It is a routine for the forensic scientist to obtain a genetic profile of an individual from nuclear DNA recovered from a biological stain deposited at the crime scene. Potential contributors of the stain must either be known to investigators and compared to a known profile or searched against a database of DNA profiles, such as those maintained in the Combined DNA Index System (CODIS).
This module describes various techniques, including single nucleotide polymorphisms (SNPs), Y-STRs, low copy number (LCN), and non-human DNA. These methods can be used to obtain more probative investigative information and may be used as an adjunct to standard autosomal short tandem repeat (STR) analysis. In some cases, information is provided that otherwise could not be obtained by standard techniques.
Certain factors may preclude the development of genetic profiles despite the presence of probative biological material in a crime stain because:
- The quantity of DNA present is below the detection limit
- and/or -
- The DNA is partially degraded, such that the number average molecular weight is less than the STR amplimer sizes
Potential remedies include:
- Low copy number (LCN) analysis techniques for minute quantities of DNA
- SNP typing for partially degraded DNA
In mixed male/female specimens, Y-chromosome specific systems are invaluable for the identification of the genetic profile of the male component in the following instances:
- The female portion is present in overwhelming quantities relative to the male
- The standard autosomal STR analysis fails to yield the male donor profile
In cases where there is no developed suspect and no match with a database sample as yet, the DNA profile presently provides no meaningful information to investigators, with the notable exception of gender determination. In these situations, it would be advantageous to law enforcement investigators if additional information could be obtained from the biological stain. SNP analysis using specially selected markers can be predictive of ethno-geographic ancestry; this is expected to play an increasing role in the investigation of crime.
Another area that will contribute to the crime scene investigation is non-human DNA analysis. As techniques are developed they will become more commonplace in forensic biology.
Author: Jack Ballantyne
Jack Ballantyne is an Associate Professor of Chemistry at the University of Central Florida (UCF) and the Associate Director for Research at the National Center for Forensic Science in Orlando, Florida. He possesses a B.Sc. (with Honours) in Biochemistry from the University of Glasgow, Scotland, a M.Sc. in Forensic Science from the University of Strathclyde, Scotland, and a PhD in Genetics from the State University of New York at Stony Brook, NY. His current duties include teaching and conducting research in forensic molecular genetics. He teaches a variety of forensic biology courses to baccalaureate and Masters-level students in the Forensic Science Program and nucleic acid biochemistry to PhD students in the Biomolecular Sciences Program.
Prior to entering academia, he was a casework forensic scientist in Scotland, Hong Kong, and New York where he proffered expert testimony in the criminal courts of these jurisdictions. He was the full-time DNA technical leader in Suffolk County, New York, and since then has served as a part-time consultant DNA technical leader for the States of Mississippi and Delaware, the City of Dallas, and Sedgwick County, Kansas. Inter alia, he is the Chair of the New York State DNA Sub-committee, a regular visiting guest at the Scientific Working Group on DNA Analysis Methods (SWGDAM), a member of the DoD Quality Assurance Oversight Committee, and was a member of the World Trade Center Kinship and Data Analysis Panel (KADAP).
His research interests include Y chromosome markers, the assessment and in vitro repair of damaged DNA templates, mRNA profiling for body fluid identification, the determination of physical characteristics by molecular genetic analysis, and single cell/low copy number analysis.
Author: Erin Hanson
Erin Hanson possesses a Bachelor of Science degree in Forensic Science, Biochemistry Track, and a Master of Science degree in Forensic Science, Biochemistry Track, from the University of Central Florida. Her graduate research included development and validation of novel Y chromosome STR multiplexes for use in forensic casework. A comprehensive annotated STR physical map of the human Y chromosome was also developed through this research and made available to the forensic community. Currently, Ms. Hanson is a PhD candidate in the Biomolecular Science Doctoral Program at the University of Central Florida, and her current research includes an assessment of damage and degradation to macromolecules in dried biological stains, and the use of laser capture microdissection and whole genome amplification strategies for the analysis of single cells.
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- What Every First Responding Officer Should Know About DNA Evidence
- Collecting DNA Evidence at Property Crime Scenes
- DNA – A Prosecutor’s Practice Notebook
- Crime Scene and DNA Basics
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- DNA Amplification
- Population Genetics and Statistics
- Non-STR DNA Markers: SNPs, Y-STRs, LCN and mtDNA
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- Forensic DNA Education for Law Enforcement Decisionmakers
- What Every Investigator and Evidence Technician Should Know About DNA Evidence
- Principles of Forensic DNA for Officers of the Court
- Law 101: Legal Guide for the Forensic Expert
- Laboratory Orientation and Testing of Body Fluids and Tissues
- DNA Extraction and Quantitation
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- Español for Law Enforcement
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