it is not true that traits controlled my dominant alleles are more common always, but in some cases that is true because there are more possibilities for a dominant trait. homozygous dominant, and heterozygous dominant, but recessive alleles are only homozygous recessive unless you want to get into incomplete dominance, co dominance, and so on. the dominant trait may not also show up if it is a sex linked trait. now the recessive ALLELE might be just ask common, but it is masked more often than shown in most cases. most cases, not all.