11.1

Mendel’s Experiments and the Foundations of Genetics

Gregor Mendel and the Scientific Study of Heredity

Modern genetics began in the mid-1800s with Gregor Mendel, who used pea plants to uncover the basic principles of heredity. Mendel’s methodical experiments and quantitative approach allowed him to identify predictable patterns in the inheritance of traits.

• Model Organism: Mendel used Pisum sativum (pea plants) due to their many varieties and ability to self- or cross-pollinate.

• True-Breeding: Plants that, when self-pollinated, produce offspring identical to themselves.

• Hybridization: The crossing of two different true-breeding varieties.

Example: Crossing purple-flowered and white-flowered pea plants produced all purple flowers in the first generation (F1), but both colors reappeared in the second generation (F2).

Mendel’s Laws of Inheritance

The Law of Segregation

Mendel’s first law states that two alleles for a heritable character segregate (separate) during gamete formation and end up in different gametes.

• Each organism inherits two alleles, one from each parent.

• Alleles separate during meiosis, so each gamete carries only one allele for each gene.

Example: A plant with genotype Pp (P = purple, p = white) produces gametes with either P or p alleles.

The Law of Independent Assortment

Mendel’s second law states that alleles of different genes assort independently of one another during gamete formation, provided the genes are on different chromosomes.

• Demonstrated by dihybrid crosses (e.g., seed color and seed shape).

• Results in new combinations of traits in offspring.

Example: Crossing plants with yellow round seeds (YYRR) and green wrinkled seeds (yyrr) produces F1 hybrids (YyRr), which can produce four phenotypic combinations in the F2 generation.

Key Genetic Concepts and Terminology

Genes, Alleles, and Chromosomes

• Gene: A unit of heredity that encodes information for a specific trait.

• Allele: Alternative versions of a gene (e.g., P for purple, p for white).

• Homozygous: Two identical alleles for a gene (PP or pp).

• Heterozygous: Two different alleles for a gene (Pp).

Example: The gene for flower color in peas has two alleles: one for purple and one for white.

Dominant and Recessive Traits

• Dominant allele: Expressed in the phenotype even if only one copy is present (e.g., purple flowers).

• Recessive allele: Expressed only when two copies are present (e.g., white flowers).

Phenotype vs. Genotype

• Phenotype: Observable traits (e.g., flower color).

• Genotype: Genetic makeup (e.g., PP, Pp, or pp).

Punnett Squares and Probability in Genetics

Monohybrid Crosses

Used to predict the outcome of a single trait cross. The F2 generation typically shows a 3:1 ratio of dominant to recessive phenotypes.

Dihybrid Crosses

Used to predict the outcome of two traits. The F2 generation shows a 9:3:3:1 phenotypic ratio if the genes assort independently.

Testcrosses

A testcross determines the genotype of an individual with a dominant phenotype by crossing it with a homozygous recessive individual.

• If any offspring display the recessive phenotype, the unknown parent is heterozygous.

Results of Mendel’s Crosses: Quantitative Data

Table: Mendel’s Crosses for Seven Characters in Pea Plants

Character	Dominant Trait	Recessive Trait	F2 Ratio (Dominant:Recessive)
Flower Color	Purple	White	705:224 (3.15:1)
Seed Color	Yellow	Green	6,022:2,001 (3.01:1)
Seed Shape	Round	Wrinkled	5,474:1,850 (2.96:1)
Pod Color	Green	Yellow	428:152 (2.82:1)
Pod Shape	Inflated	Constricted	882:299 (2.95:1)
Flower Position	Axial	Terminal	651:207 (3.14:1)
Stem Length	Tall	Dwarf	787:277 (2.84:1)

Summary of Mendel’s Model

• Traits are determined by discrete units (genes) inherited from each parent.

• Alleles segregate during gamete formation (Law of Segregation).

• Alleles for different genes assort independently (Law of Independent Assortment).

• Dominant alleles mask the expression of recessive alleles in heterozygotes.

Applications and Importance

• Mendel’s principles form the foundation of classical genetics and are essential for understanding inheritance patterns in all organisms.

• Modern genetics builds on these concepts to explore more complex inheritance, such as incomplete dominance, codominance, and polygenic traits.

Additional info: Mendel’s work was not widely recognized until decades after its publication, but it now underpins much of modern biology, agriculture, and medicine.