Introduction

Introduction

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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.

Objectives

Upon successful completion of this unit of instruction, the student shall be able to:

Understand the basis of single nucleotide polymorphisms, including the advantages and disadvantages of their use in forensic genetics
Understand the molecular genetics of the Y-chromosome and the applicability of Y-STR analysis in casework
Know the strategies used to analyze low copy number DNA
Describe the range of species and genetic markers used in non-human DNA analysis

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