To understand the answer to this question, we will use knowledge from genetics and mathematics. We will introduce you to the concepts of phenotype, genotype and probability, so the inheritance of blood groups will not seem like Spanish villages. It's easy, only when you know how. And we will introduce you to the world of inheritance from parents to children, what is transmitted and how, especially when we have so many different blood groups, plus and minus signs. But this is universal knowledge and you can use it to understand the many differences we see among us.