Laws of Inheritance: Mendelian Laws of Inheritance

Unit 1: Laws of Inheritance

Lesson 1.1: Mendelian Laws of Inheritance

Contents

  • Introduction

  • Learning Objectives

  • Warm Up

  • Learn about It!

  • An Overview of Genetics

  • Gregor Mendel and His Pea Plant Hybridization Experiments

  • Brief Background of Gregor Mendel

  • Pea Plant as Mendel’s Model Organism

  • Mendel’s Challenges and the Rediscovery of His Work

  • Review of Genetic Terminologies

  • Law of Segregation

  • Mendel’s Monohybrid Cross

  • Interpretations of the Monohybrid Cross

  • Using Punnett Squares

  • Law of Independent Assortment

  • Mendel’s Dihybrid Cross

  • Interpretations of the Dihybrid Cross

  • Laws of Inheritance and Gametogenesis

  • Key Points

  • Check Your Understanding

  • Challenge Yourself

  • Photo Credits

  • Bibliography

  • Key to Try It!

Introduction

  • Traits may run in families and distinguish them from others (e.g., hair color, complexions).

  • Inherited features may show variation even among siblings.

  • Genetic disorders can also be inherited, such as conditions that increase risks for diseases (e.g., hypertension, diabetes).

Learning Objectives

In this lesson, you will:

  • Explain the foundations and development of Mendelian genetics.

  • Describe and apply the Mendelian laws of inheritance.

  • DepEd Competency: Predict genotypes and phenotypes of parents and offspring using the laws of inheritance (STEM_BIO11/12-IIIa-b-1).

Warm Up: The Genetic Boat Is Sinking: Survival By Chance

  • Time: 15 minutes

  • Purpose: Understand genetic variation in classmates.

  • Materials: Paper lots with genetic traits (accessible via web link).

  • Activity procedure:

    1. Class will organize based on genetic traits describing how the physical traits compare.

    2. Each round, students group according to traits assigned by the teacher until few remain.

  • Guide Questions:

    1. Identify which traits mentioned are genetic and their significance.

    2. Discuss why some classmates share traits while others do not.

    3. Reflect on the importance of understanding genetics.

Learn about It! An Overview of Genetics

  • Genetics: Subdiscipline of biology focusing on heredity (inheritance) and genetic variation.

  • Heredity: Transmission of traits from parents to offspring.

  • Genetic variation: Degree of difference in DNA among individuals within a population.

  • Biological diversity is attributed to gene modification through time.

Subfields of Genetics

  • Table 1.1.1: Examples of genetics subfields

    • Molecular Genetics: Study of DNA and gene expression.

    • Cytogenetics: Study of chromosomes during meiosis.

    • Population Genetics: Influence of evolution on gene frequencies.

    • Transmission Genetics: Patterns of inheritance focused, involves classical genetics.

Gregor Mendel and His Pea Plant Hybridization Experiments
Brief Background of Gregor Mendel

  • Gregor Johann Mendel (1822-1884): Known as the father of modern genetics.

  • Born in Moravia, raised on a farm working with plants.

  • Studied failed teaching exams in physics and natural history, went to Vienna for education.

  • Started hybridization studies in pea plants (Pisum sativum) in 1856, contributing significantly to genetics.

Pea Plant as Mendel’s Model Organism

  • Chose Pisum sativum for experiments:

    • Traits: Vigorous growth; easy cross-fertilization due to presence of both male/female reproductive organs.

    • Self-fertilization: Possible via covered reproductive structures, allowing true-breeding plants.

    • Example: True-breeding violet plants produce no new traits upon mating.

Mendel’s Challenges and the Rediscovery of His Work

  • Mendel's work lay foundational but faced misunderstanding; lacked discovery of DNA.

  • Several previous theories discredited:

    • Pangenesis: Proposed by Hippocrates (400 B.C.E), suggested traits were produced as "seeds" in body organs.

    • Homunculus theory: Suggested sperm contained a miniature human.

    • Blending inheritance: Claimed traits blended with each generation.

  • Published findings in 1866—ignored until rediscoveries in 1900 by Hugo de Vries, Carl Correns, and Erich von Tschermak.

Review of Genetic Terminologies

  • Gene: Basic unit of heredity as termed by Mendel, earlier unit factors.

  • Characteristic: Heritable feature controlled by genes.

  • Alleles: Alternative forms of a gene; e.g., flower color.

  • Locus: Position gene occupies on a chromosome.

  • Homozygous: Organism has identical alleles for a trait (e.g., TT, tt).

  • Heterozygous: Organism has different alleles (e.g., Tt).

  • Genotype: Genetic makeup (combination of alleles).

  • Phenotype: Observable trait (expression of genotype).

  • Dominant allele: Overrides recessive allele expression.

  • Recessive allele: Masked by a dominant allele.

Mendel's Characteristics in Hybridization

  • Seven traits examined:

    • Plant height, flower color, flower position, seed color, seed shape, pod color, pod shape.

Law of Segregation
Mendel’s Monohybrid Cross

  • Monohybrid Cross: Involves one characteristic with contrasting phenotypes (e.g., tall vs dwarf).

  • P Generation: True-breeding plants mated (TT x tt).

  • F1 Generation: All tall offspring (Tt).

  • F2 Generation: Offspring from F1 cross (Tt x Tt) yields 3:1 tall to dwarf ratio.

Interpretations of the Monohybrid Cross

  • Observations:

    1. Tall phenotype present in F1 only.

    2. Dwarf phenotype reappears in F2 indicating masked expression by dominant trait.

  • Principle of Dominance: Dominant allele masks recessive.

  • Mendel’s notation:

    • T = Tall (dominant), t = Dwarf (recessive).

  • F1 individuals are termed heterozygous (Tt).

  • F2 genotypic ratio: 1/4 TT : 1/2 Tt : 1/4 tt, phenotypic ratio: 3/4 ext{ Tall} : 1/4 ext{ Dwarf}.

Using Punnett Squares

  • Punnett Square: Method developed by Reginald Punnett to visualize genetic crosses.

  • Steps:

    1. List parental genotypes (P Generation & F1 Generation).

    2. Determine possible gametes.

    3. Create Punnett Square to combine gametes, calculate ratios.

Law of Independent Assortment
Mendel’s Dihybrid Cross

  • Dihybrid Cross: Examines two traits; e.g., seed shape and color (e.g., RrYy).

  • F2 phenotypic ratio: 9:3:3:1 (round yellow: round green: wrinkled yellow: wrinkled green).

Interpretations of the Dihybrid Cross

  • Law of Independent Assortment: Genes independently assort, unlinked.

  • Application of Punnett Square: Cross between parental genotypes determines genotypic and phenotypic distribution.

Laws of Inheritance and Gametogenesis

  • Segregation and independent assortment essential during anaphase I of meiosis (Gamete formation).

  • Homologous chromosomes segregate during gamete formation randomly; allele distribution crucial for inheritance patterns.

Key Points

  • Genetics encompasses inheritance and variation.

  • Genesis of Mendelian genetics through Gregor Mendel.

  • Genes control organism characteristics; alleles display dominance/recessiveness.

  • Monohybrid and dihybrid crosses reveal segregation and assortment laws.

Check Your Understanding

  • True or False Statements: Assess the accuracy of textbook statements about Mendelian inheritance and characteristics.

  • Genotyping Exercises: Identifying suitable gametes produced by given parental genotypes.

Challenge Yourself

  • Questions addressing genetic principles, gamete production, and implications of dominance within breeding contexts.

Photo Credits

  • Various images credited appropriately, showcasing Mendel's work and pea plants.

Bibliography

  • Academic sources that provide foundational support for Mendel’s genetic studies and terminology.