Human blood groups are classified according to the presence of certain antigens on the surface of red blood cells. The greatest practical significance of blood group systems is to do with blood transfusions, and the most important divide human blood types according to the ABO system and according to the Rh system, although many more are recognized by the International Society of Blood Transfusion.
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The ABO blood group is controlled by a single gene, located on the long arm of the ninth chromosome and thus, as all other autosome-located traits, inherited from both parents.
The gene controlling ABO blood group occurs in three alleles: A, B and O. A and B blood groups are dominant over the O group, and co-dominant in relation to each other. Thus, the six possible genotypes are expressed as four phenotypes.
AA and AO genotype results in a blood group A.
BB and BO genotype results in a blood group B.
AB genotype results in a blood group AB.
OO genotype results in a blood group O.
Understanding of the genetic basis of the blood group inheritance makes it possible, in some cases, to work out what blood group a child of particular parents will be if we know their blood group.
And thus, if both parents have a blood group O, they can only have a child with a blood group O.
If both parents are A, the child can be O or A (but not B or AB).
If both parents are B, the child can be O or B (but not A or AB).
If both parents are AB, the child can be A, B or AB (but not O).
If one parent is O and the other is A, the child can be O or A (but not B or AB).
If one parent is O and the other is B, the child can be O or B (but not B or AB).
If one parent is O and the other is AB, the child can be A, B, O (but not AB).
If one parent is A and the other is B, the child can have any blood group.
If one parent is A and the other is AB, the child can be A, B, AB (but not O).
If one parent is B and the other is AB, the child can be A, B, AB (but not O).
Knowledge of these inheritance patterns can be helpful in disproving (but not confirming) paternity. Nowadays, DNA testing is used for such purposes, but it used to be applied more commonly. It cannot positively prove paternity, but it can, in some cases, disprove it (although there are rare cases of mutations that cause exceptional inheritance which contravenes the normal pattern of inheritance, and means it is, in rare cases, for example possible for two O blood group parents to have a AB blood group child or for two AB parents to have a O group child).
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The D antigen, which determines the blood group in the Rh system, is controlled by one gene, located on the short arm of the first chromosome. Rh+ blood group is dominant and Rh- blood group is recessive, and thus two parents with Rh+ blood can have a child that has either a Rh+ or Rh- blood group, two parents with a Rh- blood will have a child with a Rh- blood group, while one parent with a Rh+ and another with a Rh+ can have a child with either Rh+ or Rh- blood group.
This blood group is clinically very significant, as a pregnancy of a mother with a Rh- blood who is carrying a fetus with a Rh+ blood can result in a Rh D Hemolytic disease of the newborn.
