InGammarus chevreuxi there is a marked sexual difference in linkage intensity. For the factors B-G, the mean cross-over value for females is 50.4 per cent. on 2133 specimens, for males is 25.4 per cent. on 4154 specimens. It is impossible at present to decide whether the state of affairs in the female is due to true linkage with high c.o.v., or to independent chromosomes which undergo pseudo-linkage in the male; though the former is the more probable.
There is great variation in linkage intensity in both sexes; but the shape of the frequency polygons obtained is different for the two sexes. The variation appears to be due in large part to genetic causes.
The fact that absence or marked reduction of crossing-over in the autosomes of one sex has only so far been recorded for the heterogametic sex is mentioned and its theoretical bearings discussed.