DNA is deoxyribonucleic acid, the most complex molecule found in living organisms and in which the genetic information of each individual is accumulated. It is transmitted from parents to children, thus guaranteeing the continuity of the species. Given the fundamental importance of this molecule, nature arranged for it to be found in the nucleus of the cells, so that its "safe", in order to guarantee its protection. Inside the nucleus, DNA is organized into subcellular structures called chromosomes, which can be observed under a microscope by laboratory procedures.

There are 24 types of chromosomes in a human being. They are 23 so-called autosomal chromosomes or simply autosomes and the gender chromosome, which can be either X or Y. In almost all human cells the chromosomes are presented in pairs, i.e., each cell contains a pair of each type of autosomal chromosome and a pair of gender chromosomes, so that in total, they present 46 chromosomes, 44 of them autosomes and 2 genders. Exception to this are germ cells (ovules and sperm) containing only one copy of each type of chromosome and red blood cells or erythrocytes, which do not contain chromosomes (since they do not have a nucleus). The pair of gender chromosomes that an individual presents is the one that defines their gender. A normal woman presents two copies of the X chromosome (XX) while the man presents a copy of the X and another one of the Y (XY). In the zygote, which gives rise to a living being and is the product of the fertilization of an egg (from the mother) by a spermatozoon (from the father), the 23 chromosomes contained in each of the two germ cells to give it a total of 46 that are the corresponding to our species. That is why we can ensure that one person receives one half of their genetic material (DNA) from their biological mother and the other half from the biological father. That is, in each individual are present genetic characters of each of their two parents. Each parent, regardless of whether the mother or father, transmits to their children only a copy of each of their 22 autosomes. However, regarding the transmission of the gender chromosomes there are differences between the father and the mother. As the woman is XX, the mother will always transmit to her offspring an X chromosome, while the man, for being XY, can transmit the same X as the Y. If the man transmits the X chromosome, then his descendants will be a woman (XX), while if he transmits the Y will be a man (XY).

It is a fragment of the DNA that presents / displays high level of heterogeneity (or polymorphism), reason why it can appear in different forms in different people. The different ways in which a marker can be presented are called alleles. That is, a marker, being polymorphic, has different alleles in a particular population. Like any other genetic character, each individual receives an allele from each parent of each DNA marker. That is, an individual has two alleles of each DNA marker, each from a parent. As these markers are highly polymorphic (i.e., different alleles are present in a population), it is very likely that the two alleles with the same individual are different, that is, they have two different alleles, being heterozygous for this marker. However, it is also possible that both alleles are equal, i.e., it has two copies of the same allele and in this case the individual is homozygous for this marker. The set of the two alleles of a marker presenting an individual, be different alleles or the like, is known as the genotype of that marker. The combination of the genotypes of the markers analyzed in an individual is defined as their DNA profile. The markers are also identified taking into account the type of chromosome in which they are located. This is called autosomal markers to those found on autosomes and markers of the X chromosome or the Y chromosome to those found on gender chromosomes. The most commonly used DNA markers in forensic practice are the so-called short tandem repeats (STR) markers. Each of these markers has a short DNA sequence that is repeated in tandem form (one after the other) in varying numbers. In this way, in a chromosome this marker can be with 2 repetitions of this sequence, whereas in another chromosome it presents / displays with 5 repetitions. Each allele is defined as a function of the number of replicates it presents (allele 2 and allele 5 for the example used). In order for a DNA marker to be reliably used in medical-forensic studies, it must be validated in each population and the frequencies that its alleles and their genotypes present. Our laboratory has performed these validation studies for all of its markers and defined its frequencies, which are used in the studies that it performs. (See POPULATION DATABASES)

They are the markers located on the 22 types of autosomal chromosomes. These markers are the most used in forensic practice and you will find millions of them in the human species. The international forensic practice has selected a group of these markers as being the most reliable and informative and among them are the 19 markers used by our laboratory.

This type of markers is of great value in studies of filial relation because it is exclusively paternal transmission, so it allows to determine if two males are related, even if the degree of kinship is distant. Since the Y chromosome is inherited exclusively from man to man, all men in the same family line will theoretically have the same Y chromosome and therefore the same allele as the markers present on this chromosome. Many STR markers have been characterized in the Y chromosome. The international forensic community has selected among them a group of markers that by their characteristics are the most convenient for studies of forensic genetics. Among them are the 11 markers used by our laboratory. However, the fact that all markers on the Y chromosome are located on the same chromosome limits its use in probabilistic calculations. For this reason the set of these analyzed markers conforms a haplotype of the Y chromosome that is the genetic unit used in the calculations.

These markers are located specifically and exclusively on the X chromosome. The fact that X-chromosome transmission is different in men and women marks the use of these markers in forensic studies. For example, in the transmission of the mother to its children the X chromosome behaves just like the autosomes, because it can transmit any of the two copies of this chromosome that presents. However, in men, only one copy of the X chromosome is present, so all their daughters will receive the same paternal copy, while none of their children will have it. The introduction of these markers in forensic practice is very recent and is still in the process of international validation. Our laboratory participates in this validation process and has incorporated the battery of 10 markers currently selected for these purposes.

The Filial Relations studies are of enormous social and family importance. At present, it is accepted by all specialists that the execution of these studies with the use of DNA markers is much superior to when using other conventional markers such as blood groups, protein markers and even the HLA system. DNA markers report greater resolving power and have great flexibility that allows them to adjust to different situations that frequently characterize these studies.

In general, yes, but one cannot fail to consider the possibility that the exclusion is due to a rare but probable phenomenon called mutation. The mutation can cause changes in the number of replicates present in an allele of an individual STR marker, becoming another allele that does not match that of the parent that transmitted it. In a paternity study, this phenomenon would cause it to be falsely considered that the biological father is not the father. But due to the rarity of this phenomenon it is very unlikely that more than one mutation is present in an individual, so it is internationally required to find at least three markers that indicate exclusion in order to be able to define with absolute confidence that the alleged father is not the biological father.

You never get full biological security of paternity. It is probable that because of chance there are no exclusive markers even though the alleged father is not the true father. This is why a large number of markers are required. In cases of non-exclusion the final result is expressed as two parameters: The Paternity Index (PI), expressed by a number, which indicates the times that the alleged father is more likely to be the biological father than to what is any other individual. That is, an IP of 1000 indicates that the alleged father is 1000 times more likely to be the biological father than he is not. The Probability of Paternity (PP), which is usually expressed as a percentage and never reaches 100%, so there will always be a margin of doubt about paternity. How many markers are required? Impossible to predict. The number of markers required in a study in which no exclusions are found is defined on the progress of the study and as a function of the values of the previously described parameters proposed by each laboratory. For example, in our laboratory we analyze the number of markers that is required to offer at least an IP of 1000 and a PP of 99.9%. This information provided by the study of DNA markers is added to the other evidence available to the judge to conclude regarding paternity.

Paternity studies are more informative when they involve the mother of the individual whose biological father wants to know. With the participation of the mother (and starting from the premise that motherhood is guaranteed) it can be defined which of the two alleles of each marker present in the child is inherited from the mother and, therefore, is defined unambiguously what is the one that inherited of the biological father. Comparing this with those that the alleged father presents. If it is not possible to know through the analysis of the mother, which is the allele present in the child that comes from the mother, then when a paternity study is carried out, it is necessary to consider the possibility that the father transmits any of the two alleles present in the child. Nevertheless, the mother's participation is not indispensable. A paternity study can be performed in her absence, although in these cases the analysis becomes more complex and requires the study of more DNA markers. In general, studies involving the mother are more informative and have results that are more reliable.

Yes, it is possible, and that is why we have several strategies for it:

1. By studying some biological sample from the alleged father that is available. For example, a court order can be obtained for the exhumation of the body, provided that the death has not been long before the study, and samples of hair (capillary bulbs), bones, teeth or muscle are obtained, depending on the state of conservation of the corpse. Some samples of the deceased may also be preserved for sentimental reasons ("the first tooth") or for medical reasons (appendix, biopsies preserved in paraffin blocks). However, in these cases there is always no certainty that the sample will be useful. However, this possibility should be taken into account because of the advantages of carrying out the paternity study with the alleged father.

2. Reconstruction of the alleged father's genotype. This is possible if you have other children of the alleged father whose paternity is not questionable. The greater the number of these children available, the more likely it is to reconstruct the genotype of the so-called Father. In the event that it is not possible to reconstruct the genotype of the Supposed Father for a sufficient number of markers, there is still the possibility of performing the analysis of Brotherhood between the Son whose paternity is discussed and his supposed brothers (the children with no doubtful fatherhood). For these cases, it is very desirable that both the mother of the child whose parenthood is questioned and the mothers of the non-doubtful children participate.

3. From the paternal grandparents. As alleles for a marker, bearing the alleged father inherited them from their mother and father, and then such alleles must be present in their parents. Therefore, it is possible to perform the study of paternity from the supposed paternal grandparents. In these cases, it is highly recommended that the mother participate in order to be able to define more accurately, what is the allele that for each marker inherited the son of his father. Once defined the allele inherited from the father, it is sought if it is present in any of the supposed paternal grandparents. If so, the possibility of paternity cannot be excluded. Nevertheless, if it is not present in the alleged grandparents, then the possibility of paternity is excluded, since it is not possible that the alleged father transmitted an allele he could not have received from his parents. In these cases, the study of a large number of markers is required.

4. From uncles by paternal way. When the alleged paternal grandparents are not available, but other children of these (brothers of the alleged father) are. In these cases, the first thing is to reconstruct the genotypes of the alleged grandparents, but this requires the existence of several uncles (no matter the gender) and not even that is always possible. If reconstruction is not achieved, then the evaluation of the relationship between the alleged uncle and the individual whose father wishes to evaluate can be performed. If a degree of kinship between them is shown, it is indicative that the alleged father also presents the same.

5. From any male relative of the alleged father. This study is carried out by the use of markers of the Y chromosome, which, as we have seen, is inherited exclusively by the paternal route and all the male individuals of the same paternal family line have the same Y chromosome.

Of course. This doubt occurs frequently, especially in the face of the fear of changing a child in the maternal hospital where he was born. In these cases, it is not necessary to study the alleged father as well, since doubt of motherhood also leads to doubt of paternity, so the alleged father would not report much information. The analysis in these cases is performed just like a paternity study with an alleged absent father.

Yes, conceptually you can study any family bond. However, the reliability of the results will depend on many factors and is not always satisfactory. That is why it is advisable to evaluate with the specialists the feasibility of doing it. As we have already seen, the study of "grandparentage" is feasible, just like any other family relationship between men.

The potency of these studies with DNA markers is such that it generally allows the use of any type of biological material sample. However, depending on the type of sample will be the quantity and quality of the DNA extracted for the study, so it is always convenient to define which is the best option available. The main options are as follows:

* Peripheral blood obtained by venipuncture and preserved with anticoagulant. It is the best choice as to the quantity and quality of the DNA that you can obtain. Usually (with) one milliliter of blood is enough and the anticoagulant to use should be EDTA. Samples must be transported at 4 ° C (not frozen).

* Blood obtained by digital puncture and deposited on a solid support. It is the most convenient case when transporting the sample from distant places is required, since it does not require refrigeration. The solid support used are FTA sheets, material specially designed for these purposes.

* Cells of the buccal mucosa taken by swab. To be used in the case of newborn infants in whom blood collection is not possible or recommended. It presents the difficulty of demanding a fast transportation and under very strict conditions and that; the obtained DNA is usually of poor quality.

* Fetal cells obtained by amniocentesis or chorionic biopsy. They are used when the paternity study is requested in the prenatal state. The technique to be used to obtain the sample will depend on the time of pregnancy at the time of sampling. Samples must be transported at 4 ° C (not frozen).

* Solid tissues (teeth, capillary bulb, bones, muscle, skin, fragments of organs). To be assessed in cases where the individual is deceased. They can correspond to samples preserved before or post-mortem and can be obtained by exhumation of the corpse. The feasibility of its use will depend on the time it takes the individual to die or on the conservation of the sample and must be analyzed in advance with the specialists of the Laboratory.

Genomik develops studies of filial relation since the year 1999, being pioneer in this field in the national scope. Its specialists have extensive experience in carrying out these studies and guarantee their scientific update in this field by participating in the activities organized by the Spanish-Portuguese Group (GEP) of the International Society of Forensic Genetics (ISFG), of which they are members. Particular mention deserves the successful results obtained in the quality controls annually convened by the GEP. The quality and reliability of its results are further strengthened by having the latest technologies currently available for these studies in the international market and with powerful batteries of available DNA Markers (for definition click here and link to What is a DNA marker in Rationale); allowing to approach any variant of study of feasible filial relation to realize and of medical forensic identification. This battery of markers is continuously increasing to increase the possibilities of resolution of complex cases and now it is integrated by:

• 19 autosomal STR markers. These markers are the most commonly used in affiliation and medical-forensic identification studies and allow the resolution of at least 95% of the studies requested.

• 11 STR markers of the Y chromosome. Very useful in studies of filial relationship and medical-forensic identification in which a kinship relationship is evaluated by patri-linear means, that is to say, relatives through parental lines (son-father- grandfather-paternal uncle-paternal cousin-etc.).

• 10 STR markers of the X chromosome. Very useful when it comes to establishing common parenthood, regardless of gender and other complex studies. In addition, Genomik C.A. has developed software that allows it to guarantee the maximum confidentiality of the information corresponding to the studies and to perform the probabilistic calculations of filial relation on the statistical bases supported by the ISFG. The reliability of our results is expressed in the rigorous criteria with which we evaluate the conclusion of a study, all based on the recommendations of the ISFG:>

• Perform the analysis with a battery of markers that reach a probability of exclusion a priori not less than 99% and with a number of markers of at least 15, regardless of the partial results that are being obtained.

• Criteria for considering exclusion from maternity / paternity: at least 3 exclusionary markers.

• Criteria for considering a non-exclusive maternity / paternity study: an a priori exclusion probability of at least 99% and a Paternity Index (link to paternity is guaranteed? In Rationale) of at least 1000 (for which a probability of paternity of 99.9% corresponds when an a priori probability of 0.5 is considered).

• Perform the probabilistic calculations using the information corresponding to our population based on POPULATION DATABASES prepared by our Laboratory.

Description: GENOMIK LABORATORY is a specialized laboratory in Molecular Biology applied to clinical diagnosis and medical-forensic studies, using molecular amplification techniques detected by endpoint, real time, capillary electrophoresis.

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Publicado en: 2017-11-08 (93 Lecturas)


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