Further data on the polarity of the paba1 locus of Aspergillus nidulans

Newly mapped paba1 alleles are better distributed on the fine-structure map than those mapped earlier. They provide a proximal, a middle and a distal cluster, of four, one and four sites respectively (Fig. 1). The recombination fractions indicate map expansion (Fig. 2). The large samples of paba+ recombinants classified show that classes I and I III are positively correlated with increased recombination fraction, and classes I II and I II III are negatively correlated (Fig. 3). No polarity reversal was found. The results are explained in terms of a hybrid DNA model for crossing over.