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I see a family in which it has only one daughter. The blood group of the mother is AB positive and the father is B positive, but the blood group of the daughter is O negative.

When she was young, they apply to shift abroad permanently. When the Embassy test their blood group, the doctor said "She is not your daughter because of her blood group".

How is it possible? Why is she not the daughter of their parents?

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  • $\begingroup$ Have you tried reading from wikipedia > ABO blood group system? If yes, was there anything specific that was unclear to you? $\endgroup$ – Remi.b Dec 3 '18 at 8:15
  • $\begingroup$ yes because it is not clear for me thats why i ask this question i am confuse that how it is possible that a girl is not a daughter of her parents .why? $\endgroup$ – Jj Mmmm Dec 3 '18 at 8:18
  • $\begingroup$ Your comment make it unclear what is unclear to you. The girl is the daughter of her biological parents (by definition). However, the people she calls parents might not be her biological parents. Genetics (in this example, ABO groups) can help to figure whether the supposed parents might be the biological parents. $\endgroup$ – Remi.b Dec 3 '18 at 8:21
  • $\begingroup$ @Jj Mmmm: How is it possible? Obviously, either she was adopted, or the mother got pregnant by someone other than the supposed father. See e.g. Homer and "It's a wise child that knows its own father". $\endgroup$ – jamesqf Dec 3 '18 at 17:34
  • $\begingroup$ @jamesqf: The mother is AB, so based on our normal understanding of blood types (without exceptions mentioned in my answer), she cannot be the biological mother of a child with blood type O. $\endgroup$ – Frieke Dec 4 '18 at 14:06
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Each person has two alleles for the blood type. The two A and B alleles are dominant. So, you don't need A/A to end up with blood type A, it could also be A/O. Having both A and B results in blood type AB. Having none of these, results in blood type O.

Following these rules, from parents being AB and B (genetically either B/B or B/O) you would expect these possible blood types for their children: - A/B - A/O - B/B - B/O

The Rh factor (positive or negative) is inherited similarly. The positive allele is dominant, so genetically +/+ or +/- would be positive, only -/- is negative.

There is a rare case of a "cis AB" mutation, where the A and B antigens are in just one allele (wiki, paper). Then the genetic alleles of an AB type could be AB (in one allele) and O. The children of the couple could then be - AB/B - AB/O - B/O - O/O

So, there would be two possibilities for the case you describe.

  1. The parents are not the biological parents of the child.

  2. The mother is cisAB/O +/- and the father B/O -/-

edit: or a third way would be of course some de novo mutation in the child, resulting in the loss of A/B from the mother

edit2: maybe to answer your actual question "How can a child not be the child of its parents?".. adoption, in-vitro fertilization with donated eggs and/or sperm from other people, mix-up in the hospital..

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