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Mendelian Genetics Powerpoint notes

What is a Gene?

  • A gene is defined as a unit of function responsible for specific traits.

  • One Gene, One Trait: Traditionally, it's thought that one gene dictates one trait. However:

    • Genes can have multiple functions (pleiotropy).

    • Multiple genes can influence a single function (gene interaction).

    • Example: Complementation in wing development in flies.

  • Unit of Expression: Genes are also units of expression, defined by the Central Dogma, "One Gene - One Protein", although:

    • Operons can express multiple genes as a single unit.

    • Different proteins can be produced from the same gene via alternative splicing or overlapping reading frames.

Seed Variations in Peas

  • Types of Seeds:

    1. Round

    2. Wrinkled

  • Cotyledon Colors:

    1. Yellow

    2. Green

  • Color and Shape Traits:

    1. Gray coat with violet flowers vs. White coat with white flowers.

    2. Pod types: Full vs. Constricted, stem height: Tall vs. Dwarf.

    3. Pod and flower positions: Axial vs. Terminal.

Mendelian Genetics and Gregor Mendel

  • Gregor Johan Mendel (1822-1884) is known as the father of genetics.

  • Conducted experiments on pea plants, noting seven characteristics that varied:

    1. Shape of seeds (Round/Wrinkled)

    2. Color of cotyledons (Yellow/Green)

    3. Flower color (Violet/White)

    4. Pod shape (Full/Constricted)

    5. Flower position (Axial/Terminal)

    6. Stem height (Tall/Dwarf)

Drosophila Mutants

  • Various Drosophila mutants with unique appearances include:

    • Spider

    • Cyclops

    • Rogue

    • Beast

    • Ice

    • Fenix

    • Hulk

    • Wolverine

    • Storm

Blood Type Genetics

  • ABO Blood Types: Different blood types are defined by the antigens present on red blood cells:

    • Type A: Antigen A, Antibodies: Anti-B

    • Type B: Antigen B, Antibodies: Anti-A

    • Type AB: Antigen A and B, No antibodies

    • Type O: No antigens, both Anti-A and Anti-B antibodies

  • Genotype Examples:

    • Type A: IAIA or IAi

    • Type B: IBIB or IBi

    • Type AB: IAIB

    • Type O: ii

Functions of Blood Type Alleles

  • Alleles and Structures:

    • i allele: No change in H structure.

    • A allele: GalNAc added to H structure, affecting blood type.

    • B allele: Gal added to H structure.

  • Key Sugars:

    • Fucose

    • Galactose (Gal)

    • N-Acetylgalactosamine (GalNAc)

    • N-Acetylglucosamine (GlcNAc)

Dominance and Rarity of Mutations

  • Dominance refers to how an allele can mask the effect of a recessive allele:

    • Factors:

      1. Biological processes often reach saturation, leading to dominant traits being more common.

      2. Gene expression is intricately regulated.

      3. Genomic redundancy results in more loss of function mutations compared to gain of function ones.

  • Typically, only one copy of a dominant allele is needed to express the trait, leading to dominance of wild type alleles.

The Nature of Mutations

  • Dominant Mutations: Can be due to:

    • Gain of function.

    • Haploinsufficiency: Insufficient gene dosage leads to observable traits.

    • Negative-dominant alleles: Mutant proteins may interfere with normal protein function.

  • Example: Huntington's disease, where the wild type (H) is dominant over mutant (h), leading to varied expressions in offspring.

Genetic Anticipation in Huntington’s Disease

  • The gene associated with Huntington's features a triplet repeat (CAG) which can expand in subsequent generations, causing more severe manifestations.

Mendelian Ratios in Genetics

  • Understanding the ratios and outcomes of genetic crosses:

    • Complete dominance versus incomplete dominance observed in hybrid generations.

    • Use of Punnett squares and probability calculations to predict outcomes of genetic variations:

      • Differential probabilities can indicate potential offspring traits based on parental genotypes.

Probabilities and Ratios Explained

  • Probability Rules:

    • Multiplication Rule: For independent events, multiply individual probabilities.

    • Addition Rule: For mutually exclusive events, sum their probabilities.

  • Outcomes from hybrids can be predicted through:

    • Genotypic ratios: Reflect genotypes present in offspring.

    • Phenotypic ratios: Reflect observable characteristics in offspring (e.g., dominant vs. recessive traits).

  • In trihybrid crosses involving multiple traits, the calculation of combined ratios can give detailed predictions of offspring phenotypes.