9 - Mendel

Chapter 9: Mendel, Genes, and Inheritance

Why It Matters

  • Sickle-Cell Disease

    • A genetic disorder affecting red blood cells

    • Illustrates the concept of inherited diseases

Early Theories of Inheritance

Blending Theory of Inheritance (Pre-1900s)

  • Hereditary traits blend evenly in offspring through mixeding of parents' blood.

  • Failures of Blending Theory:

    • Extremes do not gradually disappear.

    • Offspring sometimes display traits not seen in either parent.

  • Conclusion: Proven FALSE.

Gregor Mendel and the Foundations of Genetics

  • Gregor Mendel: Founder of genetics, employed scientific method to study inheritance.

  • Conducted experiments with garden peas to understand heredity.

Mendel's Hypotheses

  • Four Hypotheses: Established to support two principles:

    1. Principle of Segregation: Alleles segregate during gamete formation.

    2. Principle of Independent Assortment: Alleles of different genes assort independently.

The Pea Plant Experiments

Why Pea Plants?

  • Easy to grow, produce clear traits, and enable controlled breeding.

  • Focus on discrete hereditary factors termed genes.

True-Breeding Garden Peas

  • True-breeding varieties self-fertilize producing consistent traits.

  • Cross-pollination demonstrated both consistent and different traits.

Key Terminology

  • P generation: Parental generation used in the initial cross.

  • F1 generation: First generation offspring.

  • F2 generation: Second generation offspring.

Experimental Results

Flower Colour Cross

  • P Generation: Purple flowers crossed with white flowers.

  • F1 Generation: All offspring produced purple flowers.

  • F2 Generation: Results: 75% purple and 25% white flowers (3:1 ratio).

  • Conclusion: NO blending observed.

Results of Other Pea Characters with Ratios

  • Seed Shape: Round vs. Wrinkled (2.96:1)

  • Seed Colour: Yellow vs. Green (3.01:1)

  • Pod Shape: Inflated vs. Constricted (2.95:1)

  • Pod Colour: Green vs. Yellow (2.82:1)

  • Flower Colour: Purple vs. White (3.15:1)

Mendel’s Hypotheses Explained

First Hypothesis

  1. Genes for characters occur in pairs.

  • Alleles: Different versions of a gene.

  • Organisms are diploid (two copies of each gene).

Second Hypothesis

  1. Dominant vs. Recessive Alleles

  • One allele may mask the presence of another (dominant is expressed).

  • Recessive alleles only exhibited in homozygous condition.

Third Hypothesis

  1. Genes segregate into gametes singly.

  • Gametes are haploid: carry only one allele for each gene.

Fourth Hypothesis

  1. Alleles of different characters assort independently.

  • Principle of Independent Assortment during gamete formation.

Monohybrid Cross Example

  • P Generation: Purple (PP) x White (pp).

  • F1 Generation: All offspring heterozygous (Pp).

  • F2 Generation: Resulting ratios from a monohybrid cross yield 3:1 ratio in phenotypes (purple to white).

Terminology

  • Homozygous: Same alleles (PP or pp).

  • Heterozygous: Different alleles (Pp).

  • Genotype: Genetic makeup (PP, Pp, pp).

  • Phenotype: Physical appearance (purple or white flowers).

Probability in Genetics

  • Product Rule: Calculates probability of two independent events occurring in succession.

  • Sum Rule: Calculates the probability of different events resulting in the same outcome.

Mendel’s Predictions Based on Probability

  • Predicted classes and proportions of offspring using probability rules.

Testcross Validation of Mendel's Hypotheses

  • Testcross: Cross unknown genotype with homozygous recessive to assess genotype.

  • Helps confirm if dominant phenotype is homozygous (PP) or heterozygous (Pp).

Chromosome Theory of Inheritance

  • Proposed by Walter Sutton

  • Chromosomes occur in pairs in diploid organisms.

  • Chromosomes assort independently during meiosis, corroborating Mendel’s theories.

Modifications to Mendel's Hypotheses

  1. Incomplete Dominance: Dominant alleles do not always mask recessive alleles.

  2. Codominance: Different alleles are expressed equally in heterozygotes.

  3. Multiple Alleles: Presence of more than two alleles for a gene in a population.

  4. Epistasis: Interaction between genes affecting phenotypic expression.

  5. Polygenic Inheritance: Multiple genes control the same trait resulting in continuous variation.

  6. Pleiotropy: One gene influences multiple traits.

Examples of Inheritance Patterns

Incomplete Dominance

  • Example: Snapdragons with red (CRCR), white (CWCW) flowers producing pink (CRCW) flowers when crossed.

Codominance

  • Example: ABO Blood Groups, where both A and B antigens are expressed in blood type AB.

Multiple Alleles

  • Example: Human blood types with IA, IB, and i alleles.

Epistasis

  • Example: Labrador Retriever coat colour determined by alleles interacting with pigment deposition genes.

Polygenic Inheritance

  • Human traits such as height are controlled by multiple genes leading to a bell-shaped distribution curve.

Pleiotropy

  • Example: Sickle-cell disease affecting multiple physiological functions due to one recessive allele.

Conclusions

  • Investigates how Mendel's inheritance patterns explain heritable traits and adaptations among species.

  • Links between genetic mechanisms and chromosomal behavior.

  • Exploration of ongoing modifications in genetic theory based on emerging research.