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.