Estimating the number of loci determining skin colour in a hybrid population.

A model developed by Stern to estimate the number of loci determining human skin colour in African-European hybrid populations is tested on recently collected reflectance data. Despite improved measurement scales and admixture estimates, and the use of statistical goodness-of-fit tests, contradictory results are obtained. It is shown that previous authors' use of visual inspection to compare expected and observed phenotype distributions introduced considerable bias into their results.

Admixture estimation using skin reflectance data.

Several different methods are suggested for the estimation of admixture proportions in hybrid populations based on skin reflectance data. These methods are applied to hybrid populations of known ancestry and yield results generally quite similar to those expected based on a simple genetic model. Results indicate the usefulness of these methods in hybridization studies and in the development and refinement of models of the genetics of skin color.

Skin colorimetry in Belize. I. Conversion formulae.

Comparisons between skin colorimetry reports have been hampered by the common use of two different types of portable reflectometers, which sample reflectances at different wavelengths. In an attempt to provide direct comparability between the two machines, multiple linear regression equations were derived from reflectance spectrophotometry readings on 308 Black Caribs and 175 Creoles in Belize, Central America, using both machines. Cross validation tests show the coefficients presented are applicable to independent data sets and generally applicable to other heavily pigmented populations. Comparisons with previously published conversion formulae, which were from very small samples using simple linear regression, show a definite improvement in predictive accuracy when using multiple regression equations based on a large sample.

Skin colorimetry in Belize. II. Inter- and intra-population variation.

Skin colorimetry readings taken in Belize, Central America (formerly British Honduras) with the two most commonly used portable reflectometers reveal significant differences in mean reflectance between Garifuna (Black Caribs) and Creoles, and between Garifuna in two settlements. These differences are related to variation in African, Indian, and European admixture, as estimated from serological markers. Sex differences are not evident in univariate comparisons, but females are significantly lighter than males in multivariate analyses of variance. Polynomial age trends exist in some groups for certain variables, but account for a very small amount of the variation in skin color within these groups.

The effects of aging and secular trend on adult stature in rural western Ireland.

Cross-sectional data, consisting of anthropometric measurements for 347 adults males and 261 adult females in western Ireland measured during the 1930s, were used to determine the effects of aging and secular change upon stature. Estimates of statural loss due to aging were obtained using partial regression of stature on age while controlling for subischial length, and regression of the difference between observed stature and maximum predicted stature on age. Males show the effects of aging to a greater extent than do females. After correction for the effects of aging, the adjusted values of stature were progressed on age to estimate secular trend of stature. For males, there is a general increase of stature with time, excepting those born around 1878, while females generally show random variation with time. Both male and female adjusted stature decrease sharply around 1878, for which alternative historical explanations are proposed, relating to differential migration and survival.

Population structure and anthropometric variation in rural western Ireland: migration and biological differentiation.

Models of population structure can be investigated using data on anthropometric variation among local populations. Anthropometric data collected by Dupertuis and Dawson during the 1930s were analysed from 347 males and 261 females in 12 towns in three counties of western Ireland. We hypothesized that recent migration would decrease the degree of among-group variation. To test this hypothesis, two additional samples were created by excluding known inter-county migrants from both male and female samples. Based on ethnographic data, a fifth sample was created using unmarried females only, in order to control partially for local migration upon marriage. Univariate and multivariate measures of relative differentiation were developed to compare different levels of migration and differences among the sexes. We found that the degree of among-group variation decreased as the amount of migration increased, in accordance with spatial models of population structure. Using non-parametric correlations of geographic and anthropometric distance, the observed patterns of differentiation were closely related to geography, suggesting a spatial model of gene flow to be appropriate in interpreting among-group variation. The female samples showed greater differentiation and higher correlations with geography than the males. It seems that this results from the sensitivity of males to developmental and local environmental influences, causing an increase in the relative amount of within-group variation.

PIP: The authors first introduce methods of applying the partitioned model of population structure developed by Wahlund to anthropometric data in order to assess the overall differentiation among subdivisions of a population. These methods are then applied to the analysis of data from rural western Ireland in an attempt to examine the effects of cultural variation in migration-related behavior on the degree of differentiation. The patterns of differentiation with reference to geographic distance are also briefly examined (SUMMARY IN FRE, GER)

Interobserver error in human skin colorimetry.

Twenty light-skinned adults were measured at the upper inner arm site using two commonly used reflectance spectrophotometers. Each subject was measured by each of three investigators to assess the influence of interobserver error on the reflectance readings. A repeated measures design analysis of variance showed no significant variance component due to observers.