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.


• Single mutation affects multiple organs.

Locus Heterogeneity:

• Same disease phenotype but mutations in different loci.


• 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.


• 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.

Prader-Willi Syndrome:

• Deleted or mutated locus is at 15q11-13 (paternal).

Angelman Syndrome:

• Deleted or mutated locus is at 15q11-13 (maternal).

Additional Readings:

Basic Genetics

1. Single-Gene Disorders
2. Population Genetics
3. Genetic Disease Transmission Probability
4. What is Cytogenetics?
5. Notes on Bloom's Syndrome
6. Introduction to Human Chromosomes
7. Life Cycle of Somatic Cells

Related Topics

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

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