What is Genetic Variation?
Genetic variation, a fundamental concept in genetics, encompasses the intricate differences in DNA sequences among individuals within the same species. Delving into the depths of this subject reveals a mosaic of variations occurring at various levels, including single nucleotide variants (SNV), insertions and deletions (indel), copy number variants (CNV), and structural variants (SV). Of these, SNVs, arising from single nucleotide base alterations, stand as the predominant form of genetic variation. They can be inherited from parents or emerge spontaneously due to mutations, thus shaping the genetic landscape of individuals.
The paramount significance of comprehending genetic variation lies in its direct influence on individual phenotypic dissimilarities. These dissimilarities encompass a wide range of traits, including disease susceptibility, drug response patterns, and other distinctive characteristics. Notably, genetic variation plays a pivotal role in molding protein structure and function, governing gene regulation, and orchestrating the overall machinery of cellular processes. By scrutinizing and analyzing genetic variation, researchers gain valuable insights into the vast diversity and distinctive attributes of both individuals and populations.
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Fig. 1. Common Genetic Variations. (Cardoso J G R, et al, 2015)
Types of Genetic Variation
- Single Nucleotide Variation (SNV): Among the multifaceted tapestry of genetic variations, SNVs reign as the most prevalent form. These variants manifest through the substitution of a single nucleotide base (adenine, thymine, cytosine, or guanine) with another base. Further classification reveals two subcategories: synonymous and nonsynonymous variants. Synonymous SNVs uphold the encoded amino acids' integrity, whereas nonsynonymous SNVs induce changes in amino acid composition, potentially affecting protein structure and function.
- Insertions and Deletions (Indels): Delving deeper into the intricacies of genetic variation, we encounter indels—nucleotide insertions or deletions within DNA sequences. These variants span a spectrum, ranging from single base pairs to extensive insertions or deletions encompassing multiple nucleotides. Indels possess the capability to inflict code-shifting mutations, thereby disrupting the reading frame and potentially altering the function of resultant proteins.
- Copy Number Variation (CNV): The realm of genetic variation encompasses fluctuations in the copy number of specific DNA fragments, encapsulated by CNVs. These variants span a diverse range, encompassing both subtle insertions or deletions within DNA fragments and larger repetitions or deletions that encompass entire genes or genomic regions. CNVs are linked to a myriad of genetic disorders, including developmental disorders, cancer, and neurodegenerative diseases.
- Structural Variants (SV): The intricate orchestration of genetic variation unravels through the realm of SV, which encompasses sweeping modifications to genome structure. Chromosomal rearrangements, inversions, translocations, and gene fusions constitute these large-scale SV. Their profound effects extend to gene expression and function, potentially serving as harbingers of disease development or contributing to evolutionary processes.
Analysis of Genetic Variation
The realm of bioinformatics emerges as a cornerstone in the analysis and interpretation of genetic variation data. As high-throughput sequencing technologies surge forth, the generation of large-scale genomic data becomes a rapid reality, necessitating sophisticated computational tools and algorithms for analysis. Several methods and processes have been developed to identify, annotate, and interpret genetic variants. Here, we explore two commonly employed methods:
Sanger Sequencing Analysis
Sanger sequencing, a time-honored method, assumes a significant role in DNA sequencing endeavors. Amplification and sequencing of individual DNA fragments, employing modified nucleotides that halt DNA synthesis at specific sites, constitute the essence of this technique. Capillary electrophoresis facilitates the separation of resulting DNA fragments by size, unraveling the elusive DNA sequence. Although Sanger sequencing stands as a reliable and accurate approach, its scalability and cost-effectiveness face limitations when scrutinizing extensive genomic regions or analyzing multiple samples simultaneously.
Next Generation Sequencing (NGS)
The advent of NGS technologies has ushered in a revolution in genetic variation analysis, harnessing the power of high-throughput sequencing. Illumina sequencing, Ion Torrent sequencing, and Pacific Biosciences sequencing, among others, rapidly generate millions to billions of short DNA sequence reads in parallel. By employing diverse sequencing chemistries and strategies, NGS platforms usher in a workflow encompassing DNA fragmentation, library preparation, sequencing, and bioinformatics analysis. This groundbreaking technology dramatically reduces sequencing costs while exponentially expanding the scale and speed of genetic variation analysis.
Bioinformatics analysis of NGS data revolves around comparing sequence reads with reference genomes, deploying variant calling algorithms to identify genetic variants, and annotating detected variants against databases and function prediction tools. The culmination of these analyses yields variant call format (VCF) files, which furnish vital information about genomic location, variant type, and quality of identified variants.
Applications of Genetic Variant Analysis
Genetic variant analysis serves as a versatile tool, yielding insights across diverse fields, including human health, population genetics, evolutionary biology, and personalized medicine. Highlighting a few notable applications:
Disease Genetics
Delving into the genetic variation associated with diseases unravels critical insights into disease mechanisms, aids in the identification of disease-causing mutations, and propels the development of targeted therapies. Genome-wide association studies (GWAS) stand as formidable endeavors, analyzing genetic variants in large populations to unravel associations between specific genetic variations and diseases. Through GWAS, significant strides have been made in comprehending complex diseases such as cancer, diabetes, and cardiovascular disorders.
Pharmacogenomics
Genetic variation assumes a central role in modulating an individual's response to drugs, influencing both efficacy and potential side effects. Pharmacogenomic studies delve into how genetic variation impacts drug metabolism, drug targets, and inter-individual variability in drug response. This wealth of knowledge underpins the realm of personalized medicine, empowering healthcare professionals to tailor treatment plans based on an individual's unique genetic makeup.
Forensic Genetics
Genetic variation analysis assumes an indispensable position in forensic genetics, aiding in human identification and criminal investigations. DNA analysis techniques, such as short tandem repeat (STR) analysis and single nucleotide polymorphism (SNP) genotyping, exploit genetic variation to establish individual identities or relationships, thereby assisting in criminal investigations.
Population Genetics and Evolutionary Biology
Analyzing genetic variation within and among populations unearths a treasure trove of insights into human migration patterns, ancestry, and evolutionary relationships. By comparing genetic maps, researchers can trace population histories, identify genetic markers associated with population-specific traits, and unravel the effects of natural selection on genetic variation.
References
- Cardoso J G R, Andersen M R, Herrgård M J, et al. Analysis of genetic variation and potential applications in genome-scale metabolic modeling[J]. Frontiers in bioengineering and biotechnology, 2015, 3: 13.
- Barnes MR. Genetic variation analysis for biomedical researchers: a primer. Methods Mol Biol. 2010;628:1-20.
