Abnormal tetrads inAspergillus nidulans

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Abstract
Seventeen tetrads in which one or more alleles segregate in unusual ratios have occurred among 1642 fully classifiable hybrid asci of Aspergillus nidulans. The number of asci in which these abnormal ratios could be the consequence of faulty technique has been minimized by statistical treatment. As a result of this treatment, eleven out of the seventeen tetrads could be ignored. Of the remaining six, two could be explained in terms of an extra mitotic division of one or two products of meiosis with corresponding loss of others. A third tetrad had 3:1 ratios with respect to two adjacent linked markers. These two markers entered the cross in the trans arrangement, and the ratios in the ascus were 3:1 for one marker and 1:3 for the other one. It is suggested in agreement with Mitchell (1955a) that such a tetrad could arise if a section of chromosome covering an interval of at least 9 units produced two daughter chromosomes instead of the usual one. In a section of a chromosome marked only at one locus, a process of this kind would lead to a 3:1 ratio for the alleles at that locus. The three remaining tetrads which gave 3:1 ratios at one locus might well have such an origin. An alternative possibility would be Lindegren's 'gene conversion', i.e. the directed mutation of an allele to the alternative allele in a heterozygote.
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