Mendel and the Gene Idea: Principles of Inheritance

Chapter 14: Mendel and the Gene Idea

Introduction to Mendelian Genetics

Mendel’s experiments with garden peas established the foundational principles of heredity, introducing the concepts of genes, alleles, and the predictable patterns of inheritance. His work disproved the blending hypothesis and led to the formulation of the laws of segregation and independent assortment.

Concept 14.1: Mendel’s Experimental Approach and Laws of Inheritance

Mendel’s Experimental Design

• Character: A heritable feature that varies among individuals (e.g., flower color).

• Trait: Each variant for a character (e.g., purple or white flowers).

• Mendel used garden peas due to their short generation time, large number of offspring, and controlled mating possibilities (self- or cross-pollination).

• He tracked characters with two distinct forms and started with true-breeding varieties (offspring identical to parents when self-pollinated).

Hybridization and Generations

• Hybridization: Mating two contrasting, true-breeding varieties.

• P generation: True-breeding parents.

• F1 generation: Hybrid offspring of the P generation.

• F2 generation: Offspring from self- or cross-pollination of F1 hybrids.

The Law of Segregation

• When Mendel crossed F1 hybrids, the F2 generation showed a 3:1 ratio of dominant to recessive traits (e.g., purple to white flowers).

• Dominant trait: Expressed in F1 hybrids (e.g., purple flowers).

• Recessive trait: Masked in F1 but reappears in F2 (e.g., white flowers).

• Mendel’s “heritable factor” is now known as a gene.

Mendel’s Model of Inheritance

1. Alternative versions of genes (alleles) account for variations in inherited characters. Each gene is located at a specific locus on a chromosome.

2. Each organism inherits two alleles for each gene, one from each parent. These may be identical (homozygous) or different (heterozygous).

3. If alleles differ, the dominant allele determines the phenotype; the recessive allele is masked.

4. Law of segregation: The two alleles for a heritable character segregate during gamete formation and end up in different gametes.

Genetic Vocabulary

• Homozygote: Two identical alleles for a gene (e.g., PP or pp).

• Heterozygote: Two different alleles for a gene (e.g., Pp).

• Phenotype: Physical appearance (e.g., purple flowers).

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

The Testcross

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.

The Law of Independent Assortment

• Mendel’s second law, identified by following two characters at once (dihybrid cross).

• Law of independent assortment: Each pair of alleles segregates independently during gamete formation (applies to genes on different chromosomes or far apart on the same chromosome).

Concept 14.2: Probability Laws in Mendelian Inheritance

Rules of Probability

• Multiplication rule: Probability of two independent events occurring together is the product of their individual probabilities.

• Addition rule: Probability of any one of two or more mutually exclusive events is the sum of their individual probabilities.

Concept 14.3: Complex Patterns of Inheritance

Degrees of Dominance

• Complete dominance: Heterozygote and dominant homozygote are phenotypically identical.

• Incomplete dominance: F1 hybrids have a phenotype intermediate between the two parents.

• Codominance: Both alleles affect the phenotype in distinguishable ways.

Multiple Alleles and Codominance

• Most genes exist in more than two allelic forms (e.g., ABO blood groups in humans: IA, IB, i).

• Codominance is seen in the AB blood group, where both A and B carbohydrates are present on red blood cells.

Pleiotropy

• Pleiotropy: One gene affects multiple phenotypic traits (e.g., cystic fibrosis, sickle-cell disease).

Epistasis and Polygenic Inheritance

• Epistasis: One gene affects the expression of another gene (e.g., coat color in Labrador retrievers).

• Polygenic inheritance: Multiple genes independently affect a single trait (e.g., human skin color, height).

Environmental Impact on Phenotype

• Phenotype can be influenced by both genotype and environment (multifactorial traits).

• Example: Hydrangea flower color varies with soil pH.

Concept 14.4: Mendelian Inheritance in Humans

Pedigree Analysis

• Pedigree: Family tree describing inheritance of a trait across generations.

• Used to predict probabilities of future offspring inheriting certain traits.

Recessively Inherited Disorders

• Disorders appear only in homozygous individuals (e.g., albinism, cystic fibrosis, sickle-cell disease).

• Carriers are heterozygous and phenotypically normal.

• Consanguineous matings increase the risk of recessive disorders.

Dominantly Inherited Disorders

• Dominant alleles causing lethal diseases are rare (e.g., achondroplasia, Huntington’s disease).

• Huntington’s disease has a late onset, allowing it to persist in populations.

Multifactorial Disorders and Genetic Counseling

• Many diseases (e.g., heart disease, cancer) have both genetic and environmental components.

• Genetic counselors use Mendelian rules and probability to assess risk for prospective parents.

• Fetal and newborn testing can identify genetic disorders early, allowing for informed decisions and interventions.

Summary Table: Mendel’s Seven Pea Plant Characters

Character	Dominant Trait	Recessive Trait	F2 Generation Ratio
Flower color	Purple	White	3.15:1
Seed color	Yellow	Green	3.01:1
Seed shape	Round	Wrinkled	2.96:1
Pod color	Green	Yellow	2.82:1
Pod shape	Inflated	Constricted	2.96:1
Flower position	Axial	Terminal	3.14:1
Stem length	Tall	Dwarf	2.84:1

Additional info: These notes provide a comprehensive overview of Mendelian genetics, including the experimental basis, laws of inheritance, extensions to complex traits, and applications to human genetics. Probability rules and genetic vocabulary are included to support problem-solving and exam preparation.