Biology Question #595
Michael Comeau, a 16 year old male from the Internet asks on January 25, 2002,
What must occur for two parents with blood type AB to have a child with blood type O?
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Michael Linsey, MD, Nephrologist in Pasadena, California
answered on February 16, 2005
It's nearly impossible for two AB parents to have a child with blood type O. But in very rare cases it can happen if one parent has a cis-AB gene.
The genes for the ABO blood-type system are located on chromosome 9. Everyone has two copies of each gene, one from their mother and one from their father. The “A” gene makes an enzyme which can put a sugar called 'N-acetyl-N-galactosamine' on red blood cells - we'll just call it the “A” sugar. The “B” gene makes an enzyme which puts a different sugar “D-galactose” on red blood cells; we'll call it the “B” sugar. The “O” gene makes an enzyme that doesn't put any sugars at all on the red cell.
People have type “A” blood if they only have the “A” sugar on their red cells. So people with blood type “A” either have one chromosome 9 that has the “A” gene and one chromosome 9 with the “O” gene or they have two chromosome 9s both with the “A” gene. In either case only the 'A' sugar can be put on red cells.
You have type “B” blood if you only have the “B” sugar on your red cells. So people with blood type”B” either have one chromosome 9 with the “B” gene and one with the “O” gene or they have two chromosome 9s both with the “B” gene. Again, in either case only the “B” sugar can be found on their red blood cells.
Type 'O' people have neither the 'A' nor the 'B' sugar on their red cells. So they usually have two chromosome 9s both with the “O” gene. (Note: in rare cases you can have a condition in which the enzyme that makes the sugars is present but the hook where the sugars are supposed to be attached isn't made: this is called O-Bombay and is another story.)
Type “AB” people have both “A” and “B” sugars on their red cells. In 99.999% of cases this is because they have one chromosome 9 with an “A” gene and one with a “B” gene. So most of the time when a type “O” person and a type “AB” individual have children half are “A” and half are “B” depending upon whether they get the chromosome 9 with the “A” or the “B” gene from their type “AB” parent.
There is, however, the quite rare “cis-AB” gene in which one chromosome 9 either makes BOTH the “A” and “B” enzymes or makes an enzyme which can put BOTH the “A” and “B” sugars on red blood cells. If you have a “cis-AB” gene on one chromosome 9 you could have a “O” gene on the other chromosome 9 and have type “AB” blood since the “cis-AB” gene product could put both “A” and “B” sugars on your red cells.
In this case if you were to have children with a type “O” person (who would have two chromosome 9s each with an “O”gene) - one-half of your offspring would be type “AB” and one-half type “O” depending upon whether they got your chromosome 9 with the “cis-AB gene” or the one with the “O” gene. So a type “AB” and a type “O” person can have type “O” or type “AB” children if the type “AB” person has the rare “cis-AB” gene. And theoretically two “cis-AB” people -- each with one chromosome 9 with the cis-AB gene and one with the O gene -- could have children one-fourth of whom would get the “O” gene from each parent and thus be type “O”.
There are also many other interesting combinations related to not being able to make the hook where the sugars are attached to the red cells so you could have the “A” gene or the “B” gene and still not be able to put the “A” or the “B” sugars on your red cells. (O-Bombay).
The moral of this story is that the ABO/ABH system is actually complex. Web sites that say that certain children are “impossible” are wrong and may lead people to unjustifiably question their paternity.
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