Mendel and the Gene Idea - Summary

Gregor Mendel

  • Mid-1800s: Used garden peas to study trait inheritance.
  • Identified predictable patterns of inheritance.
  • Proposed that "unit factors" (now known as genes) are passed from parent to offspring.

Keys to Mendel’s Success

  • Suitable organism: Easy, fast growth; numerous offspring.
  • Contrasting traits: Visible, with two variations each.
  • Meticulous data recording.

Mendel’s Methodology

  • Controlled matings by transferring pollen between plants.
  • Prevented self-fertilization by removing stamens or carpels.

Genetic Generations

  • P generation: True-breeding parents.
  • F1 generation: Offspring of P generation.
  • F2 generation: Offspring from F1 self-fertilization.

Mendel’s Observations

  • F1 generation showed only one trait variation.
  • F2 generation showed a 3:1 ratio of traits.

Mendel’s Conclusions

  1. Alternative gene versions (alleles) cause trait variations.
  2. Organisms inherit two alleles per trait, one from each parent.
    • Homozygous: Identical alleles.
    • Heterozygous: Different alleles.
  3. One allele is dominant, the other recessive.
    • Dominant allele determines appearance.
    • Recessive allele is masked by dominant allele.
  4. Law of Segregation: Alleles separate during gamete formation.

Genotype and Phenotype

  • Genotype: Genetic makeup.
  • Phenotype: Observable traits determined by genotype.
  • Punnett Square: Used to predict offspring genotypes and phenotypes.

Dihybrid Crosses

  • Evaluated inheritance of two traits simultaneously.
  • F2 generation exhibited a 9:3:3:1 ratio.

Law of Independent Assortment

  • Alleles for different traits are inherited independently.
  • Dominant and recessive alleles do not always stay together.

Variations of Inheritance

  • Incomplete Dominance: Heterozygotes show an intermediate phenotype.
  • Codominance: Heterozygotes express both alleles.
  • Multiple Alleles: Genes with more than two alleles (e.g., human blood groups I^A, I^B, i.)
  • Epistasis: One gene masks or modifies another (e.g., Labrador retriever coat color).
  • Polygenic Inheritance: Traits influenced by multiple genes (e.g., skin color, height).