Instead of the random activation and/or inactivation of the X-chromosome in sex determination, as suggested by the Lyon's hypothesis, a proposal is made here that crossingover between the sister- and/or nonsisterstrands at the sticky or nonsticky loci, at heterochromatinizing regions and at the inactivating centers of the centromère, be responsible for the heterochromatinization and/or heteropyknotization of the X-chromosome. (This proposal will be called the Mustafa hypothesis.)
Such would be the basis for the activation and/or inactivation of the X-chromatid(s), which would then replicate into a normal or a heterochromatic X-chromosome respectively. The heterochromatic X-chromosome may be transformed into a heteropyknotized mass of sex chromatin (Barr body). Translocation of the Y-chromosome and of some of the autosomes could also result in the same effect. Hence, the number of heterochromatinized X-chromosomes, and/or of heteropyknotized masses (Barr bodies), in each daughter-cell is directly proportional to half the number of chromatids involved in crossingover and/or translocation in the mother-cell.
