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
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