A. Classification
Everybody's blood looks the same. But under a microscope, distinct differences are visible. Two types of chemical molecules (A and B) can be observed in blood cells. Depending on what chemical molecules are present, bloods can be classified into 4 main types:
A - if the blood cells have only A molecules
B - if the blood cells have only B molecules
AB - if the blood cells have a mixture of both A and B molecules
O - if the blood cells have neither the molecules
In addition, a particular protein may also be present in the blood cells. If the blood contains that protein, it is said to be positive. if not, it is negative. Thus, the blood type can be further classified into 8 types: A+, A-, B+, B-, AB+, AB-, O+, O- accordingly.
B. Matching
If two different blood types are mixed together, the blood cells may begin to clump together in the blood vessel, causing a potential fatal condition. Therefore, it is important that blood types be matched before blood transfusions take place. As shown in Figure 1, type O blood can be given, apart from O itself, to all blood types, A, B and AB. Type A blood can be given, apart from A itself, to blood type AB. Type B blood can be given, apart from B itself, to blood type AB. But type AB blood can be given to AB itself only.
Furthermore, positive blood types can only be given to positive blood types, whereas negative blood types can be given to both positive and negative blood types. As such, the possibilities of blood types for blood transfusion are summarized as in Figure 2 accordingly. As can be seen, blood type O- can be given to all blood types, and hence is usually regarded as 'universal donor'. Blood type AB+ can take any blood types for transfusion, and hence is usually regarded as 'universal receiver'.
C. Popularity
Type O blood is the most common type found in the population (49%), followed by type A (38%) and type B (10%), with type AB being the rarest (3%). Within each of the 4 man blood types, negative blood types are much rarer than their positive counterparts. Figure 3 shows the percentages of Australians with particular blood types.
D. Inheritance
Blood type is determined by a gene (ABO) in chromosome 9. Since chromosome comes in pair; one from each parent, so the gene also comes in pair, one from each parent accordingly. The gene is called type:
A - if it contains genetic material to produce chemical molecule A in blood cells
B - if it contains genetic material to produce chemical molecule B in blood cells
O - if it contains neither of those to produce any chemical molecule in blood cells.
Since one copy of gene comes from each parent's sperm and egg respectively during conception, the resulting gene pair and thus the resulting blood type of the child can be as follows:
| Gene Type from One Parent | Gene Type from Another Parent | Gene Pair of Child | Blood Type of Child |
| A | A | AA | A |
| A | B | AB | AB |
| A | O | AO | A |
| B | B | BB | B |
| B | O | BO | B |
| O | O | OO | O |
From this table, we can then deduce, although a bit more complicated, blood type of the child as inherited from the blood types of the parents:
| Blood Type of One Parent | Gene Pair of One Parent | Blood Type of Another Parent | Gene Pair of Another Parent | Gene Pair of Child | Blood Type of Child |
| A | AA or AO | A | AA or AO | AA, AO or OO | A or O |
| A | AA or AO | B | BB or BO | AB, AO, BO or OO | A, B, AB or O |
| A | AA or AO | AB | AB | AA, AB, AO or BO | A, B or AB |
| A | AA or AO | O | OO | AO or OO | A or O |
| B | BB or BO | B | BB or BO | BB, BO or OO | B or O |
| B | BB or BO | AB | AB | AB, BB, AO or BO | A, B or AB |
| B | BB or BO | O | OO | BO or OO | B or O |
| AB | AB | AB | AB | AA, AB or BB | A, B or AB |
| AB | AB | O | OO | AO or BO | A or B |
| O | OO | O | OO | OO | O |
In addition, the positive/negative aspect of blood type is determined by a separate gene (RH) in chromosome 1. The gene is called type:
+ - if it contains genetic material to produce a particular protein in blood cells
- - if it doesn't contain genetic material to produce the particular protein in blood cells
Similar to the main blood type above. since one copy of gene comes from each parent's sperm and egg respectively during conception, the resulting gene pair and thus the resulting positive/negative blood type of the child can be as follows:
| Gene Type from One Parent | Gene Type from Another Parent | Gene Pair of Child | +/- Blood Type of Child |
| + | + | ++ | + |
| + | - | +- | + |
| - | - | -- | - |
Again, from the table, we can then deduce, albeit much simpler, the positive/negative blood type of the child as inherited from the positive/negative blood types of the parents:
| +/- Blood Type of One Parent | Gene Pair of One Parent | +/- Blood Type of Another Parent | Gene Pair of Another Parent | Gene Pair of Child | +/- Blood Type of Child |
| + | ++ or +- | + | ++ or +- | ++, +- or -- | + or - |
| + | ++ or +- | - | -- | +- or -- | + or - |
| - | -- | - | -- | -- | - |
....end
