How is blood type derived?

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Blood Type Distribution
A and O are the most common blood types worldwide. B relatively less common than A and O in population in some countries such as Germany and Spain (about 9%). AB is the rarest of the blood types: this blood type is found in about 3.4% of the US population, 4% of Europe's population but can be as high as 10% in some Asian countries like Japan and Korea.

Universal Receivers and Universal Donors
People with blood type AB are known as "universal receivers" because they can receive blood from any type but can only give blood to people with blood type AB. People with blood type O are known as "universal donors" because they can give blood to any type but can only receive type O blood. (I'm sure you have heard some people say that that those with blood type AB are selfish while those with blood type O are generous).

What is blood type?
The blood type is a description of which antigens are present in the blood. A person with A blood has A antigens on the blood cells. B blood contains B antigens. A person with AB blood has both types of antigens and O blood has no antigens.

How is blood type derived?
Blood type is determined by two sets of genes - the inheritance from the mother and father.
Like genes, some blood types are dominant over others. A and B genes are dominant over O gene. This means that if you receive an A gene from your mother and an O gene from your father, your blood type will be A (contains A-O). It is also possible for you to be of blood type A if you receive an A gene from each of your parents (contains A-A).

Let me use a few examples to explain possible scenarios and outcome.

E.g. 1
Both parents are of blood type O.

O-O and O-O

Possible permutation: O-O
Children will be 100% of blood type O.

E.g. 2
Both parents are of blood type AB (A-B).

A-B and A-B

Possible permutations: A-A, B-B, A-B
Children can be of blood type A, B or AB.

E.g. 3
Both parents are of blood type A. Blood type A means they can have both A and O genes (A-O) or only the A gene (A-A).

If parents are A-A and A-A, then child will be: A-A

If A-O and A-O, possible permutations: A-O or A-A

If A-O and A-A, possible permutations: A-O or A-A

Therefore, children can be of blood type A or O.

E.g. 4
One parents is of blood type A (can be either A-O or A-A), other parent of blood type B (can be either B-O or B-B).

If A-O, B-O, possible permutations: A-O, A-B, B-O

If A-O, B-B, possible permutations: A-B, B-O

If A-A, B-O, possible permutations: A-B, A-O

If A-A, B-B, possible permutation: A-B

Therefore, children can be of blood type A, B, AB or O.

I know for a fact that this is true because my parents have blood types A and B. I have 2 sisters: one has blood type B, another has blood type O and mine is AB.

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