Difference in the Behaviour of Eu- and Hetero-chromatin: Crossing-over

Abstract

THE recombination map-length (0.04 per cent) for the heterochromatin of the X chromosome of Drosophila melanogaster is far less than would be anticipated from its mitotic bulk (half the chromosome at metaphase) and considerably less than would be estimated from its polytene-length (17 bands of 1,024)¹. An exchange frequency comparable to the euchromatic average for the 66 map units of the X chromosome should result in a heterochromatic map-length of at least 1.6 per cent and possibly as high as 33 per cent, depending on which cytological measurement more closely approximates the actual physical length of X heterochromatin. In order to decide whether this low frequency of crossing-over reflects, a basic difference in the exchange behaviour of eu- and hetero-chromatin rather than an over-estimate of the physical length of X heterochromatin, use was made of the ability of autosomal inversions to increase crossing over on the X chromosome, the interchromosomal effect on recombination.