Mendelian genetics
An individual receives one allele from each parent, thus, in the case of a homozygous recessive individual (aa), it received one recessive allele from each parent, and therefore it is not possible that one of its parents is a homzygous dominant (AA). The parents may be either heterozygous (Aa) or homozygous recessive (aa), in both cases porters of the recessive allele that will inherit to their child.