heredity theories before Mendel - 

• classical assumption 1 (constancy of species) - heredity occurs within species
  • possible to get weird combinations by cross-breeding different species
  • soon obvious that extreme cross-breeding not possible
  • species maintained w/o much change since creation
• classical assumption 2 (direct transmission of traits)
  • gonos - “seed;” reproductive material transmitted from parents to offspring
  • information about each body part of offspring came from same body part in parent
• Charles Darwin - believed that “gemmules” (microscopic granules) passed down to offspring to guide development
• traits from parents blend and mix in the offspring
• hybridizations - carried out by Josef Koelreuter on tobacco plants
  • hybrids had different appearances than parents
  • offspring of hybrids either resembled hybrids or grandparents
  • proved the classical assumptions wrong
  • traits/characters segregated among a certain population
• T. A. Knight - did breeding experiments w/ white/purple plants
  • found that purple/white plants produced purple offspring
  • offspring of purple offspring were both purple/white
• early scientists didn’t record numbers or specific observations >> science advanced slowly

Mendel’s garden pea - same plant studied by Knight and others 

• hybrid peas created by breeders consistently in the past, ensuring segregation of traits
• large variety of peas available
• many easily identifiable traits
• small/easy to grow, short generations
• able to self-fertilize or cross-fertilize

Mendel’s procedure - studied comparable, specific differences 

• pure-breeding - produced plants that only produce plants w/ the same characteristics
  • pea plants allowed to self-fertilize over and over
  • ensured that certain pea plants would only pass down certain characteristics
• cross-bred pea varieties w/ different traits
• hybrids produced by plants w/ different traits allowed to cross-breed many times
• make quantitative observations (not done by any preceding scientist)

Mendel’s results - analyzed 7 traits each w/ 2 obvious differences  

• F1 generation (first filial generation)
  • hybrid offspring of purple/white flowered plants
  • all plants had purple flowers, the dominant trait
  • no white flowers (recessive trait)
• F2 generation (second filial generation)
  • recessive trait reappeared in 1/4 of offspring
  • dominant trait appeared in 3/4 of offspring
• 3:1 ratio of dominant to recessive (Mendelian ratio) actually a 1:2:1 ratio of pure-breeding dominant to non-pure-breeding dominant to pure-breeding recessive
• no traits ever blended/mixed, each trait inherited all together
• recessive traits latent (present but not expressed) in F1 generation