Bias in the internal coiling direction of chromosomes

Summary

• 1. The coiling of chromosome arms in Tradescantia shows L and R directions to be equally frequent (Table 6).

• 2. This is true of one chromosome arm (e.g. a unique telocentric) — and presumably of any one — in different cells. Coiling must therefore begin afresh in every cell division.

• 3. The direction of coiling in opposite arms of the same chromosome (across the centromere) is biassed: it is almost 3.5 times as frequent in the opposite as in the same direction. The combinations LL∶LR∶m RR are as 1∶7∶1 (Table 7).

• 4. The bias is perhaps greater in diploid than in tetraploid Tradescantia.

• 5. The directions of coiling in partner arms (across a terminal chiasma) where the chromosomes are similar are at random and without bias. The combinations LL∶LR∶ RR are as 1∶2∶ 1. This is true of the diploid and tetraploid in Tradescantia, of bivalents and multivalents, and probably of unpaired as well as paired, homologous arms.

• 6. Contrariwise, however, where the partner arms belong to unequal chromosomes (one-armed and two-armed) coiling across a chiasma is biassed. The bias is small but again it is in favour of unlike directions. The bias seems to be reduced when a third chromosome is added (Table 8).

• 7. A small test of the effects of varying external conditions gave negative results.

• 8. We conclude that:

  • a) The equality of L and R frequencies may be universal.

  • b) Bias of coiling relations across the centromere may be equally so.

  • c) But in degree it is likely to vary under genotypic control.

  • d) Bias across a chiasma perhaps arises only with structural hybridity.