Spontaneous crossing over was studied by means of combined cytological and genetic methods in F1 Drosophila virilis x D. texana females (series I) and in D. virilis females carrying a D. texana fifth chromosome in heterozygous condition (series II). The main criterion utilized to distinguish the oogonial crossovers from the meiotic ones is the identity of cytological positions of genetic exchange in crossovers constituting a cluster. Five clusters of crossovers with identical positions of exchange were found in the first series of experiments. In the second series of experiments not a single cluster of crossovers resulting from oogonial crossing over was found.
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