THE THEORY OF MULTIPLE-STRAND CROSSING OVER

Abstract

AC CORDING to the theory of the chiasmatype as originally formu- lated by JANSSENS (1909), interchange between homologous chromosomes takes place when each is already split longitudinally, but at any level only two of the four strands exchange parts. It was pointed out by MULLER (1916) and BRIDGES (1916) that the theory could be demonstrated genetically if it could be shown that in eggs which have retained two maternal strands, one strand may be a crossover and the other a non-crossover, or both crossovers but not at the same level. Such individuals were obtained by MULLER and BRIDGES; and although in these cases there was, as MULLER pointed out, the possibility that the extra strands had arisen by non-disjunction prior to maturation, the correctness of JANSSENS’S theory has since been demonstrated by the regular occurrence of such individuals in races with attached X’s or high non-disjunction and in triploids (ANDERSON19 25b, BRIDGESA ND ANDERSON19 25, L. V. MORGAN19 25, REDFIELD1 930, STURTEVAN19T3 1). If crossing over occurred at a two-strand stage, each chromatid would be identical with one of the other chromatids of the tetrad and the complement of the remaining two; hence the enumeration of the strands and the determination of how they are combined in the tetrad would be a simple matter. In four-strand crossing over, the strands recovered are presumably still a random sample of all the strands; but there are two complications. (1) Each of the missing strands is not necessarily either identical with or the complement of the strand recovered. (2) There is the possibility that crossing over may have occurred between sister strands; and this would not be directly detectable. A complete theory of crossing over must take into account the missing strands in each tetrad and the unrecognizable crossings over. At first This investigation was aided by a grant from the Bache Fund of the National Academy of Sciences. GENETICS 21: 155 May 1936 156 ALEXANDER WEINSTEIN sight this may seem like a search for the substance of things hoped for, the evidence of things not seen; nevertheless the nature of the strands and tetrads can be deduced by calculation from the experimental data.