Overview of Genetics

Principles Module 39


Key points

Inheritance Before Mendel

  • blending: traits of parents would combine in offspring
  • experience-dependent: traits modified through life, modified version passed to offspring

Mendel’s approach

  • used garden pea as model organism
  • identified and followed true breeding¬†traits, meaning those that were uniform among offspring
  • garden peas will self-pollinate naturally, but can also be manipulated to prevent this and forced to cross-pollinate
  • quantified the results of hundreds of crosses through many generations

Summary of Observations

  • crossing true-breeding plants produced only 1 trait of the 2 in first generation
    • crossing parent with green seeds with those having yellow seeds gave offspring with all yellow seeds
  • the ‘missing’ trait would reappear in the second generation
    • the offspring of the first generation produced mostly yellow-bearing seeds, but some with green seeds
  • the reappearance of a missing trait disproved the other mechanisms of inheritance and led to Mendel’s theory of inheritance

Principle of Segregation

  • each offspring receives 2 alleles, 1 from each parent
  • the process of the 2 alleles separating during gamete formation leads to each gamete receiving only 1 of the alleles
  • one of the alleles tends to mask the other, acting in a dominant manner
  • the other, recessive allele, is latent but not gone or blended out


  • trait
  • gene
  • allele
  • genotype
  • phenotype
  • dominant
  • recessive
  • homozygous
  • heterozygous

In-class activities

Questions for Practice

  • Be able to provide a definition for each of the vocabulary terms.
  • Provide an example result that demonstrates the principle of segregation.
  • Why did Mendel choose pea plants as a model system? What were the benefits of this organism for the study of genetics?