Thirty-four families with a child or fetus with Turner's syndrome were studied using a series of polymorphic DNA probes. Analysis of the origin of the normal X chromosome was possible in all cases. In 16 families with 45,X (four fetuses and 12 livebirths), the observed X was maternal in each case, indicating a preferential loss of the paternal sex chromosome at, or before, conception. In the remaining 18 families with a variety of karyotypes, but especially in those where the child had an isochromosome of Xq or a ring X, there was again a strong tendency for the normal X to be maternal. Analysis of parental ages was performed with known origin of each abnormality, but no evidence for an increased or decreased parental age effect was detected.Turner's syndrome is one of the most common chromosomal abnormalities with an estimated frequency at conception of 1-5%. Most affected fetuses are, however, spontaneously aborted and the residual birth frequency is 1 in 2500 to 5000 females.' In a large consecutive study the majority of fetuses were found to have monosomy X (45,X) with 45,X/46,XX in the remainder, whereas in the same study, among liveborn Turner's syndrome patients, 53% were 45,X, 14-8% were mosaic 45,X/46,XX, 90/o were 46,Xi(Xq)/ 45,X, 4 90/o were mosaic 47,XXX/45,X, and the remaining 9-7% showed five other karyotypes (46, 46, mosaic applicable to cultured cells from affected fetuses, were only informative in a proportion (31%) of families, and were biased in that informative families were more frequent for 45,Xm than for 45,XP.3 4 The development of multiple polymorphic DNA markers for the X chromosome offers an alternative approach which should avoid these difficulties. Hassold et a15 6 used this alternative approach to study 40 families with Turner's syndrome where the majority were spontaneously aborted fetuses, and were able to identify the parental origin in 35 (88%) of their families. We describe our results from a study of 34 families in which the majority were live born and where the origin of the abnormality has been determined for a variety of karyotypic abnormalities. Patients and methods ASCERTAINMENT OF CASESFamilies for this study were recruited from the records of the West of Scotland Regional Genetics Service and with the assistance ofthe United Kingdom Child Growth Foundation. CYTOGENETIC ANALYSISChromosomal analysis was undertaken using standard cytogenetic techniques on peripheral blood lymphocytes or cultured products of conception. Two families where the proband had Turner's syndrome but 45,X/46,XY (that is, a paternal defect) were excluded from DNA analysis.DNA ANALYSIS DNA was extracted from cultured fetal cells in the case of an affected fetus or from peripheral blood lymphocytes of affected children and of both parents.DNA samples were digested with the appropriate restriction enzymes under the conditions specified by the manufacturers. The resulting fragments were fractionated on 0-8% agarose gels and Southern blotted onto Hybond-N (Amersham) followed by hybridisation wi...

Spinal muscular atrophy (SMA) is a common cause of inherited morbidity and mortality in childhood. The wide range of phenotypes in SMA, uncertainty regarding its mode of inheritance, and the suggestion of linkage heterogeneity have complicated the genetic counselling of parents of affected children. The locus responsible for autosomal recessive SMA has been mapped to 5q11.2-q13.3. The most likely order of loci is cen-

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SUMMARYThe gene Tin-1 in tomato plants is dominant for suppression of mosaic symptoms caused by tobacco mosaic virus isolates designated as tomato strain o. Virus multiplication (measured either by virus RNA content or by virus coat protein content) was inhibited in plants containing Tm-l. Inhibition was greater in hosts homozygous for Tin-1 (90 to 95 %) than in hosts heterozygous for Tin-1 (65 to 75 %). Thus, inhibition of tobacco mosaic virus multiplication is Tin-1 gene dosagedependent; suppression of visible symptoms is not.Inhibition of TMV RNA accumulation occurred in both directly inoculated and systemically-infected leaves of Tin-l-containing hosts, and was consistent from experiment to experiment. Tin-1 inhibited accumulation of RNA of different isolates of strain o to different extents: two isolates causing yellow mosaic symptoms on susceptible plants were especially strongly inhibited.In Tm-I hosts, there was a delay between inoculation and the first detection of virus multiplication, measured either as increase in virus RNA content or recoverable infectivity. The delay was about 8 days in heterozygous Tm-1 hosts and about t 6 days in homozygous Tin-1 hosts. The implications of these results for the mode of action of the Tin-1 gene resistance are discussed.