linkage disequilibrium
Summary
Linkage disequilibrium (LD) is the non-random association of alleles at two or more loci in a population, occurring more often or less often than expected by chance given their individual allele frequencies. It is distinct from linkage, which refers to physical proximity of genes on a chromosome. LD is commonly tested on USMLE in the context of HLA associations with autoimmune diseases.
Detail
Linkage disequilibrium describes a population genetics phenomenon where specific alleles at two different loci occur together on the same chromosome more frequently (or less frequently) than would be predicted by random assortment based on their individual allele frequencies. This occurs because alleles that are physically close together on a chromosome tend to be inherited together, but LD also reflects population history, selection pressures, founder effects, and genetic drift, not just physical linkage.
Key distinctions: Linkage refers to genes being located near each other on a chromosome, making them likely to be inherited together (measured by recombination frequency). Linkage disequilibrium refers to allele frequencies at these loci deviating from what Hardy-Weinberg equilibrium would predict for independent loci.
Mathematically, LD is quantified by D = pAB - pApB, where pAB is the observed frequency of the AB haplotype and pApB is the expected frequency if the alleles were independent. D' or r² are normalized measures used to compare LD across studies.
Clinical/Board relevance: LD is classically tested using HLA gene associations with autoimmune and inflammatory diseases (e.g., HLA-B27 with ankylosing spondylitis, reactive arthritis, psoriatic arthritis, IBD-associated arthritis; HLA-DR3/DR4 with type 1 diabetes; HLA-DQ2/DQ8 with celiac disease). These HLA alleles are in linkage disequilibrium with other genes in the MHC region that may be the true disease-causing variants, illustrating why LD complicates identifying causal variants in genetic association studies.
LD decreases over generations due to recombination, and its extent can be used to map disease-associated genes (genome-wide association studies rely on LD blocks to narrow down candidate genomic regions). Population bottlenecks and founder effects can create strong LD patterns useful for studying disease mutations in genetically isolated populations (e.g., Ashkenazi Jewish population studies).
Sources
- First Aid for the USMLE Step 1
- Robbins and Cotran Pathologic Basis of Disease
- Thompson & Thompson Genetics in Medicine
- Kaplan USMLE Step 1 Lecture Notes: Biochemistry and Medical Genetics
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