Both my parents are positive too, but I am negative. This is how it works. Each person has 2 alleles(variant form of a gene). Only one of these alleles is inherited from each parent. A person can be either heterozygous (1 allele of a certain type), or homozygous (2 alleles of same type) for the RH D gene. Your parents were likely both heterozygous, meaning they had one Rh positive allele and also carried one Rh negative allele. When this occurs the Rh positive is always dominant, and their blood is typed this way. You happened to inherit both recessive Rh negative alleles from your parents. Making you homozygous for the Rh negative gene.