The short answer to this question is: it’s rare everywhere! This is partially because of how Rh factor is passed down from parents to their children. Our distant hominid, human-like ancestors already had positive and negative blood types! 3 Rh- blood type is probably older than modern humans! (Image via Shutterstock) Why is Rh- so rare in the United States? This genetic variation is older than humans are as a species. That’s the most common cause of Rh- blood type. It turns out that some people don’t have this gene at all. The biggest difference though is in the RHD gene. And there can be a lot of differences in these genes that affect your blood in minor ways! 3 There are a lot more than just two versions, which is one reason why Rh blood type tests are sometimes inaccurate. Together, these genes provide the instructions to make Rh factor. Humans have two different Rh genes: RHD and RHCE. And if you go even further back in evolutionary history, you can even find similar genes in bacteria! 2 Other primates have similar Rh molecules to us. This actually refers to how scientists first discovered Rh: in Rhesus macaque monkeys. Like I mentioned earlier, Rh factor is short for Rhesus factor. The short answer is that Rh factor is very, very old. Different types of antigens on blood cells (Image via Shutterstock) Where does Rh- originate from? Similarly, Rh genes determine whether you have positive or negative type. For example, the ABO gene determines whether you have type A, B, AB, or O. There are a few different genes that affect blood type. Which antigens your body makes are determined by your DNA. If you don’t have it, you have negative type blood. If you have that marker, you have positive type blood. The + or – part of blood type refers to the Rhesus factor, or Rh factor for short. You might have “A” type markers, “B” type, both, or neither! But those aren’t the only antigen types. Our blood cells are covered in little markers or flags, called antigens. These are terms describing different blood types a person can have. You may have already heard terms like A+ blood, B- blood, or O blood. Only 15% or so of Americans have this blood type! 1īefore we dive all the way in, we need to talk a bit about blood type. This can affect the newborn baby, who may need treatment after birth.These are both great questions! You’re right that negative type is fairly rare. Sometimes an incompatibility may happen when the mother is blood type O and the baby is either A or B. This will prevent being sensitized if the baby is Rh positive. If the mother hasn't already been sensitized to Rh positive blood, they may be given Rh immunoglobulin. Problems with the Rh factor happen when the mother's Rh factor is negative and the baby's is positive. If the father's genes are -, and the mother's are -, the baby will be: If the father's genes are -, and the mother's are + -, the baby can be If the father's genes are + - Rh-positive, and the mother's are + - Rh-positive, the baby can be: If a father's Rh factor genes are + +, and the mother's are -, the baby will have one + from the father and one - gene from the mother. If a father's Rh factor genes are + +, and the mother's are + +, the baby will have one + from the father and one + gene from the mother. More specifically, consider the following: If a person has the genes -, the Rh factor will be negative.Ī baby receives one gene from the father and one from the mother. If a person has the genes + -, the Rh factor will also be positive. If a person has the genes + +, the Rh factor in the blood will be positive. The Rh-positive gene is dominant (stronger) and, even when paired with an Rh-negative gene, the positive gene takes over: Rh factors follow a common pattern of genetic inheritance. A baby may have the blood type and Rh factor of either parent or a combination of both parents. The following are the possible combinations of blood types with the Rh factors: If there is no Rh factor protein, the person is Rh- negative. If the Rh factor protein is on the cells, the person is Rh-positive. The Rh factor is a protein that is found on the covering of the red blood cells. The blood type is found as proteins on red blood cells and in body fluids. The blood type and the Rh factor simply mean that a person's blood has certain specific features. Every person has a blood type (O, A, B, or AB) and an Rh factor, either positive or negative.
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