Maybe I've screwed up here, but I'm not sure how. Looking at
the Rhesus factor first since it is simplest:

For her to be Rh positive means that she is producing a
particular protein, and she must have inherited the gene for it
from one or both parents. Since it is a dominant gene, that
means that at least one of her parents must have been Rh
positive as well.

As for ABO, it is a little more complicated. A person can inherit
A, or B, or nothing (O) from each parent. The possible
genotypes are:
AA, AO, or OA, giving a type A child
BB, BO, or OB, giving a type B child
AB, or BA giving a type AB child
or finally OO, giving a type O child.

So, for your daughter to be type B+ implies that at least one
parent carried that B gene, so at least one parent must have
been B, or AB. Furthermore, at least one must have been Rh+.
That was my logic, but it hinged on your daughter actually being
B+.

Now....

You say her blood type was recorded as A- at birth, and this is
entirely consistent with the parental types you gave, both also
being A-.

I think you may be right, and a blunder was made in recording
her blood type after the recent donation. It is something you
should take up with the transfusion centre, since, if she is
carrying a card that gives her blood type as B+, when in fact it is
A-, should she be given a transfusion of B, or any Rh+ blood,
there would be major problems.

Edited by xarqi on February 28 2005 at 11:10 PM

The transfusion centre mix-up does seem most likely, but
another possibility has occurred to me; one that I am reluctant
to raise, and which I hope a specialiest here can dispel. I feel I
must at least mention it, no matter how unlikely it may be, for
the sake of completeness. I apologise if the following gives you
undue concern. It is merely a theoretical possibility.

There is a chance that the neonatal A- result, and the current
B+ result are both correct, but that the baby in the bassinette
was not the one for which the A- result was obtained. To be
blunt, identification cards, or indeed infants, may have been
mistakenly exchanged after the neonatal test was done, but
before you took your baby home. Your daughter may not
actually be your biological offspring.

I do hope this is not so, but it is a possible scenario that is
consistent with the facts, and one which you should be aware of.
One conclusive test to rule this out would be a comparison of
mitochondrial DNA between yourself and your daughter. If she
is indeed your biological offspring, then they should be
essentially identical.

Again, my apologises if this causes you undue concern.