|I often get questions about a new baby's blood type. For example, a baby who is A- might be born to two parents who are both A+ and they are wondering how that could happen.
Although many people understand how the major blood groups are inherited, the Rh factor (the + or - sign after the blood type) is more confusing.
To begin with, you normally have two alleles (out of a possible three alleles - A, B, O) for your blood type and pass one of them to your child.
A person who has a type A blood type can have either two A alleles (AA) or they can have one A and one O (AO). The same for someone of blood type B, who can be either BB or BO.
If you are type O, then you will pass an O allele to your child because you must be OO. If you are AB, you can pass either the A or the B to your child.
For parents who are both type A, they can both be either AO or AA. Therefore, they could pass an A or an O to their child and they could have a child with either type A blood or type O blood. This child could not be type B or type AB.
Just like you have two copies of the A, B or O alleles, you also have two copies of the Rh factor alleles. So for two parents who are both Rh positive, they could be either +/ - or +/+. If both of them are +/-, then they could have each passed the - allele to their child and have a negative (-/-) child.
So two A+ parents could have children with the following blood types:
A+, A-, O+, O-
How about if one parent was A- and the other was A+?
Again, the baby could be either A+, A-, O+, O-. While the A- parent would only give an Rh- factor, the other parent could donate either a + or a - depending on whether he was -/+ or +/+.
What if they were both A-?
Then their baby could only be either A- or O-, since both parents are Rh - they must be -/- and can only give a - to their child. This child could not be A+ or O+ with two parents that are A-.
How about if mom is A+ and dad is B+?
In this case, mom could be either AA or AO and have either +/+ or +/- Rh factor alleles. So she could give her baby either an A or O allele and either a + or - Rh factor allele.
Dad could be either BB or BO and will also have either +/+ or +/- Rh factor alleles, and could give his baby either a B or O allele and either a + or - Rh factor allele.
So the baby could be either:
- AB : getting A from mom and B from dad
- A (AO) : getting A from mom and O from dad
- B (BO) : getting O from mom and B from dad
- O (OO) : getting O from mom and O from dad
And his Rh factor could be either + or -, depending on which Rh factor allele he inherits from each parent. So this baby could have any of the possible blood types.
With newer DNA testing, using blood types to determine paternity, or who the father is, isn't really that useful anymore. The main problem is that this method just tells you what the blood type the father could or could not have, and since many of these blood types are common, it doesn't necessarily help narrow down who the father could be.
Still, it sometimes does come up when a baby's blood type isn't what the parents expect it to be.