Can siblings have different blood types?

Q: Can siblings have different blood types? 

Krishna: A blood group is a classification of your blood based on inherited antigens (markers) on your red blood cells and antibodies (proteins) in your plasma. The main types are A, B, AB, and O, further categorized by the presence or absence of the Rh factor, creating the common blood types A+, A-, B+, B-, AB+, AB-, O+, and O-. These classifications are crucial for safe blood transfusions, as incompatible blood types can trigger a severe immune response.  

Siblings can have parents with same or different blood groups. In such cases genetics dictate what type of blood group they can inherit. see the table below.  

So it depends on several things.

Your blood type is passed genetically from your parents. You inherit a gene from each parent so your blood type may not be the same as your parents.

Several people ask whether twins can have same blood group. That again depends on what type of twins they are. Twins can be identical or fraternal. 

Identical twins, also known as monozygotic twins, are the result of a single fertilized egg splitting into two embryos. While there have been reported cases of genetic mutations that have led to differentiated blood types among twins, these are exceedingly rare. Generally speaking, identical twins are born with exact same genetic material, including the genes that determine blood type, so yes, identical twins have the same blood type.

Fraternal twins, or dizygotic twins, are the result of two separate eggs being fertilized by two different sperm. This makes fraternal twins as genetically similar as any other siblings, meaning they share about 50% of their DNA. As a result, they can inherit different blood types, just like regular siblings.

So fraternal twins do not always have the same blood type. They might share a blood type, but they are just as likely to have different blood types depending on the specific genes passed down to them.

Rh Factor: The Rh factor (positive or negative) is inherited separately from the ABO blood type, further increasing the possible combinations. 

The Rh factor works in a similar manner.

Your Rh factor is determined by whether or not your blood contains a specific protein. If you have it, you are positive. If you don’t have it, you are negative.  

• If both your parents are Rh negative, you are probably Rh negative.
• If either parent is Rh positive, you could be either Rh negative or Rh positive.

Rare blood types emerge due to specific  genetic mutations that create uncommon combinations of blood antigens. These mutations are inherited from parents or arise through spontaneous genetic changes, with certain rare blood types becoming more common in specific ethnic or geographic groups.  Because a rare blood type lacks an antigen that is present in the majority of the population, finding a matching blood donor for these individuals can be extremely challenging. 

Some rare blood types, such as the "golden blood" (Rh-null), are caused by genetic mutations. A mutation in the RHAG gene,  for example, can prevent Rh antigens from being properly expressed on red blood cells.