blood types

     You have probably heard of blood types. You have a blood type. I have a blood type. Everyone has a blood type. It's useful to know your own blood type, but many people don't. There are four blood types: "A", "B", "AB", and "O" ( letter O ). The "A" means there is an "A" substance ( an antigen ) in the blood. The "B" means that there is a "B" substance ( an antigen ) in the blood. The "AB" has both the "A" and the "B" antigens. The "O" does not have either antigen. Blood type has nothing to do with age, race, gender or class. Everyone is born with a certain type of blood, and that's it. Each blood type also has a - ( -), for negative, or a (+), for positive. The blood is negative for ( doesn't have ) or positive for ( does have ) another substance, called a "rhesus factor", or "Rh factor".
    Each person carries a blood marker of a kind ( an allele ) from each of his or her parents. A person who has type "AB" blood has an "A" from one parent and a "B" from the other. We know what the two "markers" are. A person with type "O" blood has an "O" from one parent and an "O" from the other--or the blood wouldn't be type "O", so we know what the markers are.
   The "A" and "B" types are a bit different, detection-wise. A person with type "A" blood got an "A" from one parent, that's obvious. But the other marker may be either an "A" or an "O" ( meaning no antigen ), and the blood would still be type "A". We might call these two possibilities "AA" and "AO"--both type "A".
     The person with type "B" blood got a "B" marker from one parent, but is the other marker a "B" or an "O"? The blood would be type "B" either way. We could call these possibilities "BB" and "BO"--both type "B".
     One of the interesting things about blood type is that we can see a simple set of traits through a line of inheritance. For example, two people with type "O" blood will have only children with type "O" blood. Always. They have no "A" or "B" antigens to pass on to an offspring.
    Two people with type "AB" blood will have children with "A", "B", or "AB" blood. We might think of the possible combinations as "AA", "AB" and "BB".
    The other combinations are just as simple--one from each parent, each time. An "AA" and an "AO"?
"AA" or "AO" children--all type "A".
An "AO" and an "AO"? "AA", "AO", or "OO". Some children with "A", some with "O".
A "BB" and a "BO"? "BB" or "BO" children, all type "B".
A "BO" and a "BO"? "BB", "OO" or "BO" children--type "B" or "O".
"AB" and "AA"?  "AA" or "AB" children.
"AB" and "BB"? "AB" or "BB" children.
"AB" and "AO"? "AA", "AB", "AO", or "BO" children--type "A", "B", or "AB".
"AB" and "BO"? "AB", "AO", "BB", or "BO" children--type "A", "B", or "AB".
     The positive and negative blood factors are inherited in the same way, one from each parent. One positive makes a positive blood type, since the positive means the presence of the factor--not how much.
So it takes two negatives to make a negative. Parents may be ++, --, or +-, so we can follow the inheritance of the factor. Two parents with Rh negative blood have only Rh negative children. The "positives" may be carrying a "negative" to pass along to an offspring--when both parents contribute a "negative", the blood type is negative. When one parent contributes a positive, the blood type is positive.
      Almost as easy as following a head of red hair or a case of near-sightedness.