Mendelian Genetics

Chapter 3: Mendelian Inheritance

Overview

  • Study of Mendelian inheritance delves into how traits are inherited from one generation to the next through the principles discovered by Gregor Mendel.

  • Topics covered include Mendel's study of pea plants, laws of segregation and independent assortment, chromosome theory, and methods to study inheritance patterns in humans.

Historical Background

Inheritance Hypotheses

  • Numerous hypotheses surrounding inheritance existed before Mendel's work:

    • Pangenesis (Hippocrates, ~400 BCE): Suggested that "seeds" from all body parts are collected in reproductive organs and passed to offspring, influencing traits.

    • Blending Hypothesis: Proposed that hereditary factors were malleable and blended across generations.

Mendel's Contributions

  • Gregor Johann Mendel is acknowledged as the father of genetics due to his systematic experiments with pea plants, carried out from 1856 to 1864 in his monastery's garden.

  • His seminal work, "Experiments in Plant Hybridization," published in 1866, was overlooked for 34 years due to a lack of understanding and appreciation for genetic principles.

  • Rediscovered in 1900 by botanists Hugo de Vries, Carl Correns, and Erich von Tschermak.

Experimental Design

Choice of Organism

  • Mendel selected garden pea (Pisum sativum) for its distinct varieties and ease of controlled breeding.

Types of Breeding Experiments

  1. Self-fertilization: Occurs when pollen and egg come from the same plant, allowing Mendel to study offspring of the same genetic background.

  2. Cross-fertilization: Involves the manual transfer of pollen between different plants, showcasing how different traits can combine in hybrids.

Key Findings and Concepts

Observable Characteristics

  • Characters: Observable traits (e.g., flower color).

  • Traits: Specific variations of characters (e.g., purple or white flowers).

  • True-breeder: A variety producing the same traits over generations when self-fertilized.

Law of Segregation

  • Mendel proposed that alleles (variations of a gene) segregate independently during gamete formation:

    • Each gamete carries only one allele of a pair from the parent.

  • Derived from single-factor (monohybrid) crosses, observing one trait at a time:

    • Example: Tall (T) and short (t) plants show ratios in the offspring indicative of how traits segregate.

Experimental Results

  • Observations over generations demonstrated predictable ratios:

    • F1 generation showed predominant traits (3:1 ratio in F2 for dominant traits).

    • Established that traits didn’t blend; rather, they segregated.

Punnett Squares

  • Introduced as a tool for predicting genetic crosses:

    • Allows visualization of offspring ratios based on parental allele combinations.

Law of Independent Assortment

  • Demonstrated through two-factor crosses, showing that different traits assort independently:

    • Confirmed by observing combinations in the F2 generation, contradicting the linked assortment hypothesis.

  • Example: When crossing seed shape and color variants, novel trait combinations emerged (non-parental).

Genetic Diversity and Recombination

  • Introduction of genetic recombination: Peas may occasionally exhibit unexpected combinations due to independent assortment or crossing over during meiosis.

Advanced Concepts in Multigenic Crosses

  • As the number of factors increases, using methods like forked-line or multiplication becomes essential to manage calculations.

    • For example, to analyze three traits, computations become considerable with a 64-square Punnett square.