Mendelian Genetics

Johann Gregor Mendel

  • Lifespan: 1822–1884

  • Attributes: Lifelong learner, teacher, scientist, man of faith.

  • Early Life: Joined the Augustinian Abbey of St. Thomas in Brno (now Czech Republic).

  • Education Career:

    • Taught physics, botany, and natural science at secondary and university levels.

Mendel's Research Pursuits

  • Timeframe: 1856–1866, a decade-long investigation on inheritance patterns.

  • Model Organism: Pea plants (Pisum sativum).

    • Chose pea plants for their convenient characteristics.

    • Conducted extensive hybrids and documented traits across nearly 30,000 specimens.

  • Key Findings:

    • Traits are transmitted independently in dominant and recessive patterns.

    • Published results in 1866 in “Experiments in Plant Hybridization.”

    • Work overlooked during his lifetime; rediscovered in 1900.

Model System

  • Type: Garden pea (Pisum sativum).

  • Characteristics:

    • Self-fertilizing mechanism—flowers seal until pollination.

    • True-breeding plants produce offspring identical to parents.

    • Rapid maturity allows generation evaluations within a single season.

    • Large quantity available for statistical significance in results.

Mendelian Crosses

  • Definition: Hybridizations; mating true-breeding parents with contrasting traits.

  • Procedure:

    • Manually transfer pollen from one pollen-producing plant (anther) to another (stigma).

    • Remove anthers to prevent self-fertilization in recipient.

  • Generations:

    • P0 (parental generation): Initial true-breeding plants.

    • F1: Offspring from P0; self-fertilized for generation F2.

    • Extended experiments reached F3 and F4 generations.

Pea Plant Characteristics

  • Experiment Example: Crosses between violet and white flower color.

    • F1 Generation: All plants exhibited violet flowers.

    • F2 Generation: Resulting ratio of violet to white approximately 3.15:1.

  • Different Traits Examined:

    • Plant height, seed texture, seed color, flower color, pea pod size, pea pod color, flower position.

Dominant and Recessive Traits

  • Definitions:

    • Dominant Traits: Expressed in hybrids; do not disappear.

    • Recessive Traits: Latent in hybrids, but reappear in subsequent generations.

  • Example Traits: Violet (dominant) vs. white (recessive) flower color.

Mendel's Postulates

  1. Law of Segregation: Genes segregate equally into gametes.

  2. Law of Dominance: Dominant traits conceal the presence of recessive traits.

  3. Independent Assortment: Characteristics do not influence each other in inheritance.

  4. Units of Inheritance: Pair of unit factors (genes) transmit across generations.

Probability Basics

  • Definitions:

    • Probability: Mathematical measure of likelihood.

    • Calculations:

    • Empirical Probability: Occurrence divided by opportunities.

    • Theoretical Probability: Expected occurrences divided by potential occurrences.

  • Example:

    • Self-cross of F1 plant leads to expectation of three round seeds for every one wrinkled seed in F2.

Genetic Ratios from Crosses

  • Flower Color Cross:

    • Parental Traits: Violet vs. white.

    • F1 Generation: 100% violet.

    • F2 Generation: Approximately 3.15 violet:1 white ratio confirmed across multiple traits.

  • Dominance Characteristics Ratios:

    • Flower position, plant height, seed texture, seed color analyzed similarly, yielding generational ratios.

Using the Punnett Square

  • Definition: Method to predict offspring ratios from genetic crosses.

  • Monohybrid Cross Example:

    • Crossing YY (yellow seeds) with yy (green seeds) only yields Yy (yellow) offspring.

  • Dihybrid Cross Example:

    • Expected ratios derived from two traits. If rolling a six-sided die while flipping a coin is analogous, different combinations yield observable probabilities.

Test Cross Technique

  • Definition: Determines genotype of an organism exhibiting a dominant phenotype by crossing with homozygous recessive.

  • Outcomes: Reveals whether dominant phenotype organism is homozygous or heterozygous.

X-Linked Traits

  • Definition: Genes on the X chromosome which can lead to patterns of inheritance different from autosomal traits.

  • Example in Drosophila:

    • Eye color genes discovered to be located on the X chromosome.

  • Human Disorders: Color blindness and hemophilia are often X-linked, disproportionately affecting males due to their hemizygosity.

Epistasis

  • Definition: Interaction between genes where one gene masks the expression of another.

  • Examples:

    • Mouse pigmentation governed by epistatic genes; the C gene is crucial to pigment production.

    • Fruit color in summer squash impacted by dominant gene W masking the expression of Y alleles.

Extensions of Mendelian Genetics

  • Complex Patterns: Inheritance complexities leading to varied genetic phenomena beyond simple dominant/recessive outcomes.

  • Multiple Alleles: More than two allele forms may exist in a population for a given trait.

Applications in Evolution and Medicine

  • Drug Resistance: Multiple alleles in malaria parasites demonstrate rapid evolution due to drug resistance over generations.

  • Clinical Implications: Understanding patterns of inheritance aids in predicting potential genetic disorders in human offspring based on parental genotypes.