X-chromosome inactivation analysis.

Results obtained at the HUMARA locus. The blue peaks and the black peaks correspond to PCR products before and after HpaII digestion, respectively. The length of the PCR fragments are noted on the top of each histogram, and the father had a 203-bp fragment in his only X allele, while the mother had both a 200-bp and a 206-bp fragment in her 2 alleles. The mother's 206-bp fragment was transmitted to the healthy daughter (II.1), while the mother's other X-chromosome containing the 200-bp fragment went to the twins (II.2, II.3). The 2 black peaks in approximately equal proportion are seen in X-chromosomes randomly inactivated from females I.2 and II.1, whereas only one black peak corresponding to the mother's 200-bp inactivated allele is observed in the twins (II.2 and II.3), indicating a skewed X-chromosome inactivation toward the maternally 200-bp–derived allele.