Chapter 3: Mendelian Inheritance

Introduction to Mendelian Genetics

Mendelian genetics forms the foundation of classical genetics, describing how traits are inherited from one generation to the next. Gregor Mendel's experiments with pea plants led to the discovery of fundamental laws governing heredity, which refuted earlier hypotheses such as pangenesis and the blending theory.

• Pangenesis: An early hypothesis suggesting that all parts of the body contribute 'seeds' to offspring.

• Blending Hypothesis: Proposed that hereditary factors blend together in offspring.

• Mendel's Contribution: Demonstrated that inheritance is particulate, not blended, and that traits are governed by discrete units (now called genes).

Mendel's Study of Pea Plants

Why Pea Plants?

Mendel chose Pisum sativum (garden pea) for its distinct varieties, ease of controlled breeding, and ability to self- or cross-fertilize.

• Hybridization: Breeding between individuals with different characteristics, producing hybrids.

• Self-fertilization: Pollen and egg from the same plant; natural in peas due to flower structure.

• Cross-fertilization: Pollen and egg from different plants; requires manual manipulation.

The Seven Characters Studied by Mendel

Mendel focused on seven easily distinguishable traits, each with two contrasting forms:

Law of Segregation

Single-Factor Crosses

Mendel performed crosses involving one trait at a time (single-factor crosses) to observe inheritance patterns.

1. Cross two true-breeding plants differing in one trait (P generation).

2. Collect and plant F1 generation seeds.

3. Allow F1 plants to self-fertilize, producing F2 seeds.

4. Grow F2 plants and analyze trait distribution.

Results from Single-Factor Crosses

Overall F2 ratio: Approximately 3:1 dominant to recessive phenotype.

Interpretation and the Law of Segregation

• Dominant and Recessive: One trait (dominant) masks the other (recessive) in F1.

• Particulate Inheritance: Traits are governed by discrete units (genes) that do not blend.

• Law of Segregation: Each individual has two alleles for each gene, which segregate during gamete formation so each gamete receives only one allele.

Key Terms:

• Gene: A unit of heredity encoding a trait.

• Allele: Different versions of a gene.

• Homozygous: Two identical alleles for a gene.

• Heterozygous: Two different alleles for a gene.

• Genotype: The genetic makeup (allele combination) of an individual.

• Phenotype: The observable trait.

Punnett Squares

Punnett squares are used to predict the outcome of genetic crosses.

• For a Tt × Tt cross (T = tall, t = short):

• Genotypic ratio: 1 TT : 2 Tt : 1 tt

• Phenotypic ratio: 3 tall : 1 short

Law of Independent Assortment

Two-Factor Crosses

Mendel also studied inheritance of two traits simultaneously (dihybrid crosses), such as seed shape (round/wrinkled) and seed color (yellow/green).

• Linked Assortment Hypothesis: Alleles inherited together as a unit.

• Independent Assortment Hypothesis: Alleles of different genes assort independently during gamete formation.

Experimental Results and Ratios

• Expected F2 ratio: 9:3:3:1 (for two independently assorting genes)

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

Punnett Square for Dihybrid Cross

For a TtYy × TtYy cross (T = tall, t = short, Y = yellow, y = green):

• Phenotypic ratio: 9 tall yellow : 3 tall green : 3 short yellow : 1 short green

Predicting Outcomes for Three or More Genes

Punnett squares become impractical for more than two genes. Alternative methods include:

• Forked-Line Method: Uses branching diagrams to multiply probabilities for each trait.

• Multiplication Method: Multiplies the ratios for each gene to predict overall phenotype ratios.

Example (Three Genes):

• Each gene segregates 3:1 (dominant:recessive).

• Combined ratio: (3+1) × (3+1) × (3+1) = 64 possible combinations.

• Phenotype classes: 27/64 for all dominant traits, 9/64 for two dominant and one recessive, etc.

The Chromosome Theory of Inheritance

Principles of the Chromosome Theory

The chromosome theory of inheritance explains how chromosomes carry and transmit genetic information, accounting for Mendelian patterns of inheritance.

1. Chromosomes contain genetic material.

2. Chromosomes are replicated and passed from parent to offspring.

3. Most eukaryotic cells are diploid, containing homologous pairs of chromosomes.

4. During meiosis, homologous chromosomes segregate independently.

5. Each parent contributes one set of chromosomes, functionally equivalent, to offspring.

Relationship Between Mendel's Laws and Chromosome Behavior

• Law of Segregation: Explained by the separation of homologous chromosomes during meiosis I.

• Law of Independent Assortment: Explained by the random alignment of different chromosome pairs during meiosis I.

Meiotic Basis of Mendel's Laws

• Separation of Homologs: In meiosis I, homologous chromosomes (and thus alleles) segregate into different gametes.

• Random Alignment: The orientation of each chromosome pair is independent, leading to independent assortment of genes on different chromosomes.

X-Linked Inheritance: Morgan's Experiments

Discovery of Sex-Linked Traits

Thomas Hunt Morgan's work with Drosophila melanogaster (fruit flies) confirmed the chromosome theory by demonstrating that the gene for eye color is located on the X chromosome.

• Experimental Steps:

  1. Cross white-eyed male (XwY) with red-eyed female (Xw+Xw+).

  2. F1: All red-eyed offspring.

  3. F1 cross: F1 male (Xw+Y) × F1 female (Xw+Xw).

  4. F2: 1 red-eyed male : 1 white-eyed male : 2 red-eyed females; no white-eyed females.

  5. Testcross: F2 white-eyed male × F1 female yields all four classes, including white-eyed females.

Data Table: Morgan's Crosses

Punnett Squares for X-Linked Inheritance

For F1 cross (Xw+Y × Xw+Xw):

For testcross (XwY × Xw+Xw):

• Result: 1:1:1:1 ratio of red-eyed females, red-eyed males, white-eyed females, and white-eyed males, confirming X-linked inheritance.

Summary of Key Concepts

• Mendel's laws (segregation and independent assortment) describe how alleles are inherited.

• Chromosome theory explains the physical basis for these laws.

• Punnett squares and probability methods predict genetic outcomes.

• Sex-linked inheritance patterns differ from autosomal inheritance and can be traced using crosses and Punnett squares.

Additional info: The notes above expand on brief slide points, clarify terminology, and provide context for experimental design and interpretation. All tables have been reconstructed for clarity and completeness. Equations for probability calculations in genetics (e.g., product rule) can be expressed as:

• Probability of independent events:

• For a three-gene cross: