Week 8

Mendel's Experiments

Mendel's experiments with garden peas revealed that phenotypes are inherited in discrete units. He identified principles of inheritance and used the Punnett square to predict genotype and phenotype ratios.

Key Terminology

  • True breeding: Homozygous parents.
  • Hybridization: Cross of different genetic types.
  • Monohybrid cross: Test for a single trait.
  • P generation: Parental generation.
  • F1 generation: First generation.
  • F2 generation: Second generation.
  • Dominant: Functional gene.
  • Recessive: Non-functional gene.
  • Heterozygous: Two different alleles at the same locus.
  • Homozygous: Two of the same alleles.
  • Genotype: Genetic makeup.
  • Phenotype: Observable characteristics.

Gregor Mendel

Mendel was an Austrian biologist, meteorologist, and mathematician. In 1865, he presented his work Experiments in Plant Hybridization, detailing experiments with 30,000 pea plants. His work went largely unnoticed for about 30 years.

Experimental Approach

Mendel studied seven different phenotypes in pea plants, including seed color (yellow/green), seed shape (round/wrinkled), and flower color (purple/white).

Mendel's method:

  1. Crossed true-breeding lines (homozygous parents).
  2. Scored phenotypes in the F1 generation.
  3. Self-fertilized F1 plants to produce the F2 generation.
  4. Scored phenotypes in the F2 generation.
  5. Performed test crosses.

Monohybrid Crosses

Mendel consistently observed a 3:1 ratio in the F2 generation for monohybrid crosses. He deduced that inheritance occurs in units and that each individual has two copies of a heritable factor, one of which may be dominant over the other.

Punnett Square

Developed by R.C. Punnett, the Punnett square is a tool to articulate Mendel's findings. It maps out gametes to show the probability of different genotypes and phenotypes in offspring.

Molecular Level Understanding

Different alleles can confer different traits of the same character. Alleles reside at a specific locus on a chromosome. Each individual inherits two alleles for each character, one from each parent.

Mendel's Conclusions

  • Inheritance is particulate.
  • Heritable factors exist (genes).
  • Alternate versions of genes (alleles) account for variations.
  • An organism inherits two alleles for each character.
  • If alleles differ, one may be dominant.

Test Crosses

Test crosses determine the genotype conferring a dominant phenotype by crossing the individual in question with a homozygous recessive individual.

Dihybrid Crosses

Mendel also observed patterns with two traits (dihybrid cross). With independent assortment, dihybrid crosses result in a 9:3:3:1 phenotypic ratio.

Mendel's Laws

  1. Segregation: Two alleles separate into gametes.
  2. Independent Assortment: Genes segregate independently during gamete formation.
  3. Dominance: Phenotypes depend on dominant and recessive alleles.