Tag Archives: forensic genetics

DNA Analysis

Question: What is DNA?

Answer: The nuclei of all cells in the body, excluding sperm and eggs, contain 23 pairs of chromosomes. Each of the 46 chromosomes consists of a thread of deoxyribonucleic acid (DNA). This DNA thread is made up of two strands of bases held together by a sugar–phosphate backbone arranged in a double helix. The double helix has the configuration of a twisted ladder whose steps are formed by the four bases: adenine, thymine, guanine, and cytosine. Adenine (a purine) always binds to thymine (a pyrimidine), and guanine (a purine) always binds to cytosine (a pyrimidine). A gene is a portion of this DNA thread (chromosome) that produces a specific product. The length of this gene can range from a few thousand to tens of thousands of base pairs. The order of the four bases on the thread determines the function of the gene. Genes, however, constitute only a small fraction of the total length of the chromosome. The purpose of the rest of the chromosome is unknown. There is usually more than one form of a gene for each location or locus on the chromosome. The alternate forms are called alleles.

Question: What is DNA Analysis and what is it used for?

Answer: DNA analysis is the interpretation of genetic sequences. It can be used for medical diagnoses, determine parentage, criminal investigations, etc. It is mainly used to identify a species, but can distinguish individuals within a species.

Question: How to match DNA to suspect/victim?

Answer: At the time of Analysis If the DNA profile of evidence from a crime scene or victim is different from the suspected source, then that evidence did not come from the suspect at all. If the DNA profile of the evidence matches that of a person, the person is not excluded as the source of the evidence. One performs a statistical evaluation as the source of incriminating evidence.

There are three possible results of the analysis:

1. The specimen is either inadequate in size, degraded, or contaminated, thus resulting in an insufficient amount of DNA for analysis.
2. The DNA profiles are different and one has an exclusion.
3. The DNA profiles match.
If DNA Profile Matches; the following may indicate:
a. The samples come from the same person.
b. The samples come from different people but an error was made either in the collection of the material or in the laboratory.
c. Two individuals have the same DNA profile. This could be because they are identical twins or because an insufficient number of tests have been performed so as to differentiate between the two.

Question: What are the three types of DNA analysis?

Answer: There are multiple types of DNA tests. Three major types of genealogical DNA tests: Autosomal and X-DNA, Y-DNA and mtDNA (mitochondrial DNA) are most common.

Question: What are the steps of DNA Analysis?

Answer: The steps in DNA analysis include:

  1. Sample collection and storage,
  2. Extraction and quantitation of DNA,
  3. Genotyping to generate an individual pattern of short tandem repeat (STR) loci,
  4. Interpretation of Result and,
  5. Storage of the results.

Question: Where can DNA Analysis be Performed?

Answer: DNA analysis can be performed on any tissue or substance that contains nucleated cells. The two most common tissues examined are: Blood and Semen.

Question: What techniques are used to analyze DNA?

Answer: Following Techniques are used to analyze DNA:

Restriction Fragment Length Polymorphism (RFLP): It is also known as Southern Blot Analysis. The technique involves analysis of DNA fragments (restriction fragments), which differ in length between individuals (length polymorphisms). The basic technique of identifying such restriction fragment length polymorphisms involve fragmenting a sample of DNA by a restriction enzyme, which can recognize and cut DNA wherever a specific short sequence occurs, in a process known as a restriction digest. The use of RFLP analysis to identify hypervariable DNA reveals many genetic differences among individuals. The hypervariable DNA consists of repetitive regions, which differ in size due to differing numbers of core sequence repeats: so-called “variable number of tandem repeats” (VNTRs).

PCR (polymerase chain reaction) : The polymerase chain reaction is a method for amplifying or copying a short sequence of DNA repeatedly, thus, going from a small amount of DNA to a very large amount. In the PCR methodology, polymerase enzymes are used to copy specific regions of DNA to obtain numerous copies of these areas to perform typing. PCR-based typing systems allow alleles to be identified as discrete entities.

STR (short tandem repeats): STR is a PCR technique used in forensic labs to make DNA identifications. The repeat unit in this system is normally two to six base pairs. STR loci occur throughout the genome at an estimated frequency of 1 STR every 10,000 nucleotides (a base, a sugar, and a phosphate). While most STR loci have only 6 to 12 alleles, there are a large number of such systems that can be exploited for identification purposes. In combination, they can produce a high power of discrimination. As a general rule, use of 8 to 13 STR loci gives a discriminative power of about 1 in 1 billion. It is rapid and can be done in 2 to 3 days and it can be performed on very small quantities of DNA.
STR can be performed on wipings from full-metal-jacketed bullets that have perforated the body, even if no tissue is visible.

References

  1. Parker B, Peterson J. Physical evidence utilization in the administration of criminal justice. In: Cohn SI, McMahon WB, eds. Law Enforcement, Science and Technology III. Chicago, 111: Illinois Institute of Technology Research Institute; 1970:513-524.
  2. Jeffreys AJ, Wilson V, Thein SL. Hypervariable minisatellite regions in human DNA. Nature. 1985;314:67-72.
  3. Inman K, Rudin N. Introduction to Forensic DNA Analysis. Boca Raton, Fla: CRC Press; 1997.
  4. Kirby LT. DNA Fingerprinting: An Introduction. New York, NY: WH Freeman & Co; 1992.

Genetic Genealogy

Genetic Genealogy is also known as Genetic Ancestry Testing.

Now a days it is very popular way to know about one’s ancestry or to confirm the relationship between two people. This is a method through which people can learn about their ancestry.

Every family shares certain patterns of genetic variations and Genetic Genealogy uses DNA testing to determine the genetic relationship between peoples.

The use of DNA testing in conjunction with traditional genealogy and historical records is Genetic Genealogy.

DNA (Deoxyribonucleic acid) Test is a tool which is used by the law enforcement to know the genealogy or the relation between evidence and victim/suspect.

Following DNA Tests are commonly used to for Genetic Genealogy:

Y-Chromosome Testing:

Y-Chromosome testing can be used to know the ancestry in male line or Patrilineal line. Y-Chromosome is found only in males. It can help to identify patrilineal surname lines.

mt-DNA Testing:

Mitochondrial DNA Testing can be used to know the ancestry in matrilineal line. mt-DNA are passed from mothers to their children. Both men and women have the mt-DNA so this test is for everyone.

Autosomal DNA Testing:

Autosomal DNA Testing trace the autosomal chromosome of an individual which include the DNA segments that the person shares with others with whom they are connected.

Single Nucleotide Polymorphism Testing:

SNP testing identifies genetic variations within an individual’s genes,
This test measures a large number of variations (SNPs) in an individual’s entire genome.

Limitations of Genetic Genealogy:

There are some drawbacks of genetic genealogy. Different test providers compare the results of tests of individuals to several different databases of previous tests, so the results may vary and may not be consistent. Furthermore, because the majority of human populations have migrated multiple times from here to there and mixed with neighboring groups, estimates of ethnicity may differ from an individual’s expectations based on genetic testing.