Blood Group & Parentage
Blood is a liquid tissue that, despite appearances, has various elements that differentiate it among individuals.
This variability (or polymorphism) of the blood composition makes it impossible to mix blood during transfusion, for example between certain groups of people .
People with the same characteristic are said to belong to the same blood group.
What is a Blood Group?
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A blood group is a classification based on the presence or absence of substances grouped into 2 systems : ABO ( A, B, AB or O ) and Rhesus ( + or - ) . However, there are about thirty other classification systems in humans, but these are two that remain the most important, both in medical practice and for their genetic interests.
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The ABO system makes it possible to classify the different blood groups according to the presence or absence of A or B antigens on the surface of the red blood cells by following this principle:
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For blood group A : Red blood cells have A antigens and B antibodies .
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For blood group B : Red blood cells have B antigens and A antibodies .
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For blood group AB : Red blood cells have A and B antigens and no antibodies .
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For blood groups O : Red blood cells do not have antigens and antibodies A and B. ​
An antigen is a molecule present on the surface of a red blood cell which, recognized by antibodies or cells of an organism's immune system, is capable of triggering an immune response in it.
The Rhesus system simply refers to a specific antigen which is designated by the letter D. Although there are many others, the D antigen being the most common, it may or may not be present on the red blood cell. A person's Rhesus D status is thus described by a positive or negative suffix after the ABO type .
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The presence of antigen D corresponds to positive rhesus +
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The absence of antigen D corresponds to negative rhesus -
Relationship & Blood Group
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Blood type is passed on hereditarily, as is eye or hair color. It is an inheritance which depends on a specific combination according to the blood group of the parents whose possibilities are as follows: AA, BB, AB, AO, BO, OO .
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It is important to note that each person has 2 alleles corresponding to the creation of the blood group. But only, one of his 2 alleles is replicated on the child's profile during transmission.
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- An allele from the paternal chromosome
- An allele from the maternal chromosome
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The random combination of these alleles defines a child's new blood group.
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During this association, there are genes that can be dominant, dominated (recessive) or co-dominant.
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- In an association between AO and BO, the O gene will be surpassed by the A and B gene
- In an association between AB, the genes participate in common in determining the group
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In the example below, the carrier mother of the AO alleles and the father of the alleles AB, there are 4 possible combinations for the children:
How to check parentage with blood groups?
Here is a representation of the whole possible combinations that exist with the ABO system of blood group transmission.
The use of the Rhesus system and the D antigen is much less precise for a relation search. The possible verification can only be confirmed in a specific case, since:
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- If one of the two parents has a positive rhesus, then the child can receive either the positive or the negative.
- But if both parents are rhesus negative, then the child will automatically receive negative rh.
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In case of doubt about a relationship, it is natural to think that the verification of the blood group can be used to define a parentage link between a father and their children. It is an accessible method which already allows you a first approach to the possibilities of a relationship, even if this comparison still presents limits.
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In general, blood types are considered to rule out a paternity, rather than to confirm it. The reliability and the number of combinations do not make it possible to define a biological relationship with certainty, but by deduction you will be able to ensure that the bond of relation is simply not possible.
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It is also possible that the mutations which occur during transmission mask the true blood group of the profile sought. Thus, these mutations can distort any conclusion regarding a person's paternity.
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Please find below a table containing all the theoretically possible combinations of the ABO system that can help you and the search for paternity.
How do you define paternity with blood groups?
The paternity DNA test remains the only absolutely reliable method to determine the biological filiation link.
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When the paternity test result is positive, the accuracy and reliability is usually above 99.99% and when the test result is negative, the accuracy is 100%.
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Unfortunately, it is not possible to predict any parentage using the blood groups between members of the same family (excluding family ties: father and mother) . Indeed, the possibilities of combinations being too great, a comparison of the blood group between brother and sisters, uncle/aunt or cousins does not provide any proof of the family relationship.
The compatibility and incompatibility of blood thus depends on the ABO and Rhesus antigenic systems which compose it . In other words, during a blood transfusion, if the person does not have the antigen compatible with the donated blood, it will develop an immune reaction against the red blood cells with the antigen and destroy them.
It is therefore important to classify the possible exchanges according to the type of blood, but also to the position of Receiver or Donor :
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For blood group A:
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He can receive from group A and group O
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He can give to group A and group AB
For blood group B:
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He can receive group B and group O
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He can give to group B and group AB
For blood type AB:
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He can receive from group A, B, AB and O - (universal receiver)
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He can only give to group AB
For group O:
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He can receive group O only
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He can give to group A, B, AB and O - (universal donor)
The principle remains the same for the rh of the D antigen:
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A negative rh can only give and receive negative
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A positive rh can only give and receive positive ​
