Mendelian Genetics

Introduction to Mendelian Genetics

  • Mendelian Genetics: The study of heredity based on the principles set forth by Gregor Mendel.

Mendel's Experiments

  • Heredity: Transmission of traits from one generation to the next.

  • Gregor Mendel: Conducted experiments with pea plants to study heredity.

    • True-breeding plants: Plants that produce offspring identical to themselves.

    • Hybridization: Crossing true-breeding plants with differing traits.

    • Alleles: Alternative versions of genes determining heritable traits.

Patterns of Inheritance

  • Learning Objectives:

    • Explain the success of Mendel's experiments.

    • Describe monohybrid crosses with dominant and recessive alleles.

    • Relationship between genotypes and phenotypes.

    • Use Punnett squares to calculate genotype and phenotype ratios.

    • Explain law of segregation and independent assortment.

    • Identify examples of non-Mendelian inheritance patterns.

Mendel’s Laws of Inheritance

Monohybrid Cross

  • Monohybrid Cross: Cross between two individuals differing at one gene.

  • P Generation: Parent generation with true-breeding plants.

  • F1 Generation: All hybrid progeny showing dominant traits.

  • F2 Generation: Results of self-fertilization; phenotype ratio 3:1 (dominant to recessive).

    • Example: In pea plants, 705 purple flowers and 224 white flowers observed.

    • Alleles: Dominant (represented by an uppercase letter) and recessive (represented by a lowercase letter).

Mendel's Hypotheses

  1. Alternative versions of genes exist (alleles).

  2. Each organism inherits two alleles for each character (one from each parent).

  3. If the alleles differ, one may be dominant and the other recessive.

  4. Gametes carry only one allele per gene due to segregation in gamete formation.

Dihybrid Cross

  • Dihybrid Cross: Cross between individuals differing in two characters.

  • Example Using Seed Color and Shape:

    • F1 generation are all heterozygous, leading to a phenotypic ratio of 9:3:3:1 in the F2 generation.

    • Law of Independent Assortment: Alleles of different genes segregate independently of one another during gamete formation.

Extensions of the Laws of Inheritance

  • Incomplete Dominance: The phenotype of heterozygotes is intermediate between the two parental varieties.

  • Codominance: Both alleles are expressed in the phenotype (e.g., ABO blood types in humans).

  • Pleiotropy: One gene affects multiple phenotypic traits (e.g., sickle-cell disease affects shape of red blood cells and causes anemia).

Non-Mendelian Inheritance Patterns

  • Examples of Patterns:

    • Incomplete Dominance

    • Codominance

    • Multiple Alleles (e.g., ABO blood group)

    • Epistasis: The interaction between genes that affects development of a phenotype.

    • Sex-Linkage and Linked Genes.

Conclusion

  • Mendelian genetics provides a fundamental base for understanding inheritance and various genetic traits. Mendel's work has paved the way for modern genetics and our understanding of traits and their transmission.