THE ANATOMY OF THE HETEROCHROMATIN

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Abstract
It is difficult to evaluate the significance of the fact that a heterochromatic break will affect the action of a gene removed from the break by a distance of at least one-third the length of a metaphase chromosome, since the relative distance would be much less in a prophase chromosome where euchromatin is greatly lengthened in comparison with basal heterochromatin. The latter condition would be comparable to the salivary gland chromosomes where measurements have been made of the distance over which the spreading effect in euchromatin occurs. Demerec,2 using Notch rearrangements, showed that the position effect could spread, in the case of N264-52 from the locus of rst to at least bi a distance of about one-fifteenth the length of a salivary gland X chromosome. One of the many fundamental questions about the spreading effect which remains unanswered is the distance between the heterochromatic break and the affected locus at the time the suppression of gene action takes place. This distance may be a function only of the distance along the chromosome thread, but, on the other hand, it may depend on the external chromosome length and thus on the stage of the chromosome cycle. Granted that the latter condition is correct, one would be inclined first to think that the resting (metabolic) nucleus would be the period of action. However, it should be recalled that position-effect breaks act only on genes on the same chromosome, genes on the homologue being unaffected. This might suggest that in an organism exhibiting somatic pairing, the suppression takes place at a stage when there is a barrier (space or matrix) separating the homologues. In any event, it is not possible at the present time to state with assurance whether or not the position effect may act over longer distances of heterochromatin than euchromatin. The one fact which is clearly demonstrated by these data is that breaks in widely separated regions of the basal heterochromatin are effective in evoking variegation at the pe locus. This lends support to the hypothesis1 that the basal heterochromatin functions as a unit and that a disruption of its continuity (by rearrangement) is an important factor in producing V-type position effects.