The short answer is no, siblings do not necessarily have the same blood type. While siblings share similar DNA, each child has a unique combination of genes inherited from their parents. This means siblings can have different blood types.
How Blood Types Are Inherited
There are four main blood types: A, B, AB, and O. Your blood type is determined by the types of antigens (proteins) on the surface of your red blood cells. Each blood type contains one or more of these antigens:
- Type A has the A antigen
- Type B has the B antigen
- Type AB has both A and B antigens
- Type O has neither antigen
You inherit the A and B antigens from your parents. Each parent has two alleles (genetic variants) for blood type – one inherited from their mother and one from their father. The alleles are:
Because you get one allele from each parent, there are six possible allele combinations you can inherit: OO, AO, BO, AA, BB, AB.
|AO or OA
|BO or OB
As you can see, siblings inherit different combinations of alleles from their parents. This means they can end up with different blood types.
Blood Type Examples for Siblings
Here are some examples of how siblings can have different blood types:
Parents are both Type O
If both parents have type O blood, they only have O alleles to pass on. Any children they have will also be type O, since OO is the only possible genotype.
In this case, all siblings will have the same blood type.
One parent is Type AB, one is Type O
If one parent is AB and one is O, the possibilities get more complicated. The AB parent has one A and one B allele. The O parent has two O alleles.
Possible allele combinations the children could inherit include: AO, BO, OO.
So siblings could be blood types A, B or O in this case. Each child has a 1 in 4 chance of being type O, a 1 in 2 chance of being type A, and a 1 in 2 chance of being type B.
Both parents are Type A
If both parents are Type A, their genotypes are AO. The possibilities for their children are AA, AO, and OO.
In this case, siblings could be blood types A or O. But some may be A while others are O, depending on the alleles they inherit.
Rhesus Factor Also Varies
In addition to the A and B antigens, blood also contains the Rh factor, which is inherited separately. You can be Rh positive (+) or Rh negative (-).
Siblings can have different Rh factors since this is also inherited randomly from each parent. For example, one sibling may be A+ while their brother or sister is A-.
Extreme Rare Cases
In very rare cases, it’s possible for:
- Siblings to have different blood types than their parents
- Identical twins to have different blood types
This can happen through mutations or rare genetic phenomena like chimerism. But generally, siblings will have blood types correlating to their family ancestry.
While siblings inherit similar DNA, the mix of genes and alleles each child gets is unique. This means siblings can end up with different blood types. The possibilities are determined by the genotypes of the parents. But each child has a random assortment of alleles passed down. So having the same parents is no guarantee you’ll share the same blood type with your siblings.