All individuals with the defect/disease in pedigrees (and in population) are homozygotes of recessive defective (deleterious, nonactive, affected, mutated, etc.) alleles, aa. Two copies of a disease allele are needed for an individual to express the phenotype. You can determine the patterns of inheritance by reading the pedigrees. As the diagram suggests, Autosomal Recessive shows that heterozygotes do not show traits to skip generations. If both parents have it, all offspring will have it, and parents do not have to have it. At the same time, Autosomal Dominant shows heterozygotes do show traits, so it cannot skip generations, and even if both parents have it, then offspring do not have to show it.
