Mendel and the Gene: Foundations of Classical Genetics

Introduction to Mendel and the Gene

Historical Foundations of Genetics

The study of heredity and the transmission of traits from one generation to the next began with the pioneering work of Gregor Mendel. His experiments with garden peas laid the groundwork for the chromosome theory of inheritance, which was later formalized by Sutton and Boveri. Genetics, as a branch of biology, focuses on understanding how traits are inherited and how genetic information is passed through generations.

• Gregor Mendel: Conducted experiments that established the basic principles of inheritance.

• Chromosome Theory of Inheritance: Proposed by Sutton and Boveri, linking inheritance to meiosis and asserting that genes are located on chromosomes.

• Genetics: The scientific study of heredity and variation in organisms.

14.1 Mendel’s Experimental System

Key Concepts in Heredity

Mendel sought to understand how traits are transmitted from parents to offspring. At the time, two main hypotheses existed:

• Blending Inheritance: Parental traits blend in offspring, resulting in intermediate traits.

• Inheritance of Acquired Characteristics: Traits modified through use are passed on to offspring.

The Garden Pea as a Model Organism

• Peas were chosen for their ease of growth, short generation time, and ability to produce many seeds.

• Researchers could control mating, making peas ideal for genetic studies.

• Model Organism: A species used for research that is practical to work with and yields results applicable to other species.

• Peas exhibit several polymorphic traits—traits that appear in two or more distinct forms (e.g., purple vs. white flowers).

Controlling Matings in Peas

• Peas normally self-fertilize (self-pollinate), but Mendel could prevent this by removing male organs.

• He performed cross-fertilization (a cross) by transferring pollen from one plant to another.

Figure 14.1: Peas Can Be Self-Fertilized or Cross-Fertilized

Illustrates the difference between self-fertilization and cross-fertilization in pea plants.

Traits Studied by Mendel

• Mendel examined seven easily recognizable traits: seed shape, seed color, pod shape, pod color, flower color, flower and pod position, and stem length.

• Phenotype: The observable features of an individual.

• Each trait had two distinct phenotypes in Mendel's pea population.

Pure Lines and Hybrids

• Pure lines: True-breeding lines that produce offspring identical to themselves when self-fertilized.

• Hybrids: Offspring from mating two different pure lines that differ in one or more traits.

Summary Table: Terms Used in Mendelian Genetics

Term	Definition	Example or Comment
Autosomal inheritance	Inheritance of genes not on sex chromosomes	Mendel studied only autosomal patterns
Gene	Hereditary factor influencing a trait	Modern definition: DNA sequence coding for protein/RNA
Allele	Particular form of a gene	Alleles in diploid may be the same or different
Genotype	Listing of alleles for a gene in an individual	Genotype affects phenotype
Phenotype	Observable traits	Can be observed at different levels
Homozygous	Two of the same allele	Refers to a gene
Heterozygous	Two different alleles	Refers to a gene
Dominant allele	Phenotype in heterozygous and homozygous	Dominance ≠ high frequency
Recessive allele	Phenotype only in homozygous	Recessive 'disappears' in heterozygotes
Pure line	Same phenotype, self-fertilized	Homozygous for trait
Hybrid	Offspring from parents with different traits	Hybrids are heterozygotes
Reciprocal cross	Cross with reversed male/female phenotypes	Tests for sex influence
Testcross	Cross with homozygous recessive	Determines unknown genotype
X-linked	Gene on X chromosome	Most X-linked genes show inheritance in males
Y-linked	Gene on Y chromosome	Involved in male development

14.2 Mendel’s Experiments with a Single Trait

Monohybrid Crosses

• Mendel crossed pure lines differing in one trait (e.g., round vs. wrinkled seeds).

• Parental generation (P): Initial individuals in a cross.

• F1 generation: First filial generation, all had round seeds, contradicting blending inheritance.

• F2 generation: Resulted from self-pollination of F1, showing a 3:1 ratio of round to wrinkled seeds.

Dominant and Recessive Traits

• Dominant trait: Phenotype that appears in hybrids (e.g., round seeds).

• Recessive trait: Phenotype that is hidden in hybrids but reappears in F2 (e.g., wrinkled seeds).

Reciprocal Crosses

• Performed to test if inheritance was influenced by parent gender.

• Results were identical, indicating gender did not affect inheritance for these traits.

Summary Table: F2 Phenotypic Ratios from Monohybrid Crosses

Trait	Dominant Phenotype	Recessive Phenotype	Ratio
Seed shape	Round	Wrinkled	2.96:1
Seed color	Yellow	Green	3.01:1
Pod shape	Inflated	Constricted	2.95:1
Pod color	Green	Yellow	2.82:1
Flower color	Purple	White	3.15:1
Flower/pod position	Axial	Terminal	3.14:1
Stem length	Tall	Dwarf	2.96:1

Particulate Inheritance

• Mendel proposed that hereditary determinants (genes) do not blend or change through use, but act as discrete, unchanging particles.

Genes, Alleles, and Genotypes

• Gene: Hereditary determinant for a trait.

• Allele: Different versions of a gene.

• Genotype: Combination of alleles in an individual, which determines phenotype.

The Principle of Segregation

• Each individual has two alleles for each gene; these alleles segregate during gamete formation.

• Homozygous: Two identical alleles (RR or rr).

• Heterozygous: Two different alleles (Rr).

• Segregation explains the 3:1 phenotypic ratio in F2 generation.

Genotypic ratio from heterozygous cross:

• $rac{1}{4} RR : rac{1}{2} Rr : rac{1}{4} rr$

Summary Table: Mendel's Model for Monohybrid Crosses

Mendel's Claims	Comments
Peas have two copies of each gene	True for many organisms
Genes do not blend	Genes remain unchanged between generations
Each gamete contains one copy of each gene	Due to segregation during gamete formation
Males and females contribute equally	Offspring acquire one allele from each parent
Some alleles are dominant	Dominant allele determines phenotype in heterozygotes

Predicting Offspring with a Punnett Square

1. List unique gamete genotypes for each parent.

2. Arrange one parent's gametes along the top, the other's down the side.

3. Fill in the table with possible offspring genotypes.

4. Calculate proportions or ratios of each genotype and phenotype.

14.3 Mendel’s Experiments With Two Traits

Dihybrid Crosses and Independent Assortment

• Dihybrid cross: Mating between parents heterozygous for two traits.

• Tested whether alleles of different genes segregate independently (independent assortment) or together (dependent assortment).

• Results supported independent assortment: four phenotypes in a 9:3:3:1 ratio.

Testcrosses

• Used to determine the genotype of a parent with a dominant phenotype by crossing with a homozygous recessive individual.

• Offspring phenotypes reveal the unknown genotype.

Additional info:

• Figures referenced in the slides (e.g., 14.1–14.6) visually support the described experiments and results.