Blood groups, co-dominance and haemophilia

​​In a nutshell

Blood groups are important to understand as the mixing of blood groups when donating blood can be fatal. There are four blood groups: A, B, AB and O, named after the antigen that the red blood cells carry. Blood disorders such as haemophilia are an example of how problems with blood can be fatal. 

Blood groups

Red blood cells have antigens on them, which are proteins that our immune system uses to recognise a blood cell as our own. There are two types of antigens on blood cells: antigen A and antigen B. 

A person's blood type will be named after the antigens that they possess, which means there are four blood types: A, B, AB (where red blood cells have both antigen A and B), and O (where red blood cells have no antigens).

​Our immune system has antibodies against every antigen type apart from the one we possess. This becomes particularly important during blood transfusions, as if someone receives red blood cells that don't match their blood type, they will have a severe and potentially fatal reaction to the transfusion. The antigen on the transplanted red blood cells is recognised by the immune system as not of the same type already possessed; antibodies will attack the antigens on the red blood cells; a blood clot will form; and this will cause the patient a lot of harm.

Co-dominance and inheritance

Blood group is determined genetically, it will therefore be inherited from parents. To possess antigen A, you need to inherit allele $$I^A$$. To possess antigen B, you need to inherit allele $$I^B$$. An absence of either antigen is coded for by allele $$I^O$$. 

Both antigen A and antigen B can be expressed at the same time, as the alleles for red blood cell antigens are co-dominantly expressed. This means that both the alleles present are expressed. 

Example

Charlie's mother has blood type AB. Charlie's dad has blood type A and he is homozygous dominant for the A antigen. What are the possible genotypes and resulting blood type that Charlie could inherit?

Charlie's father has the phenotype $$I^A I^A$$ and his mother has the phenotype $$I^A I^B$$. So, Charlie will inherit the $$I^A$$ allele from his father but could inherit either the $$I^A$$ or $$I^B$$ allele from his mother. 

Therefore, Charlie has a $$50\%$$ chance of having a phenotype of $$I^A I^A$$ and having blood type A and a $$50\%$$ chance of having a phenotype of $$I^A I^B$$ and having blood type B.​

Haemophilia

Haemophilia is a genetic condition in which someone's blood does not clot easily. This means that they can bruise easily, or bleed much more than usual when they are injured. In some people, the lack of blood clotting is so severe that small injuries can become fatal. 

Haemophilia is a sex-linked recessive genetic condition. This means that the allele is carried on the X chromosome, and has to be recessive in order to cause the disorder. The presence of a dominant allele will prevent someone from having the disorder. 

Either two X chromosomes carrying the recessive allele must be inherited for females to have the disorder, or one X chromosome carrying the recessive allele and one Y chromosome must be inherited for males to have the disorder.