Blood Groups

Blood Groups and Filiation: Can Paternity Be Verified with Blood Groups?

Are you wondering about the possibility of using blood groups to verify a filiation link between a parent and child? This question arises frequently, as comparing blood groups appears to be a simple and accessible method for establishing a parental relationship.

In this comprehensive article, we explain everything you need to know about blood groups, their hereditary transmission and their use in determining or excluding paternity. You'll discover how the ABO and Rhesus systems work, what the limitations of this method are, and why DNA testing remains the most reliable solution for establishing a filiation link with certainty.

What is a blood group?

A unique biological characteristic

Blood is a complex liquid tissue that shows significant variations from one individual to another. This biological diversity, called blood polymorphism, makes direct mixing of blood between certain people impossible, particularly during blood transfusions.

Individuals who share the same blood characteristics belong to the same blood group. This classification allows donations and transfusions to be organised safely, but it also has major genetic interest for understanding hereditary transmission.

The two main classification systems

A blood group is a classification based on the presence or absence of specific substances on the surface of red blood cells. Although there are around thirty blood group systems in human beings, two systems dominate through their medical and genetic importance:

• The ABO system: which determines groups A, B, AB and O

• The Rhesus system: which defines positive (+) or negative (-) status

These two combined systems allow each individual to be classified into one of eight

main blood groups: A+, A-, B+, B-, AB+, AB-, O+ and O-.

The ABO system: understanding antigens and antibodies

How does the ABO system work?

The ABO system is based on the presence or absence of two types of antigens (A and B) on the surface of red blood cells. An antigen is a molecule capable of triggering an immune response when recognised as foreign by the body.

Alongside antigens, blood plasma contains antibodies that react against antigens absent on the person's own red blood cells. This antigen/antibody combination defines the four blood groups of the ABO system.

The four blood groups of the ABO system

Blood group A:

• Red blood cells have A antigens on their surface

• Plasma contains anti-B antibodies

Blood group B:

• Red blood cells have B antigens on their surface

• Plasma contains anti-A antibodies

Blood group AB:

• Red blood cells have both A and B antigens

• Plasma contains no anti-A or anti-B antibodies

Blood group O:

• Red blood cells have no A or B antigens

• Plasma contains anti-A and anti-B antibodies

This organisation helps understand why certain blood mixtures cause dangerous immune reactions during incompatible transfusions.

The Rhesus system: positive or negative?

Antigen D, the main Rhesus marker

The Rhesus system refers to the presence or absence of a specific antigen called D antigen on the surface of red blood cells. Although the Rhesus system actually comprises several antigens, D antigen remains the most important and most commonly analysed.

A person's Rhesus status is indicated by a positive or negative suffix added to the ABO group:

• Rhesus positive (+): presence of D antigen on red blood cells

• Rhesus negative (-): absence of D antigen on red blood cells

Thus, a person with group A with D antigen will be A+, whilst a person with group A without D antigen will be A-.

Importance of the Rhesus system

The Rhesus system is particularly important in obstetrics. Rhesus incompatibility between a Rhesus-negative mother and a Rhesus-positive foetus can cause complications during successive pregnancies, requiring appropriate medical monitoring.

Hereditary transmission of blood groups

Blood group, a genetic inheritance

Just like eye or hair colour, blood group is transmitted hereditarily from parents to children. This transmission follows precise genetic rules that depend on the alleles carried by each parent.

Each person has two alleles determining their blood group:

• One allele inherited from the paternal chromosome

• One allele inherited from the maternal chromosome

The random combination of these two alleles defines the child's blood group.

Alleles of the ABO system and their dominance

Alleles of the ABO system can be of three types: A, B or O. Each parent transmits only one of their two alleles to their child, thus creating a new combination.

The possible genetic combinations are:

• AA or AO → Blood group A

• BB or BO → Blood group B

• AB → Blood group AB

• OO → Blood group O

Concept of genetic dominance:

A and B alleles are dominant, whilst the O allele is recessive. This means that:

• A person with AO will express group A (A allele dominates O allele)

• A person with BO will express group B (B allele dominates O allele)

• A and B alleles are codominant with each other, so a person with AB expresses both antigens

Example of transmission between parents

Let's take the example of a mother carrying AO alleles (group A) and a father carrying AB alleles (group AB).

The four possible combinations for their children are:

• AA (group A): A allele from mother + A allele from father

• AO (group A): A allele from mother + O allele from father

• AB (group AB): A allele from mother + B allele from father

• BO (group B): O allele from mother + B allele from father

In this case, children can be group A, AB or B, but never group O.

Summary table of possible combinations

Here is a complete overview of possible blood groups according to parental combinations in the ABO system:

This table allows quick visualisation of transmission possibilities and impossibilities according to parents' groups.

Transmission of Rhesus: positive and negative

Rhesus transmission also follows genetic rules, although this system is less precise for filiation research.

Transmission rules:

• If at least one parent is Rhesus positive, the child can inherit either Rhesus positive or Rhesus negative

• If both parents are Rhesus negative, the child will necessarily be Rhesus negative

This rule is explained by the fact that Rhesus positive can be carried dominantly or recessively. A Rhesus-positive parent can therefore transmit a Rhesus-negative gene without this being visible in their own blood group.

Can paternity be verified with blood groups?

A method of exclusion, not confirmation

When there is doubt about a paternity link, it's natural to think that comparing blood groups can provide an answer. This method has the advantage of being accessible and inexpensive, but it has important limitations.

General principle:

Blood groups mainly allow exclusion of paternity, rarely confirmation. Indeed, if the blood groups of the presumed father, mother and child do not correspond to genetically possible combinations, this can indicate with certainty that there is no biological link.

However, reliability remains limited due to the restricted number of possible combinations and the existence of rare genetic mutations that can mask the true blood profile.

Paternity exclusion table according to the ABO system

Here is a summary table for identifying paternity exclusion situations:

Caution: Absence of exclusion does not constitute proof of paternity. Several men can share the same blood group compatible with that of the child.

Limitations of the blood group method

1. Limited number of combinations

With only eight main blood groups (A+, A-, B+, B-, AB+, AB-, O+, O-), the probability that several people share the same blood group is very high. This method can therefore never confirm paternity with certainty.

2. Rare genetic mutations

In very rare cases, genetic mutations can modify the expression of antigens on the surface of red blood cells, creating atypical blood profiles. These variations can distort any conclusion regarding filiation.

3. Impossibility for indirect relationships

It is completely impossible to determine a kinship link between siblings, uncles/aunts and nephews/nieces, or cousins using only blood groups. The possible combinations are too numerous to allow any reliable deduction.

Paternity DNA testing: the reliable and definitive method

Unrivalled precision

Paternity DNA testing remains the only absolutely reliable method for determining a biological filiation link with certainty.

Unlike blood groups which only allow an approximate approach, DNA analysis compares dozens of specific genetic markers between the presumed father and child, guaranteeing results of exceptional precision.

DNA test reliability:

• Positive result: accuracy greater than 99.99%

• Negative result: accuracy of 100%

These reliability levels eliminate all ambiguity and provide a definitive answer to the question of paternity.

Why choose DNA testing rather than blood group comparison?

DNA testing has several decisive advantages:

• Absolute certainty: unlike blood groups, DNA testing definitively confirms or excludes paternity

• In-depth analysis: examination of 15 to 21 genetic markers instead of simple surface antigens

• No ambiguity: results are clear and unequivocally interpretable

• Legal acceptance: results from a DNA test performed by an accredited laboratory are recognised by the relevant authorities

When to use DNA testing?

Paternity DNA testing is recommended in the following situations:

• Persistent doubt about biological paternity

• Need for legally recognised proof

• Cases where blood group comparison is inconclusive

• Complex family situations requiring a certain answer

Blood compatibility: understanding transfusions

Why is blood compatibility important?

Blood compatibility is based on the ABO and Rhesus antigenic systems. During a blood transfusion, the recipient's immune system reacts if the transfused blood contains antigens they do not naturally possess.

This immune reaction causes destruction of transfused red blood cells and can lead to serious, even fatal complications. This is why it's essential to respect compatibility rules during any transfusion.

Compatibility rules according to the ABO system

Blood group A:

• Can receive blood from group A and O

• Can give blood to groups A and AB

Blood group B:

• Can receive blood from group B and O

• Can give blood to groups B and AB

Blood group AB:

• Can receive blood from all groups: A, B, AB and O (universal recipient)

• Can give blood only to group AB

Blood group O:

• Can receive blood only from group O

• Can give blood to all groups: A, B, AB and O (universal donor)

Compatibility according to the Rhesus system

Rhesus compatibility rules are simpler, but equally important:

Rhesus positive (+):

• Can receive positive and negative Rhesus blood

• Can give blood only to positive Rhesus people

Rhesus negative (-):

• Can receive only negative Rhesus blood

• Can give blood to positive and negative Rhesus people

In practice, a Rhesus-negative person must never receive Rhesus-positive blood, as this could trigger a serious immune reaction.

Group O: universal donor

O negative group is particularly valuable in emergency medicine, as it can be transfused to any patient without risk of immune reaction. This is why O group donors are strongly encouraged to give blood regularly.