Revision summary
Single-gene disorders chiefly arise from a pathogenic variant at one locus. Autosomal recessive examples include sickle-cell disease, PKU and cystic fibrosis. Autosomal dominant examples include Huntington disease and achondroplasia. Haemophilia A and Duchenne muscular dystrophy are X-linked recessive. Allison linked sickle-cell carrier advantage with malaria. Penetrance, expressivity, new mutations and environment prevent simplistic genetic determinism.
Model answer
Introduction
A single-gene disorder arises mainly from a pathogenic variant at one locus and therefore tends to show a recognisable Mendelian pedigree. Its expression can still vary with penetrance, modifier genes, environment and reproductive fitness.
Body
Major inheritance patterns
- Autosomal recessive: disease usually appears in homozygotes while heterozygotes are carriers. Sickle-cell disease results from a missense variant in HBB; phenylketonuria commonly involves PAH; cystic fibrosis involves CFTR. Sickle-cell heterozygote advantage in malarial regions, identified by A. C. Allison, connects mutation to natural selection.
- Autosomal dominant: one altered allele can suffice. Huntington disease is caused by a CAG-repeat expansion in HTT and often appears after reproduction; achondroplasia commonly involves FGFR3 and many cases are new mutations.
- X-linked recessive: males are more often affected because they have one X chromosome. Haemophilia A involves F8, and Duchenne muscular dystrophy involves DMD. Carrier mothers transmit risk; fathers do not pass X-linked traits to sons.
- X-linked dominant and mitochondrial: these are single-locus patterns too, though the latter is cytoplasmic. Mitochondrial disorders pass through mothers, not affected fathers.
Founder effect and endogamy can raise particular alleles in small populations, making community-sensitive screening useful in India. Yet “one gene” does not mean one fixed outcome: variable expressivity, incomplete penetrance and allelic heterogeneity complicate counselling. Chromosomal syndromes such as Down syndrome are not single-gene disorders.
Flow diagram
flowchart TD M[Single-gene mutation] --> AR[Autosomal recessive] M --> AD[Autosomal dominant] M --> XL[X-linked] M --> MT[Mitochondrial] AR --> SC[Sickle cell / PKU] AD --> HD[Huntington / achondroplasia] XL --> H[Haemophilia / DMD]
Conclusion
Single-gene disorders make Mendelian rules visible in human pedigrees, while sickle cell demonstrates their evolutionary ecology. Diagnosis should combine pedigree and molecular evidence without converting communities into genetic stereotypes.
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