Theories of Inheritance

Early Records of Genetics

• 6,000 years ago, records of pedigrees of domesticated animals (horses, dogs, camels, sheep) and crop plants (wheat, rice, maize, date palm) were kept.

The Greek Influence

• 400 B.C.: Hippocrates theorized that male seed is thicker or more solid, while female seed is more fluid.
• 500 B.C.: Aristotle believed in the "vital heat fluid" (SEMEN), which was considered highly purified blood.
• 500 B.C.: Pythagoras proposed the "Fluids & Vapors" Theory, where vapors from various organs unite to form individuals.

Era of Awakening (17th Century)

• Jan Swammerdam proposed the Theory of Preformation, suggesting that sex cells contain a complete miniature of an adult (homunculus).

Era of Awakening (17th Century)

• William Harvey introduced Epigenesis, suggesting that an organism develops from substances present in the egg that differentiate into adult structures during embryonic development.

Era of Awakening (18th Century)

• Casper Wolff disagreed with the Theory of Preformation and provided experimental evidence that no preformed embryo existed in the chicken egg.
• Pierre Louis Maupertuis proposed the Particulate Inheritance Theory, where minute particles from each body part unite in sexual reproduction to form a new individual.

Era of Awakening (19th Century)

• Charles Darwin introduced the Theory of Pangenesis, suggesting that each body part produces minute particles (gemmules) that unite in the reproductive organs to form offspring with blended characteristics.
• August Weismann disproved the Theory of Pangenesis through an experiment involving cutting the tails of mice and observing the offspring.

Era of Awakening (19th Century)

• Gregor Mendel proposed the Four Postulates: Unit Factors in Pairs, Dominance/Recessiveness, Segregation, and Independent Assortment.
• Friedrich Miescher identified DNA (initially called "nuclein") as an acidic substance found in cell nuclei.

Era of Awakening (20th Century)

• William Bateson coined the term "genetics" in 1905.
• Wilhelm Johanssen and Herman Nilsson-Ehle conducted experiments on quantitative traits in broad beans and wheat.

Era of Awakening (20th Century)

• Thomas Morgan conducted experiments on Drosophila (fruit flies) and discovered that genes are carried on chromosomes and provided the mechanical basis for heredity.
• The eugenics movement emerged in the 1910s-30s, leading to involuntary sterilization laws.

Molecular Genetics

• 1928: Fred Griffith showed that a component of heat-killed virulent bacteria can "transform" a non-virulent strain to become virulent.
• The Avery – MacCleod – Mc Carthy Experiment proved that DNA is the genetic material.
• Karl Landsteiner discovered the ABO blood types.
• Harriet Creighton and Barbara Mcclintock demonstrated genetic recombination through physical exchange of chromosomal pieces in corn.

Test your knowledge of early theories of inheritance, including the Particulate Inheritance Theory and the Theory of Pangenesis, proposed by Pierre Louis Maupertuis and Charles Darwin respectively. Learn about the concept of gemmules and pangenes in the 18th and 19th centuries.