Can an AB and an O blood type parent give birth to an O type child?

I read a little about the ';Bombay Phenotype'; which I believe makes this possible but I wanted a little more infoCan an AB and an O blood type parent give birth to an O type child?
OK, I've already half-answered this elsewhere, so some of this is copied across.





To have an O type child when either of the parents is AB is 'impossible'. This is because ABO blood type is generally determined by 2 alleles (gene versions) per person, one each inherited from each parent, out of the possible 3 types: A, B and O. O alleles are recessive to A and B ones, such that only 2 O alleles (one from each parent) results in O-type blood. AB-type blood usually only appears in people who have 1 A allele and 1 B allele - hence no O allele to pass on to a child, hence no O type children. This applies the vast majority of the time, and an O child of an AB parent might consider a DNA test.





An exception to this is a very rare gene named cis-AB, where due to crossing over, an allele has formed conferring a version of AB-type blood with a single allele. This allows space for another allele, such that an AB-type person might also have an O allele they can pass on. Again though, this is very rare.





Not much more common, Bombay blood by contrast doesn't strictly speaking yield an O child from an AB and O, rather Bombay blood shows up as O type on the standard test. Bombay blood is due to a recessive gene at a different locus to the ABO genes. Most of us have H %26amp; H alleles at this location. Some of us have H %26amp; h, meaning we carry the Bombay blood gene such that 2 H %26amp; h people could have a child who is h %26amp; h. An h %26amp; h child does not develop even some of the antigens on their blood cells which are on all 4 normal ABO types' including O type. As a result, A and B antigens cannot form on these cells either, the person tests negative for A and B, and is labeled O by default. However, in fact a transfusion of O blood could kill them, just as with other mis-typed blood. Regardless of this, the point is that such a person could have genes for A or B blood at the ABO locus, yet this be overridden by the Bombay locus.





At least as likely is that the genes which code for A or B type in the child have failed to be expressed for some less well defined reason. O is the default option on the ABO locus after all, (since it initially just means an absense of A and B antigens) so a compromised A allele might result in O type blood. Again, this would be preferable to having Bombay blood, as those with the latter are in trouble if they need a blood transfusion.





If this is a practical question, it is also possible for the tests to be misread such that someone who really is A type (in particular) is shown as O. Some sub-types react weakly to a commonly used test.





HTHCan an AB and an O blood type parent give birth to an O type child?
Yes

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