Figure 7. Two types of dosage compensation in preimplantation embryos.

(A) For X-linked genes, Xm hyperactivation is present throughout preimplantation development in both male and female embryos. In female embryos, Xm hyperactivation is linked temporally to Xp inactivation, with both occurring on a gene-by-gene basis.
(B) Two parallel tracks of dosage compensation in preimplantation development. Old genes and LINE repeats tend to be constitutively silent on Xp. Xm hyperactivation occurs contemporaneously with Xp silencing on an individual gene basis.