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Name: Webb W.
Status: Student
Age: 15
Location: N/A
Country: N/A
Date: 2002


Question:
Hey,

Today in my Biology class at Harrisonburg High School, we went over a section on multiple alleles. In our book it noted that a heterozygous woman with type A blood and a heterozygous man with type B blood could not produce an offspring with type O blood. The book says that type A and B alleles are both dominant to the recessive allele of the O type.

Since both the people are heterozygote, they both have to posses the recessive allele for O. This is for sure because if the heterozygote has both A and B alleles the phenotype would be AB, because A and B alleles are codominant.

With this information I filled out a Punnet Square, which said that it was a 25% chance the offspring's blood type would be AB, 25% chance for the offspring's blood type to be A, 25% chance for the offspring's blood type to be B, and 25% chance for the offspring's blood type to be O.

My teacher told me to send this problem in to find out for sure if a heterozygous women with type A blood and a heterozygous man with type B blood produce an offspring with type O blood.



Replies:
Looks to me that your textbook just wrote "heterozygous" when it should have been "homozygous". Typographic errors like that have a way of creeping into documents and not being discovered for a long time.

Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois


You are absolutely correct. I'm surprised your book said this. It is true that A and B are both dominant over O (which actually means "zero" because the gene doesn't produce a functional protein). When A and B are both present neither is dominant over the other and so they both show equally. When A is present on one chromosome and O is present on the other chromosome the blood type is A, and when B is present on one and O present on the other the blood type is B. There is a 25% chance that an offspring will inherit both O alleles and therefore have the blood type O.

vanhoeck


You are absolutely right and your book is wrong. I could not have explained it better myself. Somebody at the publisher must have missed that when checking the textbook!

Christine Ticknor, Ph.D.
Case Western Reserve University
Cleveland, Ohio


You are correct. A heterozygous woman with type A blood and a heterozygous man with type B blood have, as you figured with your Punnet square, a 25% chance with each child that the child will have blood group O, 25% chance for blood group A, 25% chance for blood group B, and 25% chance for blood group AB. The same is true for the reciprocal cross: a heterozygous woman with type B blood and a heterozygous man with type A blood. In such a family, the existence of a child with blood group O is evidence that both parents are heterozygous. To further test your understanding, think about what combinations of blood groups in the grandparents would be compatible with parents who are heterozygous for blood groups A and B respectively.

Sarina Kopinsky, MSc, CGC, HED


If the information that you have provided accurately reflects the statements in your textbook, you are absolutely correct in your analysis. The blood type probabilities for the progeny of heterozygous type A & type B parents are exactly as you have reported from your Punnet Square. My only suggestion would be to re-read your textbook to be sure that the wording is exactly as you have quoted and that there are no other qualifications in the text. If not, I would guess that there is a misprint in it and that they meant to make that point for the progeny when at least one of the parents was homozygous. That would demonstrate the concept of dominance, where the progeny could not be type O because the other allele would always dominate in the heterozygotes.

Maybe they were just testing to see who was alert and reading carefully? Seriously, your textbook publisher needs to get a better proofreader and your school should probably pick a new textbook for next year. Who knows how many other mistakes are slipping by unnoticed? Congratulations on spotting their error,

Jeff Buzby, Ph.D.


If I remember correctly the ABO blood groups are caused by the presence of certain immunogenic sugar residues on blood and certain tissue cell membranes. The A antigen is coded by one allelic form, the B antigen by another and the third allelic form is the absence of either of these. The clinical ramifications of this are enormous since a AB heterozygote is therefore in the desirable position of being able to receive (in emergencies...since it is desirable to have a exact match) O, A or B blood types (but not a mix of them ) since its serum contains no "agglutinins"...antibodies to either A or B antigens). O is, (the "universal donor") in emergencies, acceptable as the donor to the other two types since it has none of the antigens that would precipitate an immune reaction to foreign antigens...i.e., A or B.

As I remember, the genetics of this is as you stated...The A and B are obviously co-dominant and the O, being the absence of these antigens is obviously recessive. Furthermore, I agree that there should be a 25% chance for each of the following genotypes from heterozygous parents A/O X B/O. A/B, A/O, B/O and O/O.

Peter Faletra Ph.D.
Office of Science
Department of Energy


Webb, if the problem is indeed as you have stated it, then I agree totally with your analysis. It sounds like the book meant to say "homozygous" and not "heterozygous". Congratulations on your sharp eyes. By the way, what textbook are you using?

Paul Mahoney, PhD



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