14.1 Basic concepts in genetic epidemiology

Genetic epidemiology explores how genes influence health and disease. It uses key concepts like genotype, phenotype, and heritability to understand the genetic basis of traits and conditions. Mendelian inheritance principles guide our understanding of how genes are passed down through generations.

Genetic markers are crucial tools in epidemiological studies. They help identify disease-related genes and assess genetic risk factors. Linkage disequilibrium, the non-random association of alleles, plays a vital role in genetic association studies by improving efficiency and power in pinpointing disease-associated regions.

Fundamentals of Genetic Epidemiology

Key terms in genetic epidemiology

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• Genotype defines genetic makeup of organism composed of alleles inherited from parents represented by letters (AA, Aa, aa)
• Phenotype describes observable characteristics or traits resulting from genotype and environmental interactions (eye color, height, disease status)
• Heritability measures proportion of phenotypic variation due to genetic factors ranging from 0 to 1 calculated using twin or family studies
• Allele refers to alternative forms of a gene at specific locus can be dominant or recessive
• Locus pinpoints specific location of gene on chromosome

Principles of Mendelian inheritance

• Mendel's Laws
  • Law of Segregation states alleles separate during gamete formation each gamete receives one allele for each trait
  • Law of Independent Assortment explains alleles for different traits assort independently applies to genes on different chromosomes
• Inheritance patterns
  • Autosomal dominant requires one copy of mutant allele causes phenotype
  • Autosomal recessive needs two copies of mutant allele for phenotype
  • X-linked inheritance involves genes located on X chromosome
• Punnett squares predict offspring genotypes
• Relevance to genetic epidemiology
  1. Identifies inheritance patterns of diseases
  2. Allows risk assessment in families
  3. Guides genetic counseling and screening programs

Genetic Markers and Association Studies

Types of genetic markers

• Single Nucleotide Polymorphisms (SNPs) represent single base pair variations in DNA sequence most common type of genetic variation used in genome-wide association studies (GWAS)
• Microsatellites consist of short tandem repeats of DNA sequences highly polymorphic used in linkage studies and forensics
• Copy Number Variations (CNVs) involve structural variations including deletions or duplications associated with complex diseases
• Restriction Fragment Length Polymorphisms (RFLPs) show variations in DNA sequence recognized by restriction enzymes used in early genetic mapping studies
• Applications in epidemiological studies
  • Identify disease susceptibility genes
  • Assess genetic risk factors
  • Study gene-environment interactions
  • Conduct pharmacogenomics research

Concept of linkage disequilibrium

• Linkage disequilibrium (LD) describes non-random association of alleles at different loci measured by D' or r² statistics
• Factors affecting LD
  • Recombination rate
  • Population history (bottlenecks, admixture)
  • Natural selection
• Haplotype blocks represent regions of high LD in genome allow for efficient genotyping strategies
• Importance in genetic association studies
  • Enables use of tag SNPs capturing information about set of SNPs in high LD
  • Improves power and efficiency of association studies
  • Helps fine-mapping of disease-associated regions
• Limitations
  • LD patterns vary across populations
  • May lead to false-positive associations if not properly accounted for