To the naked eye blood can look like a simple red fluid but in reality it is a complex living tissue containing a large number of cell types and proteins that differs from person to person. To be able to distinguish people who share the same combination of cell types and protein these have been put in different blood-group systems and are know as blood types. As with a lot of other things in the human body the blood type of every person is decided by genetics. Knowing a person’s blood type is very important in medical situations as mixing blood of different type can be potentially lethal.
There are a number of different systems used for classification of blood but the most important and most know are the ABO and the Rh (Rhesus) systems because they are the most significant when it comes to human blood transfusion. In the ABO system there are 4 different blood types: A, B, AB and O where the letter represents the red blood cell type. A person’s red blood cell type is determined by the inheritance of 1 of 3 alleles from each parent: A, B or O.
Both A and B are dominant over O meaning that if they are present together A or B decides the blood type. A and B on the other hand are co-dominant to each other meaning that if both are present they will both determine the blood type. In a simplified way that means if a person gets an A and an O from their parents their blood type will be A. If they get a B and an O the blood type will be B. An A and a B gives AB while two O alleles will make the blood type O. There is however situations inheritance of blood type does not rules these straightforward rules.
The Rh system currently consists of 50 defined blood-group antigens but is often only used to describe the presence of the D antigen also called the Rhesus factor. A person who is Rh positive does have the D antigen while a person who is Rh negative does not. Inheritance of the D antigen is determined by one gene with various alleles but for simplicity one can think of alleles that are either positive or negative for the D antigen with the positive alleles being dominant. This means that only if a person receives the alleles negative for the D antigen from both parents will the persons Rh factor be negative. It also means that if either parent only has the positive alleles the child will be Rh positive.