Notes on Single-Gene Disorders

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Variable expression:
• Variability in degree of phenotypic expression.
• Influenced by environment, allelic heterogeneity, heteroplasmy, and modifier loci.

Incomplete penetrance:
• Individuals have disease genotype, but don't display genotype.
• There is no expression at all, unlike in variable expression.

Pleiotropy:
• Single mutation affects multiple organs.

Locus heterogeneity:
• Same disease phenotype but mutations in different loci.

Anticipation:
• Individuals in recent generations develop increased severity and earlier onset of symptoms compared to individuals of earlier generations. Eg., diseases associated with abnormal triplet repeat expansions.

Imprinting:
• One allele is activated and the other is rendered inactive.
• Sometimes, the supposedly active allele is deleted or mutated, while the other allele is still normally inactivated.
• If deleted or mutated locus is at 15q11-13 (paternal): Prader-Willi syndrome.
• If deleted or mutated locus is at 15q11-13 (maternal): Angelman syndrome.

Additional Readings:

Basic Genetics

1. Single-Gene Disorders
1. Population Genetics

Related Topics

1. Genetic and Developmental Disorders
2. Genetic Code, Mutations, and Translation

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