Genetic Epi was my penultimate class, and we had a guest lecturer who included a slide devoted to Harry Potter's pedigree. The lecturer argued that family history pointed to wizardry's being an autosomal recessive trait. Being a giant loser, I raised my hand to object "What about squibs?" Turns out that was they guy's last slide. Environmental factors do often mediate genetic expression. The lecturer suggested that Argus Filch's mother drank too much Butter Beer when she was pregnant and prevented the expression of his wizarding genes.