Week 5. Genetic Variation

Genetic Variation

  • Definition: Differences in genetic makeup within and among populations.

  • Polymorphism: Multiple variants (alleles) of a gene; when a gene has only one allele, it's termed a fixed allele, indicating a lack of diversity.

Human Genome Project (HGP)

  • Timeline: 1990-2003.

  • Goal: To analyze the human genome and identify genetic variations relevant to diseases.

  • Structure: Over 3 billion nucleotides sequenced.

  • Funding: $3 billion by an international consortium including institutes from the USA, UK, China, and more.

  • Limitations: Primarily based on donors of European descent.

Organization of the Human Genome

  • Coding Sequences: Estimated 100,000 originally, revised to 21,000 (1.5% of the genome).

  • Regulatory Sequences: Approximately 25% which includes promoters and introns.

  • Noncoding RNAs: Includes tRNA, rRNA, miRNA, etc.

  • Repetitive DNA: Comprises 44%, including transposable elements.

  • Unique Noncoding DNA: About 15%.

  • Transposable Elements: Major component of genetic variability in genomes.

Sources of Genetic Variation

  1. Sexual Reproduction: Involves gamete fusion leading to new allele combinations through:

    • Crossing Over: Exchange of genetic material during meiosis.

    • Independent Segregation: Random distribution of maternal and paternal chromosomes during gamete formation.

    • Assortment of Chromosomes: Significant variation due to independent assortment of chromosomes.

  2. Horizontal Gene Transfer (HGT): Transfer of genetic material between organisms, particularly common in bacteria through:

    • Conjugation: Transfer of DNA via direct contact.

    • Transduction: Transfer via bacteriophages.

    • Transformation: Uptake of free DNA from the environment.

  3. Mutations: Various types include:

    • Single-Nucleotide Polymorphisms (SNPs): Single base changes, found throughout genomes. They often do not cause phenotypic changes but can indicate genetic diversity.

    • Copy Number Variations: Variability in the number of copies of a particular gene.

    • Structural Chromosomal Aberrations: Larger scale mutations including deletions, duplications, inversions, and translocations.

    • Transposons: Mobile DNA sequences that can insert themselves into different genomic locations, causing mutations.

    • STRs, SSRs, VNTRs: Repetitive sequences that vary greatly among individuals.

Types of Mutations

  1. Point Mutations: Affect a single nucleotide and can result in:

    • Silent Mutation: No change in the amino acid sequence.

    • Missense Mutation: Changes one amino acid to another.

    • Nonsense Mutation: Converts an amino-acid-coding codon into a stop codon.

    • Frameshift Mutation: Addition or deletion of bases that alters the reading frame.

  2. Induced Mutations: Caused by environmental factors such as UV light leading to errors in replication and DNA repair.

  3. Transposable Elements: DNA sequences that can change positions, impacting gene expression and genome architecture.

Implications and Applications of Genetic Variation

  • Genome-Wide Association Studies (GWAS): Used to find correlations between genetic variations and diseases by comparing SNPs in patients and non-patients.

  • Forensic DNA Testing: Utilizes unique STRs and VNTRs for applications in crime scene analysis and paternity testing.

STR (Short Tandem Repeats) and VNTR (Variable Number Tandem Repeats) are both types of repetitive DNA sequences used in forensic DNA testing. The primary differences are:

  • Length:

    • STRs typically consist of repeat units of 2-6 base pairs long.

    • VNTRs consist of repeat units that are longer, generally ranging from 7-20 base pairs long.

  • Variation:

    • STRs have a higher frequency of polymorphism, making them more suitable for distinguishing between individuals in forensic applications.

    • VNTRs may have fewer alleles and thus less discriminatory power compared to STRs.

Due to these differences, STRs are often preferred in forensic DNA analysis for their efficiency and effectiveness in individual identification; however, VNTRs can still provide valuable information, especially when analyzing larger genomic regions or when examining populations with specific genetic traits.

  • Anthropological Studies: Projects like the Genographic Project aim to trace human migration through DNA analysis.

Conclusion

Genetic variation is critical to understanding biology, evolution, and the development of complex traits. The interplay of mechanisms like sexual reproduction, HGT, and mutation contributes to biodiversity and adaptation, while tools like GWAS aid in medical genetics and personalized medicine.