An Experimental and Theoretical Study of Chromatid Crossing over.

The term “crossing over” is used to denote the exchange of pieces or segments between homologous chromosomes. There are many facts which indicate that the exchange of parts or segments occurs during the first meiotic prophase when the two homologous chromosomes are in intimate association. Prior to 1916 crossing over was thought to take place between the two paired chromosomes before they had divided equationally. BRIDGES, however, in that year, from his study of non-disjunction of the X chromosome of Drosophila melanogaster came to the conclusion that each chromosome was split equationally when crossing over occurred. Later L. V. MORGAN (1925), ANDERSON (1925) and BRIDGES and ANDERSON (1925) in a beautiful series of eqeriments substantiated BRIDGES’ earlier conclusion ; and ANDERSON (1925) and BRIDGES and ANDERSON (1925) further demonstrated that only two of the four strands crossed over at any one level. This was true for both diploids and triploids. REDFIELD (1930, 1932) working with triploid Drosophila found that the I1 and I11 chromosomes likewise crossed over when each chromosome was split equationally. ANDERSON used the phrase “four strand crossing over” to denote the divided condition of the chromosomes at the time crossing over took place in diploids. This term should not be applied to triploids, since it infers diploidy, even though only two of the three chromosomes (four chromatids) are involved in any one point of crossing over. The terms “double strand” or “chromatid crossing over” could be used for both diploids and polyploids since they refer only to the presence of the equational split. In addition to the chromosomes of D . melanogaster, the X chromosomes of D. simulans (STURTEVANT, 1929) and D. virilis (DEMEREC, unpublished) have been genetically proved to cross over at a double strand stage. RHOADES (1932) presented data which proved that the pr-vz chromosome in Zea, the fifth largest in the monoploid complement, crossed over after, or at the time, the equational split occurred. WHITING and GILMORE (1932) reached a similar conclusion for one chromosome of Habrobracon in their study of impaternate daughters from virgin females. As far as the writer is aware, these are the only cases where chromatid or double strand crossing