In a large Mexico City prospective study with more than 140,000 participants and thousands of sibling pairs, researchers compared between-family and within-family ancestry effects by estimating ancestry proportions (IAM, EUR, AFR, EAS) from genotype data and linking them to a range of traits.

The study finds that within-family ancestry effects explain part of height and type 2 diabetes differences between IAM and EUR, indicating a genetic component, while educational attainment appears largely driven by environmental factors, demonstrating a robust family-based approach to separate genetic from environmental influences in admixed populations.

It shows that higher IAM ancestry is associated with fewer height-increasing alleles and more T2D-increasing alleles, and when these trait-increasing alleles are accounted for, the IAM height difference shrinks notably and the T2D difference is reduced, suggesting allele-frequency differences partly drive within-family effects.

Selection analyses reveal height-related variants show differential selection between IAM and EUR, with higher genetic differentiation at height loci, hinting that selection partially shapes height differences, while evidence for selection on T2D loci remains unclear.