Human blood type is determined by two co-dominant alleles and one recessive allele. An allele is one of several different forms of genetic information that is present in our DNA at a specific location on a specific chromosome (one of the different forms of a gene for a particular type of protein).

There are three different alleles for human blood type, A, B, and O: Each of us has two ABO blood type alleles, because we each inherit one blood type allele from our biological mother and one from our biological father. A description of the pair of alleles in our DNA is called our genotype. Since there are three different blood group alleles, there are a total of six possible different genotypes at the human ABO genetic locus. These six different genotypes result in only four Phenotypes, however because the O allele is recessive to both A and B.

Each biological parent donates one of their two ABO alleles to their child. A mother who is blood type O can only pass an O allele to her son or daughter because she carries only O alleles. A father who is blood type AB could pass either an A or a B allele to his son or daughter.

People with type A and people with type B blood are a little harder to figure out because they could have either of two possible Genotypes AA or AO for type A blood, and BB or BO for type B. There is an interesting molecular basis for the Dominant/ recessive relationship between the different Blood groups .

These different blood types are caused by the presence of a protein cell-surface marker*. An antigen on the surface of the red blood cells' plasma membrane.

(*note: Actually the A and B alleles encode Glycosylation enzymes which change the patterns of sugars on the membrane proteins, but for our purposes we will just look at the proteins).

When mixed with the wrong blood type, these antigens are bound by antibodies which, causes the cells to clump.

Someone with type-AB blood can receive any type blood with no ill effects, while people with type-O blood can only take their own type. People with type-A blood can receive A or O, and people with type-B blood can take B or O. Blood donors and recipients must be typed and matched very carefully before transfusions are given.

Group A Blood has A antigen on red cells, and anti-B antibody in its plasma. Group

B Blood has B antigen on red cells, and anti-A antibody in its plasma.

Group AB Blood has both A and B antigens on red cells but neither anti-A antibody nor anti-B anti body in its plasma. AB blood cannot cause the clumping of red cells of any other groups, and therefore persons with AB blood are called universal recipients.

Group O Blood has neither A nor B antigens on red cells, and both anti-A antibody and anti-B antibody in the plasma. Group O blood cannot be clumped by any human blood, and therefore persons with Group O are called universal donors

Before a transfusion is given, it is important to know which blood group a person has because the blood plasma contains strong antibodies, called anti-A and anti-B, that react against the red cells with A or B antigens.

If anti-A antibody came in contact with A antigen (or if anti-B antibody met B antigen), the result could be a dangerous, possibly fatal, transfusion reaction. To prevent such reactions, Medical Technologists will "crossmatch" your blood with donated blood. A sample of your blood and samples from donated blood are tested to make certain they are compatible .

Most people also have an another inherited protein of the red blood cells known as the Rh factor, or antigen D. When the D antigen is present, a person's blood type is designated Rh positive. When antigen D is missing, the blood type is classified Rh negative. In general, Rh negative blood is given to Rh negative patients and Rh positive blood to Rh positive patients

Your body produces antibodies to any proteins not present on the surface of your own blood cells. If you have type A blood, your body produces antibodies which will attack type B blood and so on. The various ABO blood groups occur in the average population of the US in these various proportions 45% of the population are type O, 42% type A, 10% type B and 3% type AB.

The percentage of people with any particular blood type also varies between different ethnic groups. Besides the A,B,O group, there are also several other blood groups to be considered, including the one called Rh antigen.



On to Genetic Testing

The below are exercises for practicing Genetics. Try to envision making and combining gametes, then determining the Genotype and Phenotype of the offspring.

Here is a blood type problem arising from a previous Bio 113 class.

An Atlas of Genes

Blood Groups & Genetics (IE only used in class)