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Home > AP Biology > Topic Notes > 13 - Patterns of Inheritance > Mendel's Experiment

Mendel's Experiment

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
Subject: 
Biology [1]
Subject X2: 
Biology [1]

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