Obesity related phenotypes in families selected for extreme obesity and leanness.

Obesity is a multigenic trait, and special methods and sampling designs are needed for gene identification. To describe characteristics of families selected to increase information for genetic linkage studies of obesity. Families having extremely obese siblings with a lean parent and sibling. 594 members of 94 Caucasian families. Measured height and weight, bioelectric impedance, skinfolds, circumferences and questionnaires. Families have an extreme range of obesity phenotypes, which are bimodally distributed. The obese individuals are predominantly women with an onset of obesity early in life. Obesity onset age was negatively correlated with level of obesity, and onset ages were correlated among family members. Individual obesity measures were highly correlated. The extreme range of phenotypes within families increases family variability and presumably gene segregation. Sampling families through extremely obese sibling pairs with a lean parent and sibling results in families with an extreme range of obesity and leanness. The large within-family variance and early age of onset should make these families highly informative for gene mapping and gene identification studies.