The Viability of Near-normal Irradiated Chromosomes

Summary

After the exclusion of major mutations, both lethals and semi-lethals, the viability in the Drosophila male of X-chromosomes irradiated in sperm shows a slight reduction, which is, nevertheless, highly significant. The change in mean viability is accompanied by a barely significant increase in variance. On the other hand, irradiation of eggs, though yielding the expected major mutations, produces few if any minor mutations.

These results indicate mutations of very low magnitude and very high frequency.

The following alternative possibilities are considered.

1. A general reduction in viability of all irradiated chromosomes, by an amount proportional to dose.

2. Mutation at a single highly mutable locus.

3. Mutations distributed along the entire length of the chromosome and each reducing viability by 1 per cent. The maximum frequency after 2000 r would be greater than two mutations per chromosome.

4. Polygenic mutations with an average effect reducing viability by 1 per cent or less.