Principles of Inheritance - Mendel's Laws of Genetics

Learning Objectives

  • Understand Mendel's contributions to genetics and inheritance.
  • Familiarity with Mendel's principles.
  • Use of Punnett squares for predicting genotype and phenotype ratios.
  • Importance of test crosses in genetics.

Key Terms

  • True breeding: organisms that produce offspring identical to themselves.
  • Hybridisation: crossing of different varieties or species.
  • Monohybrid cross: a cross between two organisms focusing on a single trait.
  • P Generation: parental generation.
  • F1 Generation: first filial generation.
  • F2 Generation: second filial generation.
  • Dominant: allele that masks the effect of another variant.
  • Recessive: allele whose effects are masked in the presence of a dominant allele.
  • Heterozygous: possessing different alleles for a trait.
  • Homozygous: possessing identical alleles for a trait.
  • Genotype: the genetic makeup of an organism.
  • Phenotype: the observable traits of an organism.
  • Test Cross: breeding an individual of unknown genotype with a homozygous recessive individual.
  • Punnett Square: a diagram used to predict genetic outcomes from a cross.

Gregor Mendel: The Father of Genetics

  • Background:
    • Austrian biologist, meteorologist, and mathematician.
    • Augustinian friar and abbot at St. Thomas' Abbey in Brno.
    • Initially studied mice before focusing on pea plants.
    • Presented research in 1865; published in 1966 "Experiments in Plant Hybridization".
    • Studied ~30,000 pea plants.

Mendel's Methods

  • Experimental Design:
    • Crossed true breeding parents (P generation).
    • Recorded the phenotypes of the F1 generation after self-fertilisation.
    • Analyzed the F2 generation phenotypes.
  • Findings:
    • One trait was lost in the F1 generation but reappeared in the F2 generation.
    • Saw consistent 3:1 ratio in F2 phenotypes.
    • Observed similar results with different traits.

Mendelian Ratios in Monohybrid Crosses

  • Examples of monohybrid crosses showing a 3:1 ratio in F2:
    • Purple (PP) x White (pp): 3.15:1
    • Round x Wrinkled seeds: 2.96:1
    • Yellow x Green seeds: 3.01:1
    • Inflated x Constricted pods: 2.95:1
    • Tall x Dwarf stems: 2.84:1

Conclusions from Mendel's Work

  1. Particulate Inheritance:
    • Inheritance is determined by ‘heritable factors’ (genes).
  2. Alleles:
    • Variations of a gene (different alleles lead to different traits).
    • Organisms inherit two alleles (one from each parent, diploid).
    • Only one allele is passed to gametes (haploid).
    • Dominance can occur when alleles differ.
  3. Test Cross Utility:
    • To determine the genotype for a dominant phenotype.
    • Cross with a homozygous recessive to infer the unknown genotype.

Dihybrid Cross Observations

  • Independent Assortment:
    • Genes on different chromosomes assort independently.
    • Mendel's law states each pair of alleles segregates independently.
  • Phenotypic Ratios:
    • Dihybrid cross results in a 9:3:3:1 phenotypic ratio in F2 generation (for traits on different chromosomes).

Importance of Punnett Squares

  • Useful for predicting genetic ratios in offspring based on parental genotypes.
  • Helps illustrate Mendelian genetic principles.

Underlying Assumptions

  • Some of Mendel's genes were mapped improperly due to being on the same chromosome yet far apart, possibly affecting the inheritance patterns observed.

Key Concepts Summary

  • Mendel's principles include segregation, independent assortment, and dominance.
  • Punnett squares serve as a tool for predicting and understanding genetic inheritance.