequal segments of heterologous chromosomes. It has been proved to occur cytologically as well as genetically (see BELLING 1927, STURTEVANT and DOBZHANSKY 1930, DOBZHANSKY and STURTEVANT 1931, MCCLINTOCK 1930). It seems to originate a t both mitosis and meiosis. It may be caused by X-rays. Apparently genes may sometimes be lost in the process. “Translocation” shows a segment of one chromosome terminally attached, usually to a heterologous chromosome (see especially PAINTER and MULLER 1929). “Inversion” consists in a portion of the gene string or chromonema being turned round, end for end; genes being apparently sometimes lost in the process (see especially STURTEVANT 1931, and h4CCLINTOCK 1931). “Deletion” removes a non-terminal portion of the gene string, the gap closing up (see especially PAINTER and MULLER 1929). The portion removed may form a ring (MCCLINTOCK 1932).