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L5_Gene interaction I: Types of mutation & departure from Mendelian ratios

May 10, 2025

Gene Interaction and Types of Mutation

Key Concepts

  • Impact of Mutations: Mutations affect gene function, influencing molecular dominance.
  • Dominance Variations: Includes types other than full dominance.
  • X-Linked Inheritance: Unique patterns from genes on the X chromosome.
  • Sex-Influenced Traits: Some traits are sex-limited or sex-influenced.

Learning Outcomes

  • Departure from Mendelian Ratios: Due to mitochondrial and chloroplast DNA inheritance patterns.
  • Historical Context: Mendel's choice of traits for clear dominant-recessive patterns.
  • Molecular Basis: Understanding dominance through protein function.

Types of Mutations

Loss of Function

  • Amorphic (Null): No protein function, typically recessive. Example: CFTR mutation in cystic fibrosis.
  • Hypomorphic (Leaky): Reduced protein function, usually recessive. Example: Tyrosinase alleles affecting pigmentation.

Gain of Function

  • Hypermorphic: Increased protein activity, usually dominant. Example: Hereditary pancreatitis.
  • Antimorphic (Dominant Negative): Mutant protein interferes with normal protein. Example: FBN1 mutation causing Marfan syndrome.
  • Dominant Lethal: Causes late-onset diseases. Example: Huntington's disease.
  • Neomorphic: Alters protein function, usually dominant. Example: Antennapedia in Drosophila.

Non-Mendelian Ratios

  • Incomplete Dominance: Blend of traits. Example: Four o'clock plants.
  • Co-Dominance: Both alleles equally expressed. Example: ABO blood group.

Lethal Alleles

  • Distortion in Ratios: Yellow coat mutation in mice, Manx cats, and human genetic load.

Inheritance Patterns

  • X-Linked Inheritance: Traits such as hemophilia A often manifest in males.
  • Sex-Limited Traits: Phenotype limited to one sex.
  • Sex-Influenced Traits: Expression influenced by sex, e.g., male pattern baldness.

Cytoplasmic Inheritance

  • Mitochondrial DNA: Inherited maternally, causing specific diseases like MELAS and LHON.

Phenotypic Variation

  • Penetrance and Expressivity: Degree to which genotype is reflected in phenotype.
  • Incomplete Penetrance: Not all carriers express phenotype. Example: Polydactyly.
  • Variable Expressivity: Phenotype varies among individuals with the same genotype.

Summary

  • Mendelian vs. Non-Mendelian Ratios: Dominance, co-dominance, and inheritance types affect genetic ratios.
  • Inheritance Patterns: Include X-linked, sex-influenced, and matrilineal inheritance.