Molecular Genetics Overview

Gregor Mendel and Inheritance

  • Gregor Mendel (1822-1884)

    • Pioneer in the study of genetics through his work with pea plants.
    • Published a seminal paper on inheritance in 1865 that was largely ignored for 30 years, receiving only ~3 citations until it was rediscovered in 1900.
    • Important note: Mendel's notebooks were burned after his death, leading to the loss of some of his work.

  • Rediscovery of Mendel's Work

    • Year 1900: Three scientists independently rediscover Mendel's contributions within two months.
    • Hugo de Vries (Netherlands)
    • Carl Erich Correns (Germany)
    • Erich von Tschermak (Austria)

Early Theories of Inheritance

  • Concept of Inheritance
    • Prior to Mendel, inheritance theories were simplistic and speculative.
    • Pangenesis Theory:
    • Proposed by Aristotle; suggested used gemmules (small particles) to explain inheritance.
    • Involved the notion that offspring were a blend of parental traits.

Plant Reproduction Basics

  • Plant Reproductive Structures

    • Many plants, including pea plants, possess both male (anther, filament) and female (stigma) structures, allowing self-fertilization.
    • Flowers produce seeds, which are the offspring in an embryonic state within a protective fruit.

  • Seed Structure

    • Example: Bean seed
    • Contains plant embryo and nutrient tissue for growth until germination.

Mendel's Experimentation with Pea Plants

  • Controlled Breeding

    • Mendel carefully controlled the conditions and reproduction of pea plants.
    • Steps in Mendel's methodology:
    1. Cross-breed true-breeding white and purple flowered plants.
    2. Cross the hybrid offspring from the first generation.
    3. Grow seeds from the second generation and record the number of purple and white flowering plants.

  • Ratio Observations

    • Expectations of observed traits based on ratios.
    • Mendel observed ratios of offspring traits, primarily aiming for a 3:1 ratio between dominant and recessive traits.

Analysis of Ratios

  • Ratios in Data

    • Ratios provide a method of analyzing genetic characteristics.
    • Example from Perimeter College data (2019):
    • Total students: 18,500, International students: 2,450 yields a ratio of 7.6:1
    • At University of Georgia: Total students: 41,615, International students: 2,600 yields a ratio of 16:1.

  • Significance of Ratios

    • Ratios can help deduce patterns and potential conclusions about genetic traits.
    • Mendel’s consistent ratios across experiments reinforced the foundation of genetics.

  • The 3:1 Ratio

    • While Mendel often found near 3:1 ratios when assessing dominant and recessive traits, the slight variances were essential for understanding inheritance models.
    • Questions raised concerning the precision of ratios and what they indicate regarding genetic inheritance.

Mendelian Genetics Concepts

  • Dominant and Recessive Alleles
    • Explanation of dominant (visible traits) and recessive (hidden traits) alleles.
    • Understanding Punnett Squares to visualize genetic crossing outcomes.
  • Parental Chromosome Contribution
    • Each parent contributes one set of chromosomes to the offspring, which is critical for understanding heredity and genetic diversity.