Genetic and environmental components of chiasma control

Two selected lines of Schistocerca gregaria with mean chiasma frequencies of 17.68 and 21.66 were crossed in all possible combinations up to the F₂ generation. A genetic analysis of both generation means and within family variances reveals that the control of chiasma frequency in these lines is governed by a large additive genetic component (d) and a small dominance component (h) directed towards high frequency types. 40% of the total variance can be explained in terms of environmentally induced variation. It is argued that such significant and readily induced changes in the chiasma pattern, associated with a mean cellular difference of 4 chiasmata, can be expected to lead to an alteration in the linkage relationships within the genomes of each line. Unfortunately the intensity of the effect upon linked non-allelic associations can only be assumed since, at the present time, there is no evidence to correlate chiasma frequency variation in the parent with ensuing exophenotypic variation among the progeny.