X inactivation patterns in female monozygotic twins and their families

Abstract

X inactivation studies have been carried out on 22 pairs of female monozygotic twins, one set of female monozygotic triplets, and their mothers and singleton sisters, using the probe M27β. Forty-eight per cent of the twins, 55% of their mothers, and 42% of their singleton sisters showed skewed X inactivation. Two of the triplets and their mother had random X inactivation, while the third triplet showed skewed X inactivation. Their singleton sister was homozygous with M27β. Of the twins, six pairs showed skewed X inactivation in favour of the same X chromosome, one pair showed skewed X inactivation favouring opposite X chromosomes, in seven pairs one twin showed skewed X inactivation while her co-twin showed random X inactivation, and in eight pairs both twins were random. A higher frequency of skewed pattern of X inactivation was not observed in the monozygotic twins when compared to a series of non-twin females (mothers and singleton sisters) and, so, the results in this study do not lend support to the theory that skewed X inactivation predisposes to the twinning process.