Genetic Variations in Genomes: From SNPs to Structural Variants and Pangenomes

Vocabulary practice flashcards covering genomic variation mechanisms (SNPs, indels, structural variants, transposable elements, STRs), primate comparative genomics, non-model evolutionary adaptations, and bacterial versus eukaryotic pangenome architectures.

GRCh38

The human reference genome assembly; it is a composite sequence derived from multiple individuals rather than representing a single human genome.

Single Nucleotide Polymorphism (SNP)

A single base-pair ($1\text{ bp}$) substitution in a DNA sequence, with approximately $4\text{ to }5\text{ million}$ SNPs occurring between any two individual human genomes.

Structural Variant (SV)

A genomic alteration involving segments greater than $50\text{ bp}$ in size (such as deletions, duplications, inversions, and translocations); SVs account for over $60\%$ of differing base pairs between two humans.

Indel

An insertion or deletion of $1\text{ to }50\text{ bp}$ of DNA, commonly causing frameshift mutations or regulatory alterations.

Minor Allele Frequency (MAF)

The proportion of the less common allele at a polymorphic locus in a population, traditionally dividing variants into common ($>5\%$) and rare ($<1\%$).

Copy Number Variant (CNV)

A form of structural variant where segments of DNA are present in variable copy numbers (duplications or deletions); linked clinically to conditions such as autism, schizophrenia, and epilepsy.

Mobile Element Insertion (MEI)

A structural mutation ranging from $100\text{ bp}\text{ to }6\text{ kb}$ caused by the retrotransposition of active transposable elements, which can cause gene disruption and genome misassembly.

Alu Element

The most abundant family of Short Interspersed Nuclear Elements (SINEs) in the human genome, with over $1\text{ million}$ copies and a de novo insertion rate of approximately $1$ in every $20$ births.

LINE-1 (L1)

An actively mobile Long Interspersed Nuclear Element family present at approximately half a million copies in the human genome, with a de novo insertion rate of ~$1$ per $20$ births.

Short Tandem Repeat (STR)

A DNA segment consisting of repeating units of $1\text{ to }6\text{ bp}$ (such as $(\text{CA})_n$), exhibiting a high mutation rate ($10^{-3}\text{ to }10^{-4}$ per generation) predominantly driven by replication slippage.

Replication Slippage

The primary mutational mechanism generating variation in short tandem repeats, where misalignment of the template and newly synthesized strands during DNA replication causes unit additions or deletions.

CODIS Loci

A standardized panel of approximately $20$ short tandem repeat loci analyzed simultaneously in forensic science for human DNA profiling and individual identification.

Aneuploidy

A large-scale chromosomal abnormality characterized by an atypical chromosome number, including trisomy 21 (Down syndrome) and sex chromosome variations such as 45,X, 47,XXY, 47,XYY, and 47,XXX.

Copy-Neutral Variation

Large-scale genomic alterations that do not change the total dosage of genetic material, including balanced inversions and heterochromatin length differences at centromeres and telomeres.

Nucleotide Diversity ($\pi$)

A measure of genetic diversity defined as average heterozygosity per base pair; it is unusually low in humans (~$0.001$) compared to chimpanzees (~$0.002\text{--}0.003$) due to historical population bottlenecks.

Segmental Duplication

Long stretches of duplicated genomic DNA; human-specific segmental duplications represent a major structural difference distinguishing humans from chimpanzees and are implicated in human brain evolution.

Archaic Introgression

The retention of genetic material in modern humans through interbreeding with archaic hominins, leaving $1\text{--}4\%$ Neanderthal-derived ancestry in non-African genomes and up to $5\%$ Denisovan ancestry in Oceanic populations.

Microchromosomes

Distinctively tiny chromosomes present in the karyotypes of certain vertebrates, such as snakes and birds, coexisting alongside larger macrochromosomes.

Protocadherins

Cell-surface adhesion molecules mediating neuronal self-recognition; their gene family underwent a massive three-fold expansion in the octopus (Octopus bimaculoides, ~$168$ genes vs ~$60$ in humans), underpinning neural complexity.

A-to-I RNA Editing

An enzymatic modification where adenosine in mRNA is converted to inosine (read by translation machinery as guanosine) by ADAR enzymes, extensively recoding over $60,000$ transcriptome sites in octopuses without altering DNA sequence.

Supergene

A group of tightly linked genes inherited together as a single mendelian unit, such as the ~$600\text{-gene}$ chromosomal inversion that determines monogyne versus polygyne colony organization in the fire ant (Solenopsis invicta).

Pangenome

The entire gene and sequence repertoire across all members of a species, representing the full spectrum of population diversity beyond a single reference sequence.

Core Genome

The set of genes or genomic sequences shared by all (or nearly all) strains or individuals of a species, typically encoding essential housekeeping and cellular functions.

Accessory Genome

The dispensable portion of a pangenome present only in a subset of strains or individuals, often containing mobile elements, virulence factors, or antibiotic resistance genes.

Open Pangenome

A pangenome that increases in size without reaching an asymptote as more genomes are sequenced (such as in Escherichia coli), driven largely by high rates of horizontal gene transfer.

Closed Pangenome

A pangenome that reaches a saturation plateau after sequencing a limited number of strains, where the species possesses a fixed gene repertoire with very few private genes.

Sequence Variation Graph

A mathematical graph structure used to represent eukaryotic pangenomes, where nodes encode DNA sequence segments, directed edges represent connections, and individual genomes trace specific haplotype paths.

Human Pangenome Reference Consortium (HPRC)

A project creating reference-free human pangenome graphs from high-accuracy long-read assemblies (PacBio HiFi and ONT) across genetically diverse individuals to improve variant detection in repetitive and complex regions.

pLI Score

The probability of loss-of-function intolerance; a score greater than $0.9$ indicates a haplo-insufficient gene where loss of a single functional copy causes disease, denoting strong evolutionary constraint.