Information about the prenatal exclusion test for Huntington's disease (HD) The families Nine of the 10 couples who subsequently underwent exclusion testing were married and one couple married between their first and second tests. All couples were specifically counselled about the exclusion test at least once in the preconception period by the same geneticist and on a second occasion by the same or another geneticist. Four consultands had the exclusion test explained on three occasions before undergoing chorionic villus sampling (CVS). In addition to receiving oral explanations, consultands were give a leaflet prepared by the United Kingdom Association to Combat Huntington's Disease, which gives details on the working of both presymptomatic and prenatal exclusion tests.Details of the families and exclusion test outcomes are given in the table. It is notable that in nine out of 10 families, the mother was at 50% risk of being an HD gene carrier. As expected, the consultands were either primiparous or of low parity and the mean maternal age of 29 years was increased in keeping with an often stated preference to defer having children until it could be shown that consultands had a low chance of transmitting the
Data on Turner Syndrome from four sources were analysed for possible associations with several aetiological factors. Two classes of liveborn propositae were included, those with a non-mosaic 45, X karyotype (XO) and those with an isochromosome of the long arm of the X (iso-X). The numbers were 288 and 84 respectively and constitute the largest series of such cases to be analysed to date. For the XO's, an analysis using the liveborn full sibs of propositae as controls (method of Carothers et al. 1978) confirmed earlier studies in finding no positive association with parental age or birth order, and even suggested a small negative association. There were no significant differences between the mean parental ages of those cases shown by Xg grouping to have received a maternal X chromosome and those of the remainder. Among the iso-X's there was an exceptionally high proportion (17.5%) of parents with an age difference (paternal-maternal) of 10 or more years, raising the possibility of a paternal age effect. This agrees with earlier studies but conflicts with the finding of a negligible tendency for affected individuals to be born later within their sibships. The apparent discrepancy may be due to the relative insensitivity of the latter method to small parental age effects in samples of this size. For the XO's there were no detectable seasonal variations in the month of birth, but for the iso-X's there was a significant excess of births in the first 6 months of the year. Reviewing the conflicting evidence from the literature on seasonal variations in chromosomal aberrations, we urge caution in interpreting these results. In agreement with earlier studies, the incidence of twins among both XO's and iso-X's was higher than the population average, but the numbers were too small for statistical significance. There was no evidence for any alteration in the sex ratio among the liveborn sibs of either class.
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