- Classic (Mendelian) Inheritance:
These follow Mendel’s laws of segregation and independent assortment.
- Autosomal Dominant: A trait manifests when at least one copy of a dominant allele is present (e.g., Huntington’s disease).
- Autosomal Recessive: A trait manifests only when both alleles are recessive (e.g., cystic fibrosis).
- X-linked Dominant: Traits appear in both males and females, but more severely in males (e.g., Rett syndrome).
- X-linked Recessive: Males are more often affected since they have only one X chromosome (e.g., hemophilia).
- Y-linked (Holandric): Traits are passed from father to all sons (e.g., Y-linked hearing loss).
- Non-Classic Inheritance:
These involve mechanisms beyond Mendel’s laws.
- Mitochondrial Inheritance: Traits are passed through maternal mitochondria (e.g., Leber’s hereditary optic neuropathy).
- Multifactorial Inheritance: Traits result from interactions between multiple genes and environmental factors (e.g., diabetes, hypertension).
- Imprinting Disorders: Differential gene expression depending on the parent of origin (e.g., Prader-Willi syndrome, Angelman syndrome).
- Triplet Repeat Expansion Disorders: Diseases due to unstable repeating sequences (e.g., Fragile X syndrome, Huntington’s disease).
- Somatic Mosaicism: Genetic variation arises post-zygotically, affecting only some cells (e.g., McCune-Albright syndrome).
Characteristics of Autosomal Recessive Inheritance
- Equal Gender Distribution: Both males and females are equally likely to inherit the trait.
- Affected Offspring: Two copies of the recessive allele (one from each parent) are necessary for the trait to manifest.
- Carrier Parents: Parents of an affected individual are usually asymptomatic carriers.
- Horizontal Transmission: The condition often appears in siblings rather than in multiple generations.
- Recurrence Risk: If both parents are carriers, the risk for each child is:
- 25% (affected)
- 50% (carrier)
- 25% (unaffected)
Pedigree Chart Illustration
Below is a brief description of an autosomal recessive pedigree:
- Unaffected parents (carriers): Denoted as half-shaded circles (females) or squares (males).
- Affected individuals: Fully shaded shapes, appearing only when both parents are carriers.
- Siblings: Some affected, some unaffected, and some carriers.