SUMMARY A population based study of von Recklinghausen neurofibromatosis in south east Wales (population 668 100) identified 69 families with 135 affected members (prevalance 1/4950 of the population). In these families penetrance of the NF-1 gene was 100% by the age of five years. The genetic fitness of NF-1 sufferers was found to be reduced to 0.47, the effect being more marked in males than females (f=0.31 and 0*60, respectively). Forty-one of 135 cases were judged to represent new disease mutations and the mutation rate was estimated to lie between 3.1x10-5 and 10-4x10-5. A parental age effect for new mutations was not found, nor was a maternal effect on disease severity.

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Summary. Skin colour has been measured by refiectance spectrophotometry on 134 pairs of twins at three sites, forehead. forearm and upper arm, each at three wavelengths, 425, 545 and 685 nm. Tanning is measured most reliably at 685 nm and at this wavelength the heritability is high at the least exposed upper arm site, intermediate on the forearm, while on the forehead variation is entirely environmentally determined. The same gradient is observed, though less strikingly at 545 nm. but at 425 nm, where haemoglobin is refiecting most of the light, the degree of genetic determination is the same at all sites.

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