Those with blood type O-negative are universal donors as this type of blood can be used in transfusions for any blood type. Around 33 percent of the Caucasian population in the United States has A-positive blood type. The second most common blood type in the United States is A-positive. has blood type O-positive, while only around 37 percent of the Caucasian population has this blood type. Around 53 percent of the Latino-American population in the U.S. While perhaps this doesn’t perfectly answer the question of why Rh- is so rare in the United States, you now understand how it might be rarer.The most common blood type among the population in the United States is O-positive. A lot of random things can happen in life! Sometimes a trait will become more or less common for no particular reason, which is a concept called genetic drift. It’s also possible that the blood type frequencies are affected by random chance. 3 With that in mind, it’s maybe surprising that Rh- is as common as it is today! That could have caused Rh- to stay fairly rare. That meant that Rh- mothers might not have as many healthy babies, and wouldn’t pass on their Rh- genes as often. But before this medicine was widely available, babies could die from the condition. Today, this problem is easily prevented with a medicine called RhoGAM. Rh+ babies born to Rh- mothers can have a problem called hemolytic disease of the newborn. (Though exact numbers vary depending on ancestry.) 1īut there is another reason why Rh negative blood might be rare. Type A is the next most common at 37%, then B at 12%, then AB at 4%. Still, the fact that Rh- is recessive doesn’t fully explain why it’s so rare! It’s absolutely possible for a recessive trait to be common, while the dominant version is rare.įor example, you may be surprised to learn that while Type O blood is recessive, it’s the most common type! Around 47% of Americans have Type O blood. This is one reason why Rh- is less common. So someone with DNA for both Rh+ and Rh- will have positive type blood. Since you have two copies of your Rh gene (one from each parent!) here’s what that looks like: In contrast, a dominant trait shows up even if you only inherit it from one parent. A recessive trait is only visible when you inherit it from both parents. Rh- is rare partially because of how it is inherited: Rh- is a recessive trait. Much like other traits, eye color and hair color for example, we inherit our blood type via the DNA we get from our parents! Because we inherit half our DNA from each of our parents, their blood type plays a huge role in what blood type we have. 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.
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