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Cover A fundamental question in genetics is whether carriers of recessive disorders have distinctive phenotypes. Using genomic approaches (represented by the microarray fluorescent hybridization image in the background), a distinct gene expression phenotype (represented by a sphere as clustered output of microarray data) was identified for carriers of an autosomal recessive disease, Nijmegen Breakage Syndrome (NBS). This expression phenotype allows identification of carriers of NBS from noncarriers and also from carriers of a similar syndrome, Ataxia Telangiectasia. The result implies that there are phenotypic characteristics that define carriers of an autosomal recessive disease and suggest that heterozygosity for recessive disorders contributes to variation in human gene expression. (Cover illustration by Alethea Andrews and Michael Morley. [For details, see Cheung and Ewens, pp. 973-979.])