U01.01.054 Modes of inheritance

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Learning Objectives

  • Differentiate between Autosomal Dominant and Autosomal Recessive patterns.
  • Identify the unique transmission rules of X-linked and Mitochondrial inheritance.
  • Calculate recurrence risks for offspring based on parental genotypes.
  • Recognize classic clinical examples for each mode of inheritance.

1. Autosomal Inheritance Patterns

Mode Key Features Recurrence Risk
Autosomal Dominant Defects in structural genes. Affects multiple generations. Both sexes are affected. 50% chance for each child if one parent is a heterozygote ($Aa$).
Autosomal Recessive Defects in enzymes. Usually one generation. More severe; presents in childhood. 25% affected, 50% carriers, 25% unaffected (from carrier parents).

 

High-Yield Probability: An unaffected sibling of a child with an autosomal recessive disease has a 2/3 probability of being a carrier.

2. X-Linked Inheritance Patterns

Mode Transmission Rules Examples
X-linked Recessive No male-to-male transmission. Skips generations. More severe in males. Hemophilia, Duchenne MD, G6PD deficiency.
X-linked Dominant Affected fathers pass to 100% of daughters and 0% of sons. Fragile X, Alport syndrome, and Hypophosphatemic rickets.

3. Mitochondrial Inheritance

  • Mechanism: Transmitted only through the mother. All offspring of an affected female are at risk.
  • Heteroplasmy: Variable expression within a family due to the mix of healthy and mutated mtDNA in cells.
  • Examples: Mitochondrial myopathies (MELAS), Leber hereditary optic neuropathy.

 

 

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