Similar to the ABO blood type (see https://qa.healthtopquestions.com/120546/abo-blood-type-all-people-want-to-know), the Rh blood type is also determined by the proteins on the surface of the red blood cells.
Rh stands for rhesus, a kind of monkey. The name comes from the Rh factor, the determining element of the Rh blood type system that was discovered in the rhesus monkeys. Both the ABO blood type and the Rh blood type are critical for clinical blood transfusions.
Rh blood type is related to red blood cells
Red blood cells (RBCs) are cells in the blood that mainly function to carry oxygen to various organs. There are many proteins on the surface of the RBCs, one of which is the Rh factor.
If your blood has the Rh factor, you are Rh+ (Rh positive). If you don't have the Rh factor in your blood, you are Rh- (Rh negative).
Rh factor is determined genetically
The genes that control the Rh factor follow a common pattern of genetic inheritance. If both parents are Rh positive, the baby is Rh positive. If one of the parents is Rh positive and the other is Rh negative, the baby can be Rh positive or Rh negative. If both parents are Rh negative, the baby is Rh negative.
Rh blood type is not evenly distributed
We know that there are 4 ABO blood types, i.e., A, B, AB and O. Although these blood types are not strictly evenly distributed among a population, the number of people with each blood type won't differ too much. That is to say, in a given population, people with blood type A may be more than people with blood type B, but there won't be a huge difference in numbers.
By comparison, people that are Rh positive are far more than those who are Rh negative. In the US, for example, over 85% of people are Rh positive.
Despite that both the Rh and ABO blood types are determined by the proteins on the surface of RBCs, these two systems are controlled by different genes.
Rh blood system in clinical blood transfusions
If you are Rh positive, you can only receive Rh positive blood; similarly, if you are Rh negative, you can only receive Rh negative blood. Both ABO and Rh blood type systems are critical in blood transfusions for doctors to determine which kind of blood you can receive.