Mendel and the Gene: Foundations of Classical Genetics

Chapter 14: Mendel and the Gene

Introduction to Mendelian Genetics

Gregor Mendel's experiments with garden peas established the foundational principles of inheritance, forming the basis for modern genetics. Mendel's careful selection of the pea plant as a model organism allowed him to control mating and observe patterns of trait inheritance across generations.

• Model Organism: Peas were chosen for their ease of cultivation, short generation time, and ability to produce many offspring.

• Controlled Crosses: Mendel could manipulate which plants mated, enabling precise genetic studies.

Key Terms in Mendelian Genetics

Understanding Mendel's work requires familiarity with several essential genetic terms:

Term	Definition	Example/Comment
Gene	A hereditary factor influencing a trait	Modern: DNA region coding for protein/RNA
Allele	A specific form of a gene	Diploids may have same or different alleles
Genotype	Allele listing for an individual	Diploid: two alleles/gene; haploid: one allele/gene
Phenotype	Observable traits of an individual	From molecular to organismal level
Homozygous	Two identical alleles for a gene	e.g., RR or rr
Heterozygous	Two different alleles for a gene	e.g., Rr
Dominant allele	Expressed in both homozygous and heterozygous states	Does not imply high frequency or fitness
Recessive allele	Expressed only in homozygous state	"Recedes" in heterozygotes
Pure line	Individuals identical in phenotype, always produce same phenotype offspring	Homozygous for the gene
Hybrid	Offspring from parents with different genotypes	Heterozygous
Polymorphic trait	Trait with two or more common forms	e.g., flower color in peas
Reciprocal cross	Cross with reversed parental phenotypes	Tests for sex influence on inheritance
Testcross	Cross with homozygous recessive to determine unknown genotype	Used to reveal genotype of dominant phenotype
X-linked/Y-linked	Gene located on X or Y chromosome	Shows sex-specific inheritance patterns

Mendel’s Monohybrid Crosses and the Principle of Segregation

Mendel observed that crossing pure lines for a single trait (monohybrid cross) produced a 3:1 ratio of dominant to recessive phenotypes in the F2 generation. This led to the principle of segregation:

• Each individual has two alleles for each gene, which segregate during gamete formation.

• Each gamete receives only one allele from each gene pair.

Trait	Dominant Phenotype	Recessive Phenotype	F2 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	Short	2.96:1

Mendel’s Dihybrid Crosses and the Principle of Independent Assortment

When Mendel studied two traits simultaneously (dihybrid cross), he found a 9:3:3:1 phenotypic ratio in the F2 generation, supporting the principle of independent assortment:

• Alleles of different genes assort independently during gamete formation if they are on different chromosomes.

Testcrosses

A testcross is used to determine the genotype of an individual with a dominant phenotype by crossing it with a homozygous recessive individual. The offspring phenotypes reveal the unknown genotype.

Chromosomal Basis of Mendel’s Principles

Mendel’s principles are explained by the behavior of chromosomes during meiosis:

• Segregation: Homologous chromosomes (and thus alleles) separate during meiosis I.

• Independent Assortment: Chromosomes align independently, so genes on different chromosomes assort independently.

Extending Mendel’s Rules: Linkage and Crossing Over

Some genes do not assort independently because they are located close together on the same chromosome—a phenomenon known as linkage. However, crossing over during meiosis can separate linked genes, producing recombinant offspring. The frequency of recombination can be used to map gene positions on chromosomes.

Multiple Alleles, Codominance, and Incomplete Dominance

Many genes have more than two alleles (multiple allelism). Some alleles are codominant (both expressed in heterozygotes), while others show incomplete dominance (heterozygotes have an intermediate phenotype).

• Example (Multiple Alleles & Codominance): Human ABO blood types are determined by three alleles (IA, IB, i). IA and IB are codominant; both are dominant over i.

• Example (Incomplete Dominance): Crossing red-flowered and white-flowered plants yields pink-flowered offspring.

Environmental Effects and Quantitative Traits

Most phenotypes are influenced by both genes and the environment. Some traits, called quantitative traits, vary continuously and are controlled by multiple genes (polygenic inheritance), often resulting in a normal distribution of phenotypes.

Human Inheritance and Pedigree Analysis

Pedigrees are used to study inheritance patterns in humans. Traits can be autosomal or sex-linked, and dominant or recessive. Pedigree analysis helps determine the mode of transmission for genetic traits.

Trait Type	Key Characteristics
Autosomal Recessive	Affects males and females equally; often skips generations; affected offspring usually homozygous
Autosomal Dominant	Affects males and females equally; does not skip generations; affected offspring usually heterozygous
X-linked Recessive	Males more frequently affected; trait often skips generations; no male-to-male transmission
X-linked Dominant	Affects both sexes; does not skip generations; all daughters of affected males are affected

Additional info: These principles form the basis for understanding classical genetics and are foundational for advanced studies in molecular genetics, genomics, and evolutionary biology.