Many of us know our blood type from our parents, or after donating blood for the first time. We feel a sense of pride with our biological distinction, but don't know what it means to be type ___, aside from knowing what type of blood we can receive in transfusions. However, lingering questions remain, such as: why are some blood types more common than others?

ABO Blood System

Blood is made of the same basic elements (red blood cells, white blood cells, platelets, and plasma), but not all blood is alike. Blood types are determined by the presence or absence of certain antigens — substances that can trigger an immune response if they are foreign to the body, according to the American Red Cross. Some antigens can trigger our immune system to attack transfused blood, which is why transfusions are based on careful blood typing and crossmatching.

There are four major blood groups determined by the presence or absence of two antigens — A and B on the surface of red blood cells: Group A has only A antigen on red cells (and B antibody in the plasma); Group B has only the B antigen on red cells (and A antibody in the plasma); Group AB has both A and B antigens on red cells (but neither A nor B antibody in the plasma); and Group O has neither A nor B antigens on red cells (but both A and B antibodies in the plasma.) Aside from A and B antigens, there is a third antigen called the Rh factor, which indicates either present (+) or absent (-). This mean RH negative blood is given to RH-negative patients, and either Rh positive blood, or Rh negative blood may be given to Rh positive patients.

Type O Positive: The Most Common Blood Type And Why

O positive is the most common blood type present in 37 percent of the U.S. population. Blood types vary by ethnic group, with more Hispanic people possessing O blood type, while Asian people are more likely to be type B. Meanwhile, the universal red cell donor has type O negative blood type.

So why is type O positive the most common?

First, we need to understand clearly what 'dominant and recessive trait' means. Typically, every gene (with the exception of the Y Chromosome) comes in pairs, meaning we get two copies of every possible trait. Dominant means we only need one copy for the trait to express; recessive means we need both copies to express that trait.

When it comes to the ABO gene, there are three possible traits, including: A, B, or O. Traits A and B are dominant, and O is recessive. This means we only need one ABO gene that is type A for us to express type A blood; we only need one copy to be type B to express type B blood; but we need two copies of the O trait to have O type blood. The gene for type O is recessive, because we need to have one gene for O and one for A; then we still end up with A antigens on our cell membranes. So, in order to be type O, we need both parents to be O.

It’s important to remember alleles (different versions of a gene that determine specific traits) do not appear with equal frequency in the gene pool. This means a person expressing AA or Ai will have type A blood group; BB or Bi will be of type B blood group; AB will be of type AB blood group; and ii will be of the type O blood group.

The i allele is much more common than A, which is much more common than B. In other words, there are many more i alleles in the gene pool, so the chances of getting ii are higher than AA or Ai. Therefore, the likelihood of an ii pairing is higher than that of the Ai pairing, making O the most common blood type.

Type AB Negative: The Rarest Blood Type

People with AB blood inherited an A gene from one parent and a B gene from the other. Approximately 0.6 percent of the population is AB Negative (the rarest blood type), and 3.4 percent is AB Positive, which are potential universal plasma donors, although it's the most rare blood type.

AB plasma is universal, and can be used for all patients regardless of their blood type.

AB-negative is the rarest because it is only present in 1 out of 167 individuals. There are few combinations of alleles which can produce it, and the alleles are relatively rare. To produce AB blood, an individual must inherit an A allele and a B allele, not an O allele. The allele frequency for type O is higher, while the frequency for A and B is significantly lower. The odds of this particular combination are simply lower than any other combination.

The Other Blood Types: From O To AB

In the U.S., O positive is the most common, while AB negative is the rarest. However, there are six other blood types that are still present in the population. About 7 percent of the population is O negative; 34 percent A positive; 6 percent A negative; 9 percent B positive, 2 percent B negative; and 3 percent AB positive.

Every year, 5 million Americans need blood transfusions, and with an aging population and advances in medical treatments, there is a higher demand for blood. On any given day, an average of 36,000 units of red blood cells are needed. Therefore, knowing your blood type is vital to prevent the health consequences of a mismatched blood transfusion.

To find out your blood group, a sample of your blood must be taken and tested. Donating blood is one way to learn your blood type. It can be matched to someone with the same blood group or one that is compatible.

You can go to the American Red Cross website to find local blood drives in your neighborhood.