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There are several variants of ataxia-telangiectasia (A-T): classical A-T with marked radiation sensitivity; classical A-T with intermediate levels of radiation sensitivity; mild A-T with intermediate levels of radiation sensitivity; A-T without telangiectasia; A-T without oculomotor apraxia; and A-T with microcephaly. These disorders are probably caused by different allelic mutations, because affected sibs resemble the index patients, and because there is an association of certain haplotypes of 11q22-23 with specific phenotypes. The Nijmegen Breakage Syndrome, with its lack of ataxia, seems on clinical grounds to be a different disorder. Although A-T is almost always inherited as an autosomal recessive, there are some unusual features; an unexpectedly low parental consanguinity rate, an incidence in sibs that is < 0.25, and occurrence of disease in many different races and in the offspring of mixed race unions. Moreover, looking at haplotypes from 63 UK patients, there is a remarkably low incidence of homozygosity. An autosomal recessive condition that is deficient in parental consanguinity, and in homozygosity for the region around the gene, can be explained by J. H. Edwards' hypothesis that homozygosity for alleles at a neighbouring locus are lethal early in embryogenesis. Other possible mechanisms to explain the unusual genetic features are discussed.

Familial exudative vitreoretinopathy (FEVR) is a hereditary disorder characterized by an abnormality of the peripheral retina. Both autosomal dominant (adFEVR) and X-linked (XLFEVR) forms have been described, but the biochemical defect(s) underlying the symptoms are unknown. Molecular analysis of the Norrie gene locus (NDP) in a four generation FEVR family (shown previously to exhibit linkage to the X-chromosome markers DXS228 and MAOA (Xp11.4-p11.3)) reveals a missense mutation in the highly conserved region of the NDP gene, which caused a neutral amino acid substitution (Leu124Phe), was detected in all of the affected males, but not in the unaffected family members, nor in normal controls. The observations suggest that phenotypes of both XLFEVR and Norrie disease can result from mutations in the same gene.

Analysis of genetic heterogeneity in 40 kindreds with X-linked retinitis pigmentosa (XLRP), with 20 polymorphic markers, showed that significant heterogeneity is present (P=.001) and that 56% of kindreds are of RP3 type and that 26% are of RP2 type. The location of the RP3 locus was found to be 0.4 cM distal to OTC in the Xp21.1 region, and that of the RP2 locus was 6.5 cM proximal to DXS7 in Xp11.2-p11.3. Bayesian probabilities of linkage to RP2, RP3, or to neither locus were calculated. This showed that 20 of 40 kindreds could be assigned to one or the other locus, with a probability >.70 (14 kindreds with RP3 and 6 kindreds with RP2 disease). A further three kindreds were found to be unlinked to either locus, with a probability >.8. The remaining 17 kindreds could not be classified unambiguously. This highlights the difficulty of classifying families in the presence of genetic heterogeneity, where the two loci are separated by an estimated 16 cM.

A large family with X-linked mental retardation, originally reported in 1944 by Allan, Herndon, and Dudley, has been reinvestigated. Twenty-nine males have been affected in seven generations. Clinical features include severe mental retardation, dysarthria, ataxia, athetoid movements, muscle hypoplasia, and spastic paraplegia with hyperreflexia, clonus, and Babinski reflexes. The facies appear elongated with normal head circumference, bitemporal narrowing, and large, simple ears. Contractures develop at both small and large joint. Statural growth is normal and macroorchidism does not occur. Longevity is not impaired. High-resolution chromosomes, serum creatine kinase, and amino acids are normal. This condition, termed the Allan-Herndon syndrome, appears distinct from other X-linked disorders having mental retardation, muscle hypoplasia, and spastic paraplegia.