Chromosome 9 discusses disease, blood-types of ABO and their impact on evolution. Ridley believes diseases like cystic fibrosis and different blood types have different disease opposing characteristics. “The great epidemic diseases of the past - plague, measles, smallpox, typhus, influenza, syphilis, typhoid, chicken pox, and others - left behind their imprint on our genes. Mutations which granted resistance thrived, but that resistance often came at a price, the price varying from severe (sickle-cell anemia) to theoretical (the inability to receive transfusions of the wrong type of blood)” (Ridley). As a result of mutations resisting diseases, “Individuals with type O blood are universal donors. Individuals with type AB are universal recipients”
