Blood Type

There are 4 possible blood types: A, B, AB, and 0.

To understand blood types, you have to know a little bit about genetics, so here goes...

Blood types have three alleles: A, B, and o. Each person only has two of these alleles.

• Allele- a form of a gene that can be used to control a trait.
• The possible combinations of alleles for blood type are: AA, Ao, BB, Bo, AB, and oo.
• A and B are dominant alleles, meaning they will express their characteristics over the o allele. Dominant alleles are normally shown in CAPITAL letters.
• o is recessive, if Ao or Bo is present, the blood type will be Type A or B respectively. This person will carry the allele for type o blood, but will not have type o blood. Two recessive genes must be present in order for the recessive trait to be expressed. So, the only way someone can have Type O blood is if their alleles are oo. Recessive alleles are usually shown in lowercase letters.
• Now, in blood typing, A and B are actually co-dominant genes, meaning that one doesn't hide the presence of the other, they are both expressed. This codominance is why someone can have Type AB blood.

Okay, so if you're with me so far, then here are the combinations of alleles that produce each blood type:

Type A- AA or Ao	Type AB- AB
Type B- BB or Bo	Type O- oo