A. Genetics: Host genetics influence immune factors, mucosal receptors and other host traits that shape which microbes colonize and persist. B. Eye Color: Eye color is a physical trait determined by pigment and is not known to be a determinant of overall microbiome composition. C. Diet: Diet strongly shapes the gut and oral microbiomes by providing different substrates for microbial growth. D. Age: Age influences microbiome composition across the lifespan (infant → adult → elderly shifts).