Typically, cells have certain parts that are can be called antigens. These are parts of the cell that the immune system can target - if the antigen is not from the body ('self'), the immune system will attack the cell. When it comes to red blood cells, there are two principle antigenic sets - the ABO group, and Rhesus group. There are many other antigenic sites, but those two are by far the most important. That's why when you are informed of your 'blood' group, only these two systems are mentioned (if you get blood from the wrong group, your body might well attack it, causing all sorts of problems).
As for the ABO system, you inherit one version of the gene (allele) from each parent. Each parent contributes one of three options - A, B, or O. Thus, the gene possibilities (genotypes) in an INDIVIDUAL (who has one allele from each parent) are AA, AB, AO, BB, BO, OO.
The A gene makes an A antigen, and a B gene makes a B antigen. But there is a minor catch. The O gene is fundamentally functionless, and doesn't code for any O antigen. This is important in understanding the answer to your question, and the following should explain why.
Since the O gene is non-functional, a person with OO genes produces no antigens, and therefore is blood type O.
A person with OA or AA genes produces only type A antigens, and, therefore, has blood type A.
Similarly, the genes OB and BB give type B blood, and the genes AB give blood type AB.
Don't worry about the + or - part of the blood group for now. This simply refers to the presence (+) or absence (-) of the other major antigen group: Rhesus.
So, ignoring the Rhesus part, if a mother is A, she could have the genotype AA or AO (as shown above). Similarly, the father could have a BB or a BO genotype. Thus, If the first child got the A from the mother and the B from the father, (s)he could be AB. And, similarly, if the second child got O from the mother and O from the father, (s)he could be O.
And, inserting the Rhesus groups back into the equation, both parents are Rhesus +, and so are the kids, which is what you would expect, since their genes come from their parents.
So, the answer to your question is 'yes'.
Whew! OK, if you're satisfied that you understand, let's summarise:
- If the both parents have blood type A, the children must be either type A or type O [Why type O? Remember that type A blood could be due to AO genotype. If both parents happen to pass on their O gene, the child will have an OO genotype, and will be type O].
- Similarly, if both the mother and father are type B, the children must either be type B or type O.
- And, if both parents are type O, the children can only by type O.
So far so good. What happens if the parents have different blood types?
- If one parent is A and the other B, then the child could be A, B, AB or O. In other words, any possibility is an option here.
- If one parent is A and the other O, then the options are A or O.
- Similarly, if one parent is B and the other O, then the options are B or O.
Lastly, let's consider the tricky AB blood type:
- If one parent is AB and the other O, the options are A or B.
- If one parent is AB and the other is anything but O, the options are A, B or AB.
And the rhesus group? This is dominantly expressed, meaning that one 'positive' allele is all you need to be "rhesus positive". The only way to be rhesus negative is to have received a negative allele from each parent. Note that the parents don't have to be Rh- themselves in order to produce Rh- children. It is enough that they simply had one Rh- allele each (even if their other allele was Rh+ and so they were Rh+ overall) and that they passed that one on to their child.
Feel free to email me if you've got questions, or want to check your answers. HOWEVER, please note that this information is not enough to conclusively establish parentage, since there are various exceptions to the above. I'll deal with establishing parentage in a future post.