Frequency of Translocation in Control. The spontaneous frequency of translocation is of interest in determining whether the occurrence of chromosome interchanges following ultraviolet treatment is an effect of the treatment. Among the translocations observed in UV-treated progenies to date, although as previously mentioned the majority are deficiency-translocations, there are two or possibly three which appear to be regular segmental interchanges. Although such translocations have previously been found in untreated maize populations, there is no basis for an estimate of their spontaneous frequency. The large control in this experiment included only nine plants with segregating defective pollen; the progeny tests from these showed that two of them transmitted through pollen the factor for aborted pollen segregation. Diakinesis examination in these progenies showed in both cases the presence of chromosome interchange producing a ring-of-four at diakinesis. The spontaneous frequency of chromosome interchange thus appears to be appreciable, and the number of interchanges observed following ultraviolet treatment is not significantly higher than that in untreated material.
The results suggest that UV treatments produce a significant increase in the frequency of deficiency-translocations, without appreciable effect upon the frequency of segmental interchanges.