DNA Polymorphisms and Human Identification

Polymorphisms in Genetics

Types of Polymorphisms

Definition

Polymorphisms are variations in DNA sequences that differ from the majority sequence in a population, yet are shared by a significant percentage of individuals. They can play a crucial role in genetic diversity and studies.

1. Single Nucleotide Polymorphisms (SNPs)

  • Description: Variations that occur at a single nucleotide in a gene or non-coding region.

  • Frequency: They occur frequently, about every 1,000 to 1,500 base pairs, making them abundant throughout the human genome.

  • Scientist: Discovered in the 1990s, SNPs are now widely referred to in genetics, but no single scientist is credited with their discovery as it was a collective advancement in genomics.

2. Long Interspersed Nucleotide Sequences (LINES)

  • Description: Larger repetitive sequences that contribute to genetic variation.

  • Scientist: First identified through molecular cloning techniques, but specific scientists are not associated with their discovery.

3. Short Interspersed Nucleotide Sequences (SINES)

  • Description: Smaller repetitive sequences, such as Alu elements, present in many copies in the genome.

  • Scientist: The Alu sequence was discovered by Dr. Marshall Nirenberg in the 1970s.

4. Short Tandem Repeats (STRs)

  • Description: Sequences of DNA consisting of short (1-10 base pairs) repeated segments; very useful in genetic analysis and forensic investigations.

  • Scientist: Discovered by Dr. Alec Jeffreys in 1984.

RFLP Typing

RFLP Definition

RFLPs (Restriction Fragment Length Polymorphisms) are variations in the lengths of DNA fragments produced by the digestion of DNA with restriction enzymes.

Purpose

RFLPs serve as molecular markers in gene mapping, forensic identification, and parentage testing.

Detection Method

Involves cutting DNA with restriction enzymes, analyzing fragments using Southern blotting, and probing with specific indicators.

Applications

RFLPs laid the groundwork for modern genetic profiling and significantly improved the ability to differentiate among individuals.

Genetic Mapping with RFLPs

RFLPs are genetic markers that help locate genes within the genome, following Mendelian inheritance patterns, linking markers to diseases.

Parentage Testing with RFLPs

This testing compares a child's DNA profile to both parents to check for shared genetic markers, helping to establish paternity.

Human Identification Using RFLPs

Originally focused on blood group antigens, the development of DNA analysis through RFLPs significantly enhanced individual discrimination.

STR Typing by PCR

  • Method: STR analysis is effective due to its high discrimination capacity and low DNA requirement.

  • Process: STRs are amplified through Polymerase Chain Reaction (PCR), allowing for sizing that determines allele combinations.

Gender Identification

The Amelogenin locus helps differentiate between male (XY) and female (XX) individuals based on amplified fragment patterns.

Y-STR Matching

Y-STRs, inherited through the male line, are essential in forensic and lineage studies for tracing male ancestry.

Engraftment Testing

Essential for ensuring donor-recipient compatibility post-transplant, DNA polymorphisms can differentiate recipient and donor cells effectively.

Linkage Analysis

Relies on known chromosome locations of STRs to uncover associations with genetic diseases, aiding familial linkage studies.

Quality Assurance of Tissue Sections

STRs help confirm the origin and DNA purity of surgical samples for diagnostics.

Single Nucleotide Polymorphisms (SNPs)

  • Most frequent form of genetic diversity; insights into diseases and traits occur about once every 1000 base pairs across the genome.

Mitochondrial DNA Polymorphisms

Passed down maternally, mitochondrial DNA is used for ancestry verification due to its stable circular genome structure.

Summary Table

Type of Polymorphism

Description

Frequency/Occurrence

Applications

Detecting Methods

Scientist

SNPs

Variations that occur at a single nucleotide in a gene or non-coding region.

Approximately every 1,000 to 1,500 base pairs throughout the genome.

Disease gene mapping, population studies, understanding genetic predisposition.

Genome sequencing and genotyping technologies.

Discovered in the 1990s as a collective effort.

LINES

Long repetitive sequences that are interspersed throughout the genome.

Varies greatly; contributes to genetic diversity.

Contributing to genetic variation, evolutionary studies, and comparative genomics.

Molecular cloning techniques; presence inferred through genomic studies.

No specific discoverer known.

SINES

Short repetitive sequences like Alu elements, which occur in many copies.

Commonly present in primate genomes, millions of copies in humans.

Evolutionary biology, population genetics, and studies on gene regulation.

PCR amplification and sequencing of specific regions.

Dr. Marshall Nirenberg in the 1970s.

STRs

Sequences of DNA consisting of short (1-10 base pairs) repeated segments.

Typically found in various loci across the genome, often analyzed in sets.

Forensics, paternity testing, identity verification, and ancestry analysis.

PCR amplification for sizing and fragment analysis.

Dr. Alec Jeffreys in 1984.

RFLPs

Variations in the lengths of DNA fragments after restriction enzyme digestion.

Varies with the size of the sample and enzyme used; several polymorphisms present.

Genetic mapping, human identification, paternity testing, tracking genetic diseases.

Southern blotting and DNA fragment analysis post-digestion.

No specific discoverer known.