 We explain the basic principles behind the heredity of blood groups. This is Felix. He's been in an accident and urgently needs a blood donation. Even though his whole family wants to help him, he can only receive blood from one of his family members. But why is that? The answer is related to the heredity of blood groups. The topic of heredity is very complex and has a lot to do with chance, which is why we are only going to look at a few of the basic principles here. Felix and his brother Eric are the result of genes inherited from their parents. The genes contain important hereditary information, which determines what the brothers look like. Although both of their parents have passed on the same number of characteristics to each of their children, however, varying strengths of these features have been passed on to each child. This is reflected in their appearance. The different blends of genetic information mean that Felix looks more like his mom, while Eric shares more of his dad's features. However, there's one feature they have in common, their brown eyes. This is due to dominant and recessive inheritance patterns. This means that the dominant brown of their dad is stronger than the recessive blue from their mom, and this is what gets passed on. So, something similar should also happen when we look at blood groups, right? Unfortunately, it's not quite that simple. But let's start from the beginning. We classify four different blood types A, B, O, and AB. The letters represent the antigens on the red blood cells. These are part of our body's defenses that protect us from hostile attacks. All of us have two antigens, but we only pass on one to our children, which one is inherited is left to chance. The A and B types are stronger than the O blood type, so when it comes to heredity, they overpower the subordinate O blood type. Only when both parents have blood group O is it possible for their child to inherit this blood type. A and B are equally strong in the fight for hereditary dominance. If they are both inherited, the child's blood group is called AB. But what does this have to do with Felix's blood donation? Felix's defense system immediately combats any enemies, which is why in a blood transfusion, it is important to check which antigens are in the transferred blood. If the wrong type is used, it will be rejected by the body and can lead to life-threatening consequences. But which members of his family might be able to help? Felix has blood group A. His body's defenses will fight antigens from the B blood group, which is why his mom and his brother Eric cannot donate their blood. His mom is type AB and his brother is type B. Felix's dad belongs to the group of people with type O blood. In comparison with the AB blood type from his mom, this cannot win the inheritance draw. But it does have another crucial advantage. It doesn't have any antigens. Felix's dad is a so-called universal donor and can be used for the transfusion. Now Felix will soon be feeling much better.