Position effect variegation (due to inactivation of an autosomal segment attached to X-chromosome)
The inactivation is not confined to the facultative heterochromatin mainly present in X-chromosome but may also extend to autosomal chromosome segments, if attached to X-chromosome. For instance, it has been observed that when a translocation occurs between a euchromatin segment from an autosome to an X-chromosome, inactivation seems to spread from the X-chromosome to the translocated autosomal segment. In mouse also it has been observed that when an autosomal gene is transferred to an X-chromosome, its action may be suppressed in some cells alongwith the inactivation of X-chromosome. In a translocated chromosome, having segments from an autosome and an X chromosome, it has been observed that the inactivation of a chromosomal region depends on its proximity to X chromosome, because the effect has been found more pronounced in regions close to the heterochromatin. The autosomal regions which are located away from heterochromatin may remain active. Variegation has been observed in mice which were heterozygous for a translocation between X chromosome and an autosome, the autosomal segment carrying a gene for coat colour. Due to inactivation of the coat colour gene located on autosomal segment, translocated to the X chromosome, variegation has been observed. This kind of variegation has been described as position effect variegation.