Lecture 3: Mendelian Inheritance - Monohybrid

Overview of Mendelian Inheritance

Distribution of Genes

  • Focus on how genes are passed from generation to generation.

  • Includes the law of segregation and recombination.

Key Learning Outcomes

  • Understand gene expression in relation to dominant and recessive phenotypes.

  • Solve problems involving Mendelian inheritance patterns.

Mendel's Contributions

  • Gregor Mendel: The father of genetics.

  • Conducted pioneering experiments (1856-1864) using pea plants.

  • Maintained detailed records and quantitative analysis of traits.

Experimental Organism - Pea Plants

  • Conducted hybridization (crossing different characteristics).

  • Observed hybrids often display traits of one parent rather than blending.

Traits Studied

  • Seven plant traits observed include:

    • Tall/Dwarf

    • Purple/White flowers

    • Round/Wrinkled seeds

    • Yellow/Green seeds

    • Inflated/Constricted pods

    • Axial/Terminal flower positions

Results of Mendel’s Experiments

  1. F1 Generation: Show only one parental trait.

  2. F2 Generation: Show a ~3:1 ratio of traits, indicating no blending but discrete inheritance.

Law of Segregation

  • Two hereditary factors for a trait segregate randomly during gamete formation.

  • One factor is dominant while the other is recessive.

Modern Terminology

  • Mendelian factors = genes; Alleles = different versions of a gene.

  • Homozygous: identical alleles; Heterozygous: different alleles.

  • Genotype = allelic composition; Phenotype = observable traits.

Punnett Squares

  • A method to predict genetic crosses.

  • Enables calculation of genotype and phenotype ratios.

Basic Crosses in Mendelian Genetics

  • Six basic crosses result in various progeny based on homozygosity and heterozygosity.

  • Monohybrid cross involves a single pair of contrasting traits.

Monohybrid Cross Results

  • Cross of Aa individuals yields:

    • Genotypic ratio: 1 homozygous dominant : 2 heterozygous : 1 homozygous recessive.

    • Phenotypic ratio: 3 dominant : 1 recessive.

Backcross and Testcross

  • Backcross: Cross F1 with a parent to determine genotype.

  • Testcross: Cross unknown dominance with homozygous recessive to identify genotype.