1231 fully classifiable asci of
Aspergillus nidulans
were analyzed for chiasma and chromatid interference. Cross 1 (392 asci) and cross 3 (575 asci) covered the same three intervals in the right arm of the
BI
chromosome, while cross 2 (264 asci) covered six intervals (the same three in the right arm, two in the left arm and the sixth spanning the centromere). Chromatid and chiasma interference may be obscured if the analysis is based on data which include double and multiple exchanges within the intervals. The data from the present three crosses included at least double exchanges within the intervals. When these double exchanges (revealed by the presence of 4-strand doubles within the intervals) were ignored, no chromatid or chiasma interference was detectable with one exception. One of the crosses gave a suggestion of negative chromatid interference. Allowance may be made for undetected double exchanges by the use of Whitehouse’s (1956) correction. This correction is based on the occurrence of detected double exchanges within intervals, i. e. 4-strand double exchanges. Since the relation of these 4-strand doubles within intervals to the 2-, 3- and 4-strand doubles between intervals cannot be stated exactly, Whitehouse’s (1956) formula must be used with reservations. When the undetected double exchanges within intervals are taken into account by Whitehouse’s formula, strong negative chromatid interference is detected in two of the crosses but there is still no chromatid interference in the third cross. An attempt has been made to explain these results in terms of a two phase model of exchange. The corrected proportions of 2-, 3- and 4-strand doubles between intervals may be used to estimate the numbers of 2-, 3- and 4-strand doubles within the intervals. This gives a corrected distribution of the exchanges within intervals among the tetrads and when this distribution is compared to a Poisson distribution, a very high degree of negative chiasma interference is shown. However, this high degree of negative interference depends on the verity of the corrected proportions of 2-, 3- and 4-strand double exchanges between intervals which are in turn subject to the reservations on the use of Whitehouse’s formula.
