Mendel’s Laws


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•  Mendel discovered the principles of inheritance with experiments in which a large number of pea plants

   were crossed

Gregor Mendel was an Austrian monk who developed the principles of inheritance by performing experiments on pea plants

  • First, he crossed different varieties of purebred pea plants, then collected and grew the seeds to determine their characteristics
  • Next, he crossed the offspring with each other (self-fertilization) and grew their seeds to similarly determine their characteristics
  • These crosses were performed many times to establish reliable data trends (over 5,000 crosses were performed)

As a result of these experiments, Mendel discovered the following thing

  1. When he crossed two different purebred varieties together the results were not a blend – only one feature would be expressed
    • E.g. When purebred tall and short pea plants were crossed, all offspring developed into tall growing plants
  2. When Mendel self-fertilised the offspring, the resulting progeny expressed the two different traits in a ratio of ~ 3:1
    • E.g. When the tall growing progeny were crossed, tall and short pea plants were produced in a ratio of ~ 3:1 

From these findings, Mendel drew the following conclusions:

  • Organisms have discrete factors that determine its features (these ‘factors’ are now recognised as genes)
  • Furthermore, organisms possess two versions of each factor (these ‘versions’ are now recognised as alleles)
  • Each gamete contains only one version of each factor (sex cells are now recognised to be haploid)
  • Parents contribute equally to the inheritance of offspring as a result of the fusion between randomly selected egg and sperm
  • For each factor, one version is dominant over another and will be completely expressed if present

While there are caveats to Mendel’s conclusions, certain rules can be established:

  1. Law of Segregation: When gametes form, alleles are separated so that each gamete carries only one allele for each gene
  2. Law of Independent Assortment: The segregation of alleles for one gene occurs independently to that of any other gene*
  3. Principle of Dominance: Recessive alleles will be masked by dominant alleles

*  The law of independent assortment does not hold true for genes located on the same chromosome (i.e. linked genes)
  Not all genes show a complete dominance hierarchy – some genes show co-dominance or incomplete dominance

Mendel’s Garden Pea Plant Experiment

Mendel's pea plants