Mendel and the Gene Idea: Principles of Inheritance and Variations

Chapter 14: Mendel and the Gene Idea

Introduction

This chapter explores the foundational principles of genetics established by Gregor Mendel through his experiments with pea plants. Mendel's work laid the groundwork for understanding how traits are inherited and how genetic variation arises.

Gregor Mendel and His Experiments

Historical Context

• Gregor Mendel (1822–1884) was an Austrian monk who published his groundbreaking paper on inheritance in 1865.

• He used Pisum sativum (pea plants) to study patterns of inheritance.

Experimental Technique

• Cross-fertilization ("crossing") involved transferring pollen from one plant to another to control parentage.

• Parental generation (P): True-breeding plants with distinct traits (e.g., purple vs. white flowers).

• First filial generation (F1): Offspring of the cross, all showing the dominant trait.

Mendel's Principles of Inheritance

Principle of Segregation

Demonstrated by monohybrid crosses, this principle states that allele pairs separate during gamete formation, and randomly unite at fertilization.

• P Generation: Purple (PP) × White (pp)

• F1 Generation: All purple (Pp)

• F2 Generation: 3 purple : 1 white (phenotypic ratio); 1 PP : 2 Pp : 1 pp (genotypic ratio)

Principle of Independent Assortment

Demonstrated by dihybrid crosses, this principle states that genes for different traits assort independently during gamete formation.

• Example: Seed color (yellow/green) and seed shape (round/wrinkled)

• Phenotypic ratio in F2: 9:3:3:1

Testcross

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

• If all offspring show the dominant trait, the parent is homozygous dominant.

• If offspring show a 1:1 ratio, the parent is heterozygous.

Alleles and Genetic Variation

Definition of Allele

• Alleles are alternative forms of a gene found at the same locus on homologous chromosomes.

• Dominant alleles produce functional enzymes (e.g., purple pigment), while recessive alleles may result in nonfunctional enzymes (e.g., white flowers).

Rules of Probability in Genetics

Chance Events in Segregation and Fertilization

• Segregation and fertilization are random events.

• Rule of Multiplication (Product Rule): Probability of two independent events occurring together is the product of their individual probabilities.

• Rule of Addition (Sum Rule): Probability of either of two mutually exclusive events occurring is the sum of their individual probabilities.

Variations of Mendelian Inheritance

Complete Dominance

• Heterozygote phenotype is identical to homozygous dominant.

• Example: PP and Pp both produce purple flowers.

Incomplete Dominance

• Heterozygote phenotype is intermediate between the two homozygotes.

• Example: Red (CRCR) × White (CWCW) → Pink (CRCW)

Codominance

• Both alleles are fully expressed in heterozygotes.

• Example: ABO blood group alleles (IA, IB)

Multiple Alleles

• More than two alleles exist for a gene in a population.

• Example: ABO blood group system

Pleiotropy

• One gene affects multiple phenotypic traits.

• Example: Sickle-cell disease affects red blood cell shape and multiple body systems.

Epistasis

• The expression of one gene affects the expression of another gene.

• Example: Coat color in Labrador retrievers (modified 9:3:3:1 ratio)

Polygenic (Multifactorial) Inheritance

• Multiple genes contribute to a single trait, resulting in continuous variation.

• Example: Skin color in humans

Phenotypic Plasticity

• Phenotype can vary depending on environmental conditions, not just genotype.

• Example: Hydrangea flower color varies with soil acidity.

Pedigree Analysis

Tracing Inheritance in Families

• Pedigrees are diagrams that show inheritance patterns across generations.

• Used to determine whether traits are dominant or recessive.

• Example: Widow's peak and ability to taste PTC

Prenatal Genetic Testing

Techniques

• Amniocentesis: Sampling amniotic fluid for fetal cells and genetic testing; results in several weeks.

• Chorionic villus sampling (CVS): Sampling placental tissue for genetic analysis; results in several hours.

• Both methods allow for karyotyping and biochemical tests to detect genetic disorders.

Summary Table: Mendel's F2 Crosses for Seven Characters in Pea Plants

Character	Dominant Trait	Recessive Trait	F2 Generation Dominant:Recessive Ratio
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 Table: Relationships Among Alleles

Type	Description	Example
Complete dominance	Heterozygote phenotype same as homozygous dominant	PP, Pp (purple flowers)
Incomplete dominance	Heterozygote phenotype intermediate	CRCW (pink flowers)
Codominance	Both phenotypes expressed	IAIB (AB blood type)
Multiple alleles	More than two alleles in population	ABO blood group
Pleiotropy	One gene affects multiple traits	Sickle-cell disease

Summary Table: Relationships Among Two or More Genes

Type	Description	Example
Epistasis	Expression of one gene affects another	Coat color in dogs
Polygenic inheritance	Trait affected by two or more genes	Skin color in humans

Key Equations

• Probability of independent events: $P(A \text{ and } B) = P(A) \times P(B)$

• Probability of mutually exclusive events: $P(A \text{ or } B) = P(A) + P(B)$

Conclusion

Mendel's principles form the basis of classical genetics, explaining how traits are inherited and how genetic variation arises. Modern genetics builds on these concepts, incorporating molecular biology and environmental influences to provide a comprehensive understanding of heredity.