Sexing of preimplantation mouse embryos by measurement of X-linked gene dosage in a single blastomere

Single blastomeres were isolated from zona-free 8-cell mouse embryos and assayed for X-linked hypoxanthine phosphoribosyl transferase (HPRT) activity and autosome-linked adenine phosphoribosyl transferase (APRT) activity. At this stage of development both X chromosomes are active in female embryos. Hence, a bimodal distribution of HPRT: APRT ratios, corresponding to male (XY) and female (XX) biopsied samples, was observed due to the 2-fold difference in gene dosage for HPRT activity. Batches of putative male and female embryos identified in this way were transferred to pseudopregnant recipient females. Development of the seven-eighths embryos was equivalent to that of control zona-free intact embryos. Sex determination by measurement of X-linked gene dosage was accurate and rapid enough to allow transfer of embryos of known sex without the need for cryopreservation.