Reprocessed post ID 514021 (12 June 2026) summarises the shift from the 1983 mapping of HTT (Huntington’s disease) to current polygenic models and the limited individual predictive value of genetic risk testing.
Genetic architectures
- Monogenic vs polygenic: HTT on chromosome 4 (1983) exemplifies monogenic, high‑penetrance disease; most common non‑communicable diseases (type 2 diabetes, coronary artery disease, schizophrenia, obesity) are polygenic.
- SNPs and effect sizes: Polygenic traits reflect hundreds–thousands of SNPs with very small per‑variant effects; no single causative “obesity gene”.
Gene expression and epigenetics
- DNA methylation: Methyl groups at CpG sites typically reduce gene transcription.
- Histone modification: Acetylation/methylation alters chromatin accessibility and transcriptional output.
- Non‑coding RNAs: microRNAs cause translational repression or mRNA degradation.
- Environmental modulation: Diet, stress, sleep and pollutants modify epigenetic marks and alter relative disease risk.
Predictive testing and limitations
- Polygenic Risk Score (PRS): Aggregate of weighted risk alleles from GWAS that assigns relative population risk percentiles.
- Ancestry bias: >80% of GWAS samples are of European ancestry, reducing PRS transferability to other populations.
- Clinical translation: PRS yields probabilistic risk; it cannot predict individual disease certainty or future environmental exposures.
- Missing heritability: GWAS common variants account for a small portion of familial heritability; rare variants and epigenetic interactions remain important.
IASPOINT Booster Facts
- Penetrance: Proportion of variant carriers who express the phenotype; pathogenic HTT CAG expansions show near‑complete penetrance.
- Pharmacogenomics: CYP450 genotypes guide drug selection and dosing to reduce adverse reactions.
- GINA (US): Genetic Information Nondiscrimination Act bars health insurers and employers from using genetic data for adverse decisions.
- Air‑pollution epigenetics: Chronic PM2.5 exposure is associated with systemic DNA hypomethylation affecting inflammatory pathways.