Purpose: Multicolour FISH has been used for the preimplantation diagnosis of sex for X-linked disorders and to examine the chromosome constitution of early human embryos.
Materials and methods: Single blastomeres and whole embryos were spread using HCI and Tween 20. Multicolour FISH was performed using directly-labelled human DNA probes for chromosomes X, Y, and 1 in a two hour FISH procedure.
Results: Four groups of chromosome arrangements have been found in human preimplantation embryos (i) normal, all nuclei uniformly diploid, (ii) diploid mosaics, majority of the nuclei diploid, with a small number of nuclei aneuploid (iii) chromosomally abnormal, all nuclei uniformly chromosomally abnormal, e.g. XO, XXY, XXX and (iv) chaotic, all nuclei showing different chromosome complements.
Conclusions: For the preimplantation diagnosis of sex, an XX nucleus has always been representative of a female embryo. However, for the diagnosis of dominant disorders or chromosome abnormalities, two cells should be analysed to reduce the chance of misdiagnosis which may arise from chromosomal mosaicism. Implantation and further embryo development may be possible from mosaic or chromosomally abnormal embryos, but those showing chaotic chromosome arrangements would be unlikely to implant.