Rowling once stated in an interview that magical genes were “strongly dominant” in which case two marrying Muggleborns would constantly be throwing non-magical offspring. There are entire papers trying to reconstruct how magical genes could realistically work instead, albeit I think they tend to come up with answers that make bad fiction compared to HPMOR. Eg https://pmc.ncbi.nlm.nih.gov/articles/PMC2151141/. But ultimately it is a truth of the final origins of the observed phenomena of Harry Potter that Rowling did not understand genes and was running on Folk Heredity.
Rowling once stated in an interview that magical genes were “strongly dominant” in which case two marrying Muggleborns would constantly be throwing non-magical offspring. There are entire papers trying to reconstruct how magical genes could realistically work instead, albeit I think they tend to come up with answers that make bad fiction compared to HPMOR. Eg https://pmc.ncbi.nlm.nih.gov/articles/PMC2151141/. But ultimately it is a truth of the final origins of the observed phenomena of Harry Potter that Rowling did not understand genes and was running on Folk Heredity.