Recent studies in large human populations of the range of phenotypes observed in individuals heterozygous for CFTR (cystic fibrosis transmembrane conductance regulator) mutations (Barton et al. 2022), mutations in genes associated with developmental delay (Kingdom et al. 2022), and mutations in genes associated with maturity onset diabetes of the young (Mirshahi et al. 2022) provide striking illustrations of the complex interactions that occur between a mutation in 1 gene and normal genomic polymorphisms resulting in an affected individual's phenotype. This evidence concerns the gene CFTR and Global developmental delay.