Friday, February 23, 2007

Genetics of ABO Blood Types

Now that we understand the biochemistry behind the ABO blood types [ABO Blood Types] it's time to look at the genetics. Recall that the human ABO gene on chromosome 9 has three common variants of the gene. Different variants are called alleles. The A allele encodes N-acetylaminogalactosyltransferase and this enzyme makes the A antigen that confers blood type A. The B allele encodes a variant enzyme that makes B antigen and gives rise to blood type B. The O allele encodes a defective enzyme that doesn't make either antigen. In the absence of both A antigen and B antigen your blood type will be O.

>Everyone has two copies of chromosome 9 so you have two ABO genes. If both of them are the A alleles then your genotype (genetic makeup) will be AA. Your blood type will be A. If one of your ABO genes is the A allele and the other is the O allele then your genotype will be AO. In this case your blood type will be A as well since there are only three possibilities: (1) you have A antigen, (2) you have B antigen, (3) you have no antigen (you can have both antigens, see below). As long as you have one A allele, you will produce the A antigen on the surface of your blood cells.

Now let's look at the possibilities for inheritance of blood types. If someone has blood type A then their genotype might be AA or AO. Imagine that they mate with someone having blood type O. That person must have genotype OO. The manifestation of the genes is called the phenotype. The phenotype is related to the genotype but you can't always predict the genotype from the phenotype.

We can examine the possible genotypes and phenotypes of the children by constructing a Punnett Square where the alleles from one parent are listed on the side and the alleles from the other parent are listed on the top. You can think of this as being two different kinds of sperm or two different kinds of eggs; keeping in mind that sperm and eggs have only one copy of chromosome 9.


The results for the AA parent are shown on the left. All four of the possible combinations are shown in the matrix. There is only one genotype that will show up in the children (AO). All of the children will have blood type A, shown as purple boxes.

If the blood type A parent has the AO genotype then the Punnett square calculations look like the diagram on the right. In this case, there are two possible combinations; AO and OO. Half the children will have blood type A and half will have blood type O.

Since the O phenotype is masked by the presence of the A phenotype, we say that O is recessive to A and A is dominant with respect to O. The only way to see the O phenotype is when the genotype is OO. We refer to this as the homozygous recessive state. AO individuals are heterozygous because they have two different alleles.

Now let's look at combinations with the B allele. If you only express the B allele then you will only have B antigen on the surface of your cells and you will be blood type B. If you are heterozygous for the A and B alleles then you express both A antigen and B antigen and your blood type is AB. Note that the A phenotype is not recessive to the B phenotype or vice versa. The A and B alleles are co-dominant. This result is very common even thought we don't hear about it as often as dominant and recessive alleles.


A mating between an AB father and an OO mother will produce children who are either blood type A or blood type B as shown in the Punnett square on the left. None of the children will be blood type O.

A mating between two AB individuals will result in 50% of the children with AB blood type like their parents, 25% with blood type A, and 25% with blood type B. The percentages are derived from the matrix. Every combination of sperm and egg is random with respect to the genotype so the matrix represents the probabilities and not the definitive outcome whenever you have four children. It's quite possible for two AB parents to have four children with blood type A although the probability of this happening is low (0.254=0.4%).

The concept of probabilities is extremely important in genetics and it's also important in understanding evolution. The fact that every allele isn't necessarily passed on to the next generation gives rise to fluctuations in the frequency of alleles in a population. Over time, these fluctuations can result in the loss of an allele by chance. This is evolution by random genetic drift.

Now lets try a more difficult case that will test your understanding of genes and genetics. This example is based on an actual paternity case from several years ago. The mother has blood type AB and so does the child. The man who claims to be the father has blood type O. All genetic tests indicate that he really is the father of the child. How do you explain the blood type results?

UPDATE: Simple Mendelian genetics accounts for the vast majority of blood type inherited by children from their parents. However, in a population as large as humans with multiple alleles segregating independently, there will always be anomalies. Some are due to strange alleles that don't exactly conform to the recessive and dominant phenotypes and some are due to various recombination events. Some children can even show new, spontaneous mutations that were not present in either parent. The important lesson is that you can't rule out parental relationships based on ABO blood types alone.

Check Online Mendelian Inheritance in Man for some examples of rare alleles.


37 comments:

  1. hmmmmm...
    Some sort of crossing-over error that put both alleles on one of the maternal chromosomes?

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  2. Maybe the father could be a human chimera who developed from the fusion of two fraternal twin embryos, one with genotype OO and one with genotype OA, OB or AB. Part of his body could produce his O blood, and another part could produce his sperm as A or B.

    If this is true, then his parents must have been some combination of genotypes OA and OB, so that they could produce both an OO embryo and one with OA, OB, or AB.

    I suspect there's probably a flaw in my reasoning and the correct answer is simpler. Genetic tests in the case of a human chimera could produce contradictory results depending on which part of his body was sampled.

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  3. I don't think CCP has it right, i'm pretty sure cross overs can't happen like that. Or if they did, I doubt blood type would be the only thing the child had to worry about.

    Fred, I had the same thought as you and I almost posted it, but something called me away from my computer--in fact, I still think you might be right, but I also have another idea...

    If the father's blood type is O, it's possible that he actually inherited an O allele from one of his parents and either an "A" or "B" allele from the other parent. Except there was a single base mutation in the (let's just call it) "A" allele from his parent and the "A" allele no longer functions. Instead, it's a pseudo-O allele. Well, if that happened, then the father would have an O-"O" genotype and Type O blood. However, if there was a single base-pair mutation when his DNA was being copied during meiosis it's possible that the pseudo-O allele could revert back to an active A allele. I have to imagine the odds of this happening are phenomonal, but then again, there are ~6.5 Billion people alive...that's a lot of trials.

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  4. I read about this in Steve Jones' book Y- The Descent of Men.

    The O-antigen is a precursor to the A and B antigens, another gene product catalyses the reaction and so the carrier has to be O, even if they have a different genotype. The child therefore inherits a working copy of that gene from the mother and the ABO genes in the normal way.

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  5. "...another gene product catalyses the reaction and so if this gene is damaged the carrier has to be O..."

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  6. Follow the link "human ABO gene." This takes you to a page with a link to the OMIM site for this gene. That's where you find information about human variants including the one relevant to the question.

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  7. I am ab+ and my husband was O, I believe, I had a set of maternal twins, one lived one died being only 1 lb 15 oz each and 3 months early. The twins inherited my blood type. What is the answer?


    Adjustersinc@aol.com

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  8. ab blood type is too dominate over the o blood type they will never have a o blood type

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  9. this program is so cool keep going and god bless you guys

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  10. Please help solve a debate:

    My boyfriends Grandma insists that she has AB positive blood, and yet ALL of her children are O. His grandpa says that he is O neg. I maintain that for all of the chilren to be O (various negative and positive Rh factors) it's impossible for his grandma to be AB. Am I right? Is there ANY way she could me right?

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  11. Yes, your grandmother could be right. There is a rare allele called "cis-AB" in which people exhibit both the A and B phenotypes from a single allele.

    Your grandmother was cis-AB/O and your grandfather was O/O. By chance, all of their children inherited the O allele from their grandmother.

    You can read all about it at ...
    ABO Blood Groups.

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  12. It also could be a mutation in the H allele in the father, known as the Bombay Phenotype....

    Without H you can't show A or B. So the father really could be AO or BO and be hh, while the mother is AB HH (or Hh)

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  13. My mother has O blood & my father AB & I have O. There must be like a 0.0000001% or something like that chance that their types could produce O or AB.

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  14. i was always told that both my brother and i were born (two years apart) with a rare blood type-disorder... we were both born with ABO blood. I was born in Germany in 1972. The physician cured my problem by administering antibodies into the bloodstream and I am now AB and healthy. My brother however, born with the same problem two years later, was born in the United States and was given some sort of light therapy and has had all sorts of health problems his entire life.
    So what is this ABO blood type? I can't seem to find it anywhere. And is it possible that my brother's treatment is what led to his ailments?

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  15. Strange where a google search leads you. Yes, i too would have said its some sort of mutation or similer, to me it definatly doesnt seem like anything common, but what do i know, i'm still in school.

    i must know the answer =] but i dont know how this site works, will a answer blog be posted up or somthing? Whatever the answer be may you kindly give me a email..

    thamodi@optusnet.com.au

    thankyou.

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  16. I would like to know if a mother has O+ blood-type and the child has O+ blood-type then what blood-type would the father have if she isn't sure who fathered her child? Please send me an answer at Michelle_Lee_Shafer@yahoo.com Thank-you for your time!!

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  17. http://www.selfgrowth.com/experts/bob_afamasaga.htmlSunday, May 04, 2008 12:10:00 AM

    Larry,

    The information on the Genetic of the ABO is outdated. The Blood Type of the fetus or baby is not determined by the parents. The Blood Type is determined by the fetus choosing 1 out 24 sub-brain recombinations to protect itself from the mother's diet, environment and lifestyle. I discussed this in my webiste below.
    http://www.selfgrowth.com/experts/bob_afamasaga.html. The information is free.

    Appreciate any feedback.

    Bob Afamasaga

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  18. If both my perants genotype are both O and my blood type is A
    is it possible to get A type it they are both O blood types

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  19. I see all these comments about rare genetic occurances. There is a saying in science that plainly stated says the simplest answer is probably the right answer. Or not plainly stated (with its counter);

    Ockman's Razor: "entities must not be multiplied beyond necessity."

    Principle of Plenitude: "That everything that can happen will happen."

    Why did no one hardly fathom that "mom" may not have been faithful or "grandma" doesn't know what she is talking about?

    I for example went around thinking that I was AB- for years based on a high school science class blood analysis until I went to donate blood and found out they loved me because I was O+. I could by some stretch of plentitude have switched my blood type during my lifetime OR even simpler, the high school blood analysis was wrong. Just a thought for those looking for sand in the depths of the ocean when there is plenty on the beach.

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  20. so if male has blood type A and a female has blood type B and they have a type O child, what is the probability that their 15th child will have blood type O? explain?

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  21. Both Mother and Child are AB and the Father is "typing" O, but the father is NOT An "O". The father does carry an A, B, or AB gene, but he's lacking the H antigen on his red cells to express the terminal oligosaccharide. This is the Bombay phenotype, which is very rare. The father will type as an "O" because he has no antigens on his red cells, but it's due to the lack of "H" antigen, not the lack of A or B antigens. So he still carries the A, B, or AB genes to pass on to his children.

    The father would also make antibodies to all other blood types, including "O", and could only receive blood from another Bombay.

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  22. my grandpa is blood type O and my grandma is blood type B. My mom is blood type A. is that possible?

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  23. My fraternal twins, whom are adopted, have two different blood types. My daughter has AB and my son has O. Their biological mother has O neg blood. Was trying to figure out how my twins could have these two blood types.

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  24. I am SO glad you asked this question,Prof. Moran. I am a psychology professor, with a biological background.

    As it happens, my blood type is AB and my (also professor) husband's blood type is O. Guess what blood type BOTH of our children have?

    You guessed it.....AB!!

    I am surely going to follow your link to quell my curiosity...and to put my husband's mind at ease. ;-)

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  25. My mom is O + and my dad is A +. I am O - and my two sisters are: O + and A +.
    Isn't that weird?
    The negative allele is recessive, so idk how I got negative when my mom, dad, and sibilings are +.

    My grandma is O - though and her grandmother was O -
    Do you think it can skip a generation?

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  26. I am A+ and my husband is B+ or B- (don't remember which)....anyway both of our children are 0+. My father is also 0+. How could both of my children be 0+?

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    Replies
    1. both blood types have the O recessive, the genotype is either AO or BO

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  27. anonymous mar 21 2012 3.03pm

    my father from what i have been told was AB they said he was a rare blood type, my mother was O something, I have O rh negative phenotype according to the blood donor clinic. is this possible ?

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    Replies
    1. can anyone tell me anything?

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    2. It is possible. My brother have the same situation. It is about cis-AB (explained above).

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  28. O allele is recessive to both A and B but how is this possible that 60% of human have blood group O

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  29. thanks this helped me understand inheritance of blood types for my gcse biology unlike the stupid revision guide which makes no sense to me. ^^

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  30. I am type AB, have three siblings, type A, type B and type O. Mother type O and father type AB. Four children four different blood groups?

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  31. type A parent with B type parent --which are the corresponding probabilities ( and %)

    Thank you

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  32. To add something recent, a paper was out last year claiming that ABO is a trans-species polymorphism that has been around through a big slice of primate evolution because of balancing selection. PNAS 109 18493 2012

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  33. I have a question can rh factor negative blood skip a generation from grandparents to grandchildren if both parents don't carry the rh negative recessive allele

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