Except in rare cases, you can only inherit one allele per parent per trait (for those of you that don't know, that's two alleles). A recessive trait will be shown if both alleles are for said recessive trait. If one of the alleles is for a dominant trait, then the dominant trait will show. Some genes are tied to environment and exposure to certain things, so a trait may be 'masked' for the duration of a person/organism's life, or at least until they are exposed to it. For example, an allergy to plastic would not be apparent in someone living in Africa unless by chance they came into contact with a plastic bucket, plastic hairbrush, etc., from, say, a visiting doctor from America.