Genetics of Blood Types
Co-Dominance and Multiple Alleles
- Co-Dominance occurs when both genes/alleles in the genotype are equally dominant.
- Multiple Alleles occurs when more than 2 genes/alleles determine a characteristic, such as in ABO blood groups.
- Example of ABO Blood Types
There are 4 different blood types - A, B, AB and O.
BLOOD TYPE OR PHENOTYPE | GENOTYPE | ANTIGEN PRESENT | ANTIBODY PRODUCED |
A | AA or AO | A | Anti-B |
B | BB or BO | B | Anti-A |
AB | AB | A and B | none |
O | OO | none | Anti-A and Anti-B |
- Example 1 - Blood Types
Mum has blood type AB and Dad has blood type O. The possible blood types of the children are...
Possible genotypes = 1 AO : 1 BO
Possible phenotypes = 1 A : 1 B
1⁄2 the children will be A blood type, and the other 1⁄2 will be B blood type.
- Example 2 - Blood Types
Mum has A blood type and Dad has AB blood type. The possible children's blood types are...
First Possibility
Possible genotypes = 1 AA : 1 AB
Possible phenotypes = 1 A : 1 AB
1⁄2 the children will have blood type A, and the other 1⁄2 will have blood type AB.
Second Possibility
Possible genotypes =1AA:1AO:1AB:1BO
Possible phenotypes = 2 A : 1 AB: 1 B
1⁄2 will have blood type A, 1⁄4 will have blood type AB, and 1⁄4 will have blood type B.
Antigens and Antibodies in ABO Blood Types
- The blood type is so-called because the blood contains particular antigens - A, B, both A and B, or neither A nor B.
- The body produces antibodies to neutralise any particle (e.g. bacteria, dust, foreign blood in transfusions) that it recognises as foreign. For example, if blood type A contains Antigen A, then it will produce antibodies against B blood type (Anti-B Antibody), because B antigens are foreign. Similarly, if blood type O contains neither antigens A nor B, then a person with blood type O would produce anti-A and anti-B antibodies.
- Agglutination or 'Clumping' - If antigen-A came in contact with the antibody against it (Anti-A), then the blood would clump or clot. This could occur in an incorrect blood transfusion.
- Universal Recipient - This is a person with blood type AB who can receive a blood transfusion from any of the other blood types.
- Universal Donor - This is a person with blood type O who can donate blood to any other blood type.
Complete Dominance
(Dominant-Recessive Inheritance)
- An Example of Rhesus Factor in ABO Blood Types
- The ABO blood types are sub-divided into positive and negative types also, depending on whether that blood type does or does not contain the Rhesus Factor.
- If the Rhesus Factor is present, the genotype contains one or two R genes/alleles. If the Rhesus Factor is absent, the genotype is rr.
Blood Type | ABO Antigens Present | Rhesus Antigens Present | ABO Genotype | Rhesus Genotype |
A+ | A | yes | Aa or AO | RR or Rr |
A- | A | no | AA or AO | rr |
B+ | B | yes | BB or BO | RR or Rr |
B- | B | no | BB or BO | rr |
AB+ | A and B | yes | AB | RR or Rr |
AB- | A and B | no | AB | rr |
O+ | none | yes | OO | RR or Rr |
O- | none | no | OO | rr |