First, the identification principle of paternity test The theoretical basis for judging the biological relationship is separation phenomenon inherited by Mendel. According to this rule, when gamete cells are formed, pairs of alleles are separated from each other and enter their respective gamete cells respectively. Sperm and egg cells are fertilized to form offspring, and the two genomes of the child come from mother and father; Therefore, one allele of the same pair comes from the mother and the other from the father. If the identification results conform to this law, biological relationship is not excluded; If not, biological relationship (except variation) will be ruled out. In most cases, the mother-child relationship is known, and it is necessary to identify whether the hypothetical father and son are biological. At this time, from the comparison of maternal and offspring genotypes, we can determine the gene (OG) that may come from the father among the children's genes. Then observe the genotype of the hypothetical father. If there is no gene of the biological father, the biological relationship between the hypothetical father and the child can be ruled out. If it is assumed that the father also has the biological father gene, this result cannot be ruled out. If in a case, the mother is FGA-22/23 and the child is 22/25, then it can be determined that the biological father gene is FGA-25 by comparison. In this case, assume that the father 1 is FGA-22/24; Suppose the type of parent 2 is 24/25. Assuming that the father 1 does not have the biological father gene 25, then the biological relationship between him and the child can be ruled out; In contrast, assuming that father 2 has FGA-25, it is not excluded that he is related to his children. II. Steps of Paternity Identification The operation steps of DNA paternity experiment are as follows: 1. DNA extraction The DNA contained in the sample nucleus is extracted, and then it is purified to remove impurities in the sample. 2.PCR amplification The Chinese name of PCR is polymerase chain reaction. Simply put, the PCR amplification step of Huake gene is to copy a large number of required fragments on PCR instrument through enzymatic reaction, and enlarge them to the extent that some special instruments can see them. 3. The post-PCR reaction step is mainly the preparation stage of ABI sequencer detection. Open the double-stranded DNA and add some internal standards for detection, which is mainly used to mark the length of the detection fragment. 4. The capillary sequencer detected that DNA was charged. Through capillary electrophoresis, the electrophoresis speed of different DNA fragments is different. Under the same voltage and electrophoresis time, the swimming distance is different. These different distances can be distinguished by the internal standard measurement added in the early stage, and displayed on the computer through certain software, which is convenient for testers to process and analyze the data. 5. Analyze the data and issue a report, mainly to let the inspectors analyze, summarize and calculate the obtained results, and then make an appraisal conclusion and report. Third, the modern method of paternity test 1, blood type identification paternity test is to confirm the parent-child relationship through the comparison of blood types. According to Mendel's genetic law confirmed at the end of 19, people realize that human blood types are passed on to the next generation according to genetic genes, so children born to parents of a certain blood type also have corresponding blood types, which lays the foundation for blood type identification of parent-child relationship. The more blood group systems tested, the higher the accuracy. If the blood type test results show that there is no kinship, the conclusion of negative paternity can be made, but the result of kinship cannot be completely determined as paternity. In 1970s, it was found that the antigens of white blood cells could be used for paternity testing, with an accuracy rate of 80%. Combined with blood type test, it can achieve high accuracy. 2. Chromosome polymorphism In the 1980s, medical scientists took the lead in using chromosome polymorphism to identify parent-child relationship. Chromosome polymorphism, also known as heteromorphism, refers to the slight variation of various chromosome shapes common to normal people (such as the increase, repetition or deletion with the body, the variation of fluorescence intensity in chromosome regions, etc.). ), and this polymorphism is heritable. This technology uses its form to identify parent-child relationship, which depends on the subjective judgment of technicians, and its accuracy is not satisfactory. 3. DNA identification The most commonly used paternity test using domestic preparations is DNA typing. Human blood, hair, saliva, oral cells and bones can all be used for paternity testing, which is very convenient. A person has 23 pairs (46 chromosomes), and a pair of genes in the same position of the same pair of chromosomes are called alleles, usually one from the father and one from the mother. If the allele of a DNA locus is detected, one is the same as the mother and the other should be the same as the father, otherwise there is doubt. Using DNA for paternity testing, as long as ten to dozens of DNA loci are detected, if they are all the same, the paternity can be determined; If more than three loci are different, the parent-child relationship can be ruled out; If one or two loci are different, we should consider the possibility of gene mutation and detect some loci to distinguish them. DNA paternity, the accuracy of denying parent-child relationship is nearly 100%, and the accuracy of affirming parent-child relationship can reach 99.99%.