Blood Groups

Blood is composed of cells suspended in a liquid-like substance called plasma; these cells are red blood cells (carry oxygen), white blood cells (fight infection) and platelets (stop bleeding in injuries). Human blood is divided into various systems; these need to be matched for blood transfusions.

The ABO system: the four groups are A, B, AB and O.

• The gene on chromosome 9 can be A, B or neither (O). 
• A and B are dominant, while O is recessive.
• You inherit one gene from your mother and one from your father.
• O individuals have two copies of the O version (OO)
• AB individuals have one A and one B gene.
• A individuals have either two A versions or an AO combination
• B individuals either two B versions or a BO combination.

There is an incompatibility of O group mother and AO foetus; the mother has an anti-A which can pass across the placenta and damage an AO foetus causing miscarriage (often so early as to noticed only as a 'missed period') or severe haemolytic anaemia at birth.

The Rhesus factor: Each of the groups A, B, AB and O can be positive or negative for a specific protein, known as the Rhesus factor.

• If the mother is Rh negative and the father Rh positive, the foetus is at risk of haemolysis and jaundice, usually resulting in death (late foetal or perinatal). Surviving foetuses are often mentally retarded. Foetal red blood cells pass into the mother's blood circulation; firstborn are infrequently affected (5%) but sensitize mother for subsequent pregnancies (50% in 3rd or later births). To protect against this effect, after birth an anti-rhesus serum is now injected into the mother to destroy any foetal cells still in her blood; the serum itself is destroyed in a short time.

Blood donation

• AB can receive blood from any other blood type.
• O can donate blood to all other blood types. 
• A can receive A and O blood types but not B. 
• B can receive B and O blood types but not A.

Frequencies
Frequencies of each blood group vary between human populations. In the UK, the frequency of type A increases as you go north.

• UK: O = 48%   A = 38%   B = 10% and AB = 3%
• Southern England: O = 44%    A = 45%    B = 8% and AB = 3%

• Group B is nearly three times as common in Oriental populations as in Europeans. 
• High frequencies of group B in European gypsies (50% as opposed to 10% in Northern Europe) indicate their recent origins in India.

• A gene map of rhesus negative distribution shows a clear gradient from 25% of the West European population to 5% in the Middle East. It is thought that it may be the European norm but bred out by intermarriage with waves of settlers from the east who have a low frequency of the gene.

Rhesus negative blood group

• 2% of modern Indo-Europeans
• 4% to 8% of African Americans
• 35% of Basque people
• 15% of Caucasians of southern Russia 

Sources

• E.J. Clegg: The Study of Man.
• Walter Bodmer & Robin McKie: The Book of Man: the quest to discover our genetic heritage. Scribner, 1995.