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Blood and Genetics
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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