Use of Loosely Linked Genes to estimate Chromatid Interference by Tetrad Analysis

Abstract

STRICKLAND¹ has applied to tetrad data for Aspergillus nidulans the method I have suggested² for estimating the magnitude of chromatid interference between cross-overs when loosely linked genes are used. The principle underlying this method is as follows : Consider three linked loci X, Y and Z spaced along a chromosome in that order. By means of tetrad analysis one can distinguish a number of tetrad genotypes. Certain of these, called (a), (b) and (c), respectively, are most simply interpreted as resulting from the following cross-overs at meiosis : (a) One cross-over between X and Y and another between Y and Z involving the same chromatids, that is, a two-strand double cross-over. (b) One cross-over between X and Y and another between Y and Z, with a three-strand relationship. (c) One cross-over between X and Y and another between Y and Z involving the other two chromatids, that is, a four-strand double cross-over. With closely linked genes, the frequencies of (a), (b) and (c) provide accurate information about the frequencies of two-, three-and four-strand relationships between cross-overs. If the genes are loosely linked, these tetrad genotypes can also arise as a consequence of a three-strand double cross-over between X and Y (or Y and Z) and a third cross-over between Y and Z (or X and Y), but the strand-relationships with the third cross-over are two- or four-strand for genotype (b) and three-strand for genotypes (a) and (c). In other words, the occurrence of these triple cross-over tetrads will hinder the detection of chromatid interference by tending to obscure differences from the 1 : 2 : 1 proportion of genotypes (a), (b) and (c). This is the ratio expected with no chromatid interference. The frequency of these inconvenient triple cross-over tetrads can be estimated from the observed frequency of another tetrad genotype (d), which results from a four-strand double cross-over between X and Y (or Y and Z) and a third cross-over between Y and Z (or X and Y). In this way, allowance can be made for the effects of triple cross-overs.