Multiple metachromatic leucodystrophy alleles in an unaffected subject: a case of dispermic chimaerism

Descriptions of chimaeras and how they are generated have been reported over many years. 8 18-22 In the more recent reports, the use of DNA polymorphisms to distinguish haematological from dispermic chimaerism is particularly emphasised. 21 22 The most likely events appear to be fusion of a fertilised ovum with a fertilised polar body, either the one arising from meiosis II and directly associated with the secondary oocyte or one of those arising from meiosis I. The former would be expected to result in the presence of one maternal allele for all markers tested. 19 22 The latter would yield both maternal alleles for all markers barring recombination. Fusion of two distinct embryos would be expected to yield representation of both alleles of some markers and not others. 18 20 In the case presented here, all the maternal alleles were represented for all markers examined except for DXS987 (Xp22) where only one was found. Since we cannot rule out recombination in this case, the chimaera may have been generated by fusion of two distinct zygotes as in two earlier cases 18 19 or by fertilisation of a meiosis I polar body.