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History of Paternity Testing

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Before DNA testing became available, several blood testing methods were used to determine paternity. These tests based on different blood group systems (BGS) were difficult to perform and often produced inconclusive results. Most courts now accept only DNA test results as evidence for paternity cases, and the discussion below explains why.

 

1920    1930    1970    1980    1990

 

1920s—Blood Typing

Power of Exclusion: 30%

Blood typing, based on the ABO blood group system, is not an accurate method for determining paternity. It eliminates (excludes) only 30% of the entire male population from being the possible father. It cannot be used to prove paternity.

The Science Behind Blood Typing
Proteins found on the surface of your red blood cells determine whether you have a blood type of A, B, AB, or O. These proteins are called ABO antigens.

Your blood type is medically significant: when you need a blood transfusion or organ transplant, your blood type needs to be compatible with that of the donor. If it is not compatible, your body will reject the donated blood or organ.

You inherit ABO antigens from your father and mother. Thus, it’s possible to predict blood types a child may have if you know the parents’ blood types. The following table lists the possible blood types that a child could have based on the parents’ blood types:

 

Father's Blood Type

A

B

AB

O

Mother's Blood Type

A

A or O

A, B, AB, or O

A, B, or AB

A or O

Child's Blood Type

B

A, B, AB, or O

B or O

A, B, or AB

B or O

AB

A, B, or AB

A, B, or AB

A, B, or AB

A or B

O

A or O

B or O

A or B

O

From the table above, you could determine what blood type would be impossible for the child to have if you knew the parents’ blood types. For example, a child born from an A father and O mother could not have a B blood type.

However, the table also shows that a child could have the blood types A, AB, or B in several cases. For example, a child could have an A blood type if the mother is AB regardless of what the father’s blood type is.

Furthermore, other factors can cause the child to have unexpected blood types. Please visit the ABO Blood Type and Bombay Phenotype pages to learn more. (You will be taken to an educational site not affiliated with GeneSys.)

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1930s—Serological Testing

Power of Exclusion: 40%

Serological testing, a type of blood testing based on the Rh, Kell, and Duffy blood group systems, eliminates only 40% of the entire male population from being the possible father. Serum testing cannot be used to prove paternity.

The Science Behind Serum Testing
Other proteins found on the surface of red blood cells can cause reactions in blood transfusions and organ transplants between those with incompatible proteins. Serum testing uses the Rh, Kell, and Duffy blood group systems.

The way a child inherits the Rh, Kell, and Duffy proteins are slightly more complicated than ABO blood system inheritance. Thus, serum testing based on these proteins was used in the 1930s to achieve a higher power of exclusion. Still, most members of the male population cannot be excluded from being the father.

To learn more about inheritance of the Rh, Kell, and Duffy blood group systems, please visit Rh Inheritance and Inheritance in Other BGS. (You will be taken to an educational site not affiliated with GeneSys.)

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1970s—HLA Typing

Power of Exclusion: 80%

It was only in the 1970s that a significantly higher power of exclusion was achieved for a paternity testing method. HLA testing eliminates 80% of the male population from being the possible father, and in some cases it is possible to produce a probability of paternity of up to 90%. However, HLA testing cannot differentiate between related alleged fathers.

The Science Behind HLA Typing
Unlike previously discussed blood testing methods, HLA typing is based on proteins called Human Leukocyte Antigens, which are found in most cells of your body (except red blood cells). White blood cells contain the most amounts of these proteins. HLA antigens are used by your body’s immune system to detect foreign cells, such as those coming from organ transplants.

There are many varieties of HLA proteins and each person has a small, relatively unique set of HLA antigens inherited from the biological parents. Those who need organ transplants (for example, kidney or bone marrow) have to find a donor with the same or closely similar HLA type, usually a close relative.

The effectiveness of HLA typing in determining paternity depends on how rare an alleged father’s HLA type is in the population. Some individuals have rarer types, which would result in a more conclusive test. Others, on the other hand, may have HLA types that are more common in the population or are shared by close relatives. HLA typing cannot differentiate among individuals in the latter case. Thus, HLA typing cannot be guaranteed to produce conclusive results.

Another disadvantage of HLA typing is the large amount of blood sample required. Thus, infants younger than 6 months cannot be tested, and small children would find the test uncomfortable.

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1980s—DNA Testing via RFLP      

Power of Exclusion: 99.99% and higher

In the 1980s, DNA testing became available. DNA, the genetic material, is found in all cells of the body. You inherit a unique combination of DNA from your mother and father, and no two persons have the same DNA, except for identical twins. Thus, DNA can be used to conclusively determine paternity.

The Science of DNA Testing via RFLP
In RFLP (Restriction Fragment Length Polymorphism), blood samples are taken from child, mother, and alleged father. DNA is purified from the blood samples in the form of a long, stringlike molecule.

The purified DNA samples are placed into a “digestion mix” with biological molecules called restriction enzymes. The enzymes cut the DNA into different-sized pieces, called fragments.

The size of each fragment depends on the type of DNA that is inherited from the mother and father. Half of the child’s fragments should match DNA fragments from the mother, and the rest should match DNA fragments from the father.

If a fragment is found that matches neither the mother’s nor the father’s DNA fragments, additional analysis is required—the mismatch might be a result of mutation. Mutations are random changes that rarely occur in DNA. Statistical analysis is used to determine the chance that a mutation has caused the mismatch. If too many fragments do not have a match, however, the alleged father is excluded.

DNA testing via RFLP is conclusive, but it is an old technique that requires larger amounts of samples and longer processing time. New developments in DNA technology have paved the way for DNA testing via PCR, discussed below.

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1990s—DNA Testing via PCR

Power of Exclusion: 99.99% and higher

The Polymerase Chain Reaction (PCR) became established in the 1990s as the standard method for paternity testing. Like RFLP, this method uses DNA, which is found in all cells of your body. You inherit a unique combination of DNA from your parents.

Because scientists have extensively used PCR for DNA testing, a greater amount of information has been accumulated to form a database for accurate DNA analysis. This large database enables paternity testing via PCR to have the highest power of exclusion.

The Science of DNA Testing via PCR
The Polymerase Chain Reaction (PCR) is a powerful method for analyzing DNA. PCR can be performed on very small amounts of biological samples, from almost every part of the body. All cells in the body have the same DNA, so the results are the same regardless of the type of sample taken.

PCR allows scientists to make billions of copies of DNA from a small sample, such as a buccal swab (a cotton swab rubbed against a patient’s inner cheek). DNA is extracted from the swab as a long, stringlike molecule. PCR creates copies of only a small fragment from this molecule—scientists can control which part of the DNA molecule is copied. Once the copies are made, the DNA fragment can be easily analyzed.

In paternity testing, 16 different DNA fragments are copied at the same time. These fragments are often referred to as loci (singular locus), locations on the DNA that have been found to be useful for human identification. These fragments form the DNA profile.

The DNA profile is interpreted in a manner similar to RFLP. Half of a child’s DNA profile (8 fragments) matches fragments on the mother’s DNA profile, and the other half matches fragments on the father’s DNA profile. If a mismatch is observed, statistical analysis and additional tests will show whether or not the mismatch is a result of mutation (a rare, random change in the DNA). If too many fragments do not have a match, however, the alleged father is excluded (he is not the father).

DNA Testing via PCR is the fastest, most accurate method for determining paternity. Please visit our DNA Testing Services page to learn more about how we use this technology to determine paternity and other family relationships.

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