In the HP wiki I was perusing the other day I came across a listing for the word squib (born to at least one magicking parent, but having no magicking ability) which is not I think a mentioned bit of vocabulary in the movies. Rowling mentions that squibs are, "very rare indeed given that the gene for magicking is a dominant gene." So, assuming that magicking is a single allele trait, lets look at how the distribution would really play out. Homozygous dominant + Homozygous dominant = 100% homozygous dominant children Homozygous dominant + Heterozygous = 50% homozygous dominant, 50% heterozygous Heterozygous + Heterozygous = 25% homozygous dominant, 50% heterozygous, 25 % homozygous recessive Heterozygous + Homozygous recessive = 50% heterozygous, 50% homozygous recessive All of the end resulting offspring in red are squibs by definition since they have been born to at least one magicking parent. Muggleborns would have to be, by genetic definition, spontaneous mutations. While one can see that squibs have a smaller representation given that the magicking gene is dominant, and the heterozygous condition is sufficient for the magicking phenotype to be expressed, as spontaneous mutations muggleborns should have a much lower representation than all of the other geno/phenotypes, yet they are mentioned much more often than squibs.