Learning Objective: At the end of this lesson, the learner will be able to describe the patterns of inheritance for common genetic disorders, understand risk probabilities, and recognize key clinical examples.
High-Yield Modes of Inheritance
| Mode | Pattern / Key Features | Genetic Risk / Notes | Examples |
|---|---|---|---|
| Autosomal Dominant (AD) | Often due to structural gene defects, pleiotropic, variably expressive, seen in multiple generations; both sexes affected | Each child of an affected heterozygous parent has: 50% chance of being affected |
|
| Autosomal Recessive (AR) | Often due to enzyme deficiencies, usually only one generation affected; more severe; childhood presentation | Two heterozygous parents: 25% affected, 50% carrier, 25% unaffected; risk is higher in consanguinity; 2/3 chance of carrier if sibling affected |
|
| X-linked Recessive (XLR) | Males are more severely affected; no male-to-male transmission, and it can skip generations | Sons of heterozygous mothers: 50% affected; daughters are usually carriers unless homozygous |
|
| X-linked Dominant (XLD) | Transmitted by both parents; variable severity; females often less severely affected | Children of affected mothers: 50% chance of being affected; children of affected fathers: 100% daughters, 0% sons affected |
|
| Mitochondrial | Transmitted only through the mother; heteroplasmy → variable expression | All children of affected females may be affected; children of affected males are unaffected |
|
High-Yield Tips
- Autosomal dominant: vertical inheritance; pleiotropy & variable expressivity.
- Autosomal recessive: horizontal inheritance; enzyme deficiencies; consanguinity increases risk.
- X-linked recessive: mostly males affected; no father-to-son transmission.
- X-linked dominant: affected fathers pass to all daughters, no sons; mothers transmit 50% risk.
- Mitochondrial: maternal inheritance; heteroplasmy → variable phenotype.
