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What is the principle behind paternity testing?

Modern DNA paternity testing combines some of the best achievements of biological research in 19 and 20 century. There are four principles behind the testing:

 

a) Mendel’s Law. In mid 19 century, when studying pea, Mr. Gregor Mendel (1822 – 1884) discovered that biological father and mother independently pass their genes to their offspring; the parental genes stay separately in the offspring; they segregate again during offspring’s gametes genesis.

 

b) The genes are located on chromosomes inside cell nucleus. A normal human cell has 46 or 23 pairs of chromosomes, 23 of them come from mother’s ovum, and other 23 come from father’s sperm. 

 

c) The genetic material on chromosomes is deoxyribonucleic acid or DNA. DNA is a long-chain organic molecule that consists of 4 kinds of deoxyribonucleotides, which are Adenosine, Guanosine, Thymidine, and Cytidine. Genetic information is determined by the nucleotide sequence of A, G, T and C

 

d) DNA sequences are different among human beings. Even though 99.9% of human DNA sequences are the same, slight variations always occur (except identical twins), which in turn make each individual unique.

 

The DNA sequence variations are not evenly distributed along entire DNA chains. In some areas, they are very frequent; there are several dozens of different genotypes among all people. The short tandem repeat (STR) is one of them. Large number of STR variations provides sound scientific basis for Ningren genetic identification and paternity testing.

 

When performing paternity testing we will do following reasoning: according to Mendel’s Law, a pair of the child’s STR genes (alleles) must come from its biological mother and father. If the results turn out to be that none of the child’s two allelic STR could come from alleged father (incompatible), and then our conclusion must be: The alleged father is NOT the biological of the child. In the case where each child’s STR gene is compatible with the alleged father’s, we cannot rule out they have biological father/child relationship; but we cannot be 100% sure they do have, because in this big world it is possible that someone having no biological relationship with the child happens to harbor a set of compatible STR genes instead. In order to make this coincidence to be extremely small, Ningren simultaneously tests 16 independent STR loci.

 

Ningren paternity testing can be better explained with following diagram:

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