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The Rh Blood Type System 


Related Article: Rhesus Disease


When we are told our blood type, it is usually expressed as a letter followed by either a positive (+) or negative (-).  This + or - refers to the Rh system of blood

testing and is usually quoted in combination with the ABO system described earlier in this article, e.g. AB+ or O-.


The Rh system is actually much more complicated than the ABO system because there are more than 30 combinations possible when inherited, however for general usage, the Rh proteins are grouped into two families - either positive or negative.


As with the ABO system, there is a dominant allele which happens to be the positive family.  This means that the genetic pairs that can exist in humans are as follows:


Genetic makeup Blood type
++ Rh positive
+- Rh positive
-- Rh Negative


The same inheritance rules apply as on the previous page:


Parents' Rh types Possible allele combinations Possible Rh in the children
Both  + ++  &  ++ ++ positive
Both  + ++  &  +- ++ or +- positive
Both  + +-  &  +- ++ or +- or -- poitive or negative
Both   - --  &  -- -- negative
One + & One - ++  &  -- +- positive
One + & One - +-  &  -- +- or -- poitive or negative


1. What is a blood type? 

2. How blood types are inherited

3. The Rh blood type system - positive or negative

4. Paternity testing and blood types

5. Examples of blood type inheritance

6. A quick history lesson




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