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.

Medical costs of chickenpox in children are small relative to costs incurred as a result of parental work absenteeism and to costs of foregone childcare. Ascribing precise work-related costs should take into account some capacity to make up lost work time. Such data will be required when determining the cost-benefit of childhood varicella immunisation.

C. C. Li showed that Hardy-Weinberg proportions (HWP) can be maintained in a large population by nonrandom mating as well as random mating. In particular he gave the mating matrix for the symmetric case in the most general form possible. Thus Li showed that, once HWP are attained, the same proportions can be maintained by what he called pseudorandom mating. This article shows that, starting from any genotypic distribution at a single locus with two alleles, the same in each sex, HWP can be reached in one round of nonrandom mating with no change in allele frequency. In the model that demonstrates this fact, random mating is represented by a single point in a continuum of nonrandom possibilities.

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Biometrical genetical techniques have been applied to the analysis of certain anthropometric characters measured in 134 pairs of adult twins. After allowing for assortative mating it appears that there is a family environment (E2 ) component for variation in height larger than previously reported. "Fatness" traits -weight, ponderal index, and skinfold thickness -all show higher heritabilities in males and substantial Ez components in females, and reasons for this are discussed. The same is true for cephalic index and forearm length but the reason for these differences is not so obvious. Head length shows a much higher heritability than head breadth. A larger sample of DZ opposite-sex pairs would allow more powerful discrimination, but the variety of patterns of variation revealed by the modelfitting approach used here justify its use over more traditional techniques.