Can a paternity test be done if there is a blood relationship? Not necessarily...

Recently, the old series sitcom "Country Love 15" is on the air. One of the plots has aroused heated discussion among netizens. The character Xie Tengfei in the play was found by his biological grandmother, who asked him to do a paternity test with him to determine whether they are related by blood.

Xie Tengfei signed the paternity test application form at the hospital

Source: TV series screenshots

At first glance, there seems to be nothing wrong with a grandmother and her grandson doing a paternity test. But the fact is that medically, the accuracy of a paternity test between a grandmother and her grandson is not high, which means that a paternity test cannot actually prove that the grandmother and her grandson are related by blood .

Why is this happening? Today we are going to talk about paternity testing.

Source: Baidu Encyclopedia

1

How is paternity testing done?

Paternity testing refers to the forensic identification of the blood relationship between parents and children by detecting and analyzing human genetic markers with the help of theoretical techniques from applied medicine, genetics and biology.

Paternity testing actually has a long history. In ancient times, there was a method of "blood test" to determine blood relationship, but this method is not scientific.

In modern times, the relationship between the two can be determined through blood type testing, chromosome polymorphism (including cytoplasmic inheritance) identification and DNA identification. The blood type test is to check whether the blood type pairing of parents and children conforms to the genetic law. However, due to the large number of blood type pairing combinations and the existence of subtypes and variant blood type individuals, the accuracy of this method is not high and can only be used as a rough identification result for reference.

Source: Baidu Encyclopedia

At present, the most widely used paternity test method in China is the DNA polymorphism genetic marker test method based on STR . How does this method detect kinship? First of all, we need to understand its principle.

DNA stands for deoxyribonucleic acid. It carries the genetic information necessary for the synthesis of RNA and proteins in the body and is an indispensable substance for the growth, development and normal functioning of the human body. DNA is mainly found in the cell nucleus, and a small part is found in the mitochondria in the cytoplasm. It is composed of four types of deoxynucleotides. The order of arrangement of deoxynucleotides determines the fragments in DNA that have genetic effects, thereby reflecting different genetic expressions.

Source: pixabay

STR, the full name of which is Short Tandem Repeats, is the continuous repetition of some shorter DNA base sequences in the genome. Different numbers of repetitions will result in differences in the length of the DNA bases.

To use the STR method for paternity testing, you first need to obtain DNA samples from the test subject, which can be hair, blood, skin, or even nails, semen, saliva, etc. When taking hair, you need to choose hair with roots, because the nuclear DNA is concentrated in the hair roots. If you choose cut hair without roots, no DNA can be detected.

Source: pexels

Once the DNA sample is obtained, testing can be performed.

We know that the human body contains 23 pairs (46) chromosomes, each chromosome contains genes. Biologically, a pair of genes at the same position on the same pair of chromosomes is called alleles. One allele comes from the father and the other from the mother.

Source: Baidu Encyclopedia

During DNA testing, the obtained gene fragments of the children containing multiple STR loci are amplified exponentially, and then these amplified DNA fragments of different lengths are tested to see if they are the same as the genes on the DNA fragments of the parents to determine blood relationship.

For example, if the son’s genotype at a certain STR locus is (9, 8) and the biological mother’s genotype is (9, 6), then the biological father’s genotype at this STR locus must be 8.

Of course, the genetic match at one site cannot determine the parent-child relationship. In actual identification, different sites will be selected for testing. If the genes of the children and the parents at the site are the same, the parent-child relationship is confirmed ; if there are more than three different sites, the parent-child relationship is ruled out.

Source: pixabay

2

Why can't a paternity test be done between a grandmother and her grandson?

Speaking of this, we can think about why the grandmother and grandson in the drama mentioned at the beginning could not do a paternity test?

Paternity testing generally refers to a DNA test done on both parents and their children, while a DNA test between grandparents and grandchildren is called a kinship test .

As mentioned above, ½ of a child's genes come from the father and ½ from the mother. ½ of the father's genes come from the grandfather and ½ from the grandmother. Therefore, in theory, ¼ of the grandson's genes should come from the grandmother. However, currently supported kinship testing is limited to kinship testing between people of the same sex , that is, grandfather and grandson, grandmother and granddaughter, because of the specific DNA genetic markers between people of the same sex.

Source: pexels

We know that the genes in the human body consist of two parts. The 23 pairs of chromosome DNA are the genome in the cell nucleus, and the other part comes from the mitochondrial DNA (also known as mtDNA) in the mitochondria of the cytoplasm. In heredity, the mtDNA in the fertilized egg is only inherited from the mother, and the Y chromosome is only inherited from the father.

In the case of the "Fengxian girl who gave birth to eight children", mitochondrial DNA testing was used to determine blood relationship

Source: News screenshot

Therefore, by tracing the Y chromosome, we can accurately determine the paternal genetic relationship, and by tracing the mtDNA, we can accurately determine the maternal genetic relationship.

However, the credibility of the kinship identification between opposite sexes is not high because there is no decisive genetic marker to support it, and it is not supported in blood relationship identification.

In general, paternity testing is nothing new. DNA identification technology has become mature and its accuracy rate can reach over 99.999%. It is widely used, including missing persons inquiries, suspect identification, immigration, and settlement.

However, as demand increases, the paternity testing market has also spawned many chaos. According to the Ministry of Justice, testing agencies are not allowed to use intermediaries or individuals to solicit business, but this phenomenon continues despite repeated prohibitions, and even hierarchical agency, a business model similar to "micro-business", has emerged. In some areas, there is even a demand for prenatal paternity testing, which is also expressly prohibited.

For many ordinary people, paternity testing seems to be a little far away from life. It can usually only be seen on TV dramas. We know this knowledge, which may not be of much use, but it can help us avoid touching the boundaries of the law and be a sober person when watching TV dramas.