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. These are both great questions! You’re right that negative type is fairly rare.

Percent of o negative blood type