Chapter 3 - Mendelian Inheritance

Chapter Overview

• Key Topics:

  • Mendel’s study of pea plants

  • Flowering plant fertilization

  • Seven characters studied by Mendel

  • Law of segregation

  • Crossing and prediction of crossing outcomes

  • Generations (P, F1, F2)

  • Dominant and recessive traits

  • Law of independent assortment

  • Complex crossing outcomes

  • Chromosome theory of inheritance

  • Inheritance patterns in humans

  • Pedigree analysis

  • Probability and statistics

Early Theories of Inheritance

• Panagenesis:

  • Contributed by Hippocrates; all body parts produce seeds transmitted to offspring during conception

• Spermists vs. Ovists:

  • Late 1600s debate on whether only men or women contributed to offspring

• Blending Hypothesis:

  • Late 1700s theory where offspring traits are intermediate between parents, implying equal genetic contribution

Mendel’s Contribution

• Mendel's Research:

  • Refuted previous inheritance theories through his studies.

  • Conducted experiments on pea plants using quantitative analysis to derive the natural laws of genetics.

  • Rediscovered in the early 1900s.

Use of Pea Plant in Experiments

• Pisum sativum:

  • Contains both male and female structures.

  • Capable of self-fertilization or cross-fertilization.

  • Available in various traits useful for studying inherited traits.

Types of Fertilization

• Self-Fertilization:

  • Pollen and egg from the same plant, resulting in genetically identical offspring.

• Cross-Fertilization:

  • Uses pollen and egg from different plants to create hybrids (offspring with different traits).

Mendel's Experimental Procedure

• Creating Hybrids:

  • Mendel conducted single-factor crosses to analyze one character.

  • Generations defined as:

    • P Generation: Parents

    • F1 Generation: First filial offspring

    • F2 Generation: Second filial offspring

Mendel’s Law of Segregation

• Principle:

  • Two copies of a gene segregate from each other during gamete formation.

• Impacts:

  • Results in differential allele distributions among offspring.

  • Empirical data led to the conclusion on the particulate nature of inheritance.

Terminology Review

• Gene: Unit of heredity

• Allele: Variant of a gene

• Homozygous: Two identical alleles

• Heterozygous: Two different alleles

• Genotype: Genetic composition

• Phenotype: Observable trait

Mendel's Two-Factor Cross

• Description:

  • Examined two different characters (e.g., seed shape and color) simultaneously.

  • Explored patterns of inheritance - linked traits vs. independent assortment.

Chi-Square Test in Genetics

• Purpose:

  • A statistical method to test genetic hypotheses and evaluate observed vs. expected results.

• Formula:

  • 𝜒² = ∑ (O - E)² / E, where:

    • O = observed data

    • E = expected data

Probability in Genetic Crosses

• Basic Probability:

  • The likelihood of specific outcomes

• Examples of Probability Rules:

  • Sum Rule: Calculating the probability of mutually exclusive events (e.g., combinations of traits).

  • Product Rule: Calculating probabilities for independent events occurring simultaneously.

  • Binomial Expansion: Used to determine probabilities of specific proportions of offspring with certain traits.

Conclusion

• Mendel’s work laid the foundational principles for understanding genetics and inheritance that still affects modern biology and genetics today.