Trying to figure out the genetics of human blood types can be confusing. Check out Dr. Greene’s Guide To Understanding Blood Types for a description of the ABO blood types and the Rh system.

Human blood genetics are quite complex because at each point there are a number of possible characteristics. Nevertheless, the genetics of human blood is far better understood than that of any other human tissue. To make things a little easier to understand, here are some guidelines for understanding how both the ABO blood types and the Rh system work.

The ABO Blood Types

• Each person receives an A, a B, or an O gene from each parent.
• The A and B genes are co-dominant, and the O gene is recessive.
• A person whose genetic type is either AA or AO will have blood type A, those with genetic type BB or BO will have blood type B, and only those with genetic type OO will have blood type O.
• A child with type O blood can have parents with type A, type B, or type O blood, but not type AB. Conversely, if two parents both have type O blood, all their children will have type O blood.

The Rh System

The Rh system is actually far more complex than the ABO system in that there are 35 different possibilities that one could inherit from each parent. These, however, are roughly grouped into positive and negative types. In this system the positive are dominant over the negative.

• If your genetic type is ++ or +-, your blood type will be Rh positive.
• Only if your genetic type is -- will you be Rh negative.
• If both parents have Rh+ blood with the +- genes, they could have children who are ++, +-, or --. In other words, their children could be either Rh positive or Rh negative.
• Most children who are O negative have parents who are positive, since the +- combination is so much more common than the -- combination.