Chapter 14: Mendel and the Gene Idea

Big Idea: Mendel's Work Revealed the Fundamental Principles of Heredity

Gregor Mendel's experiments with pea plants established the foundation for modern genetics, demonstrating how traits are inherited from one generation to the next. His scientific approach led to the discovery of two key laws of inheritance: the law of segregation and the law of independent assortment.

Concept 14.1: Mendel's Scientific Approach and Laws of Inheritance

Mendel's Experimental, Quantitative Approach

Mendel used garden peas for his experiments due to their distinct traits, short generation time, and ability to control mating. He focused on characters with two alternative forms and started with true-breeding varieties.

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

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

• True-breeding: Plants that produce offspring of the same variety when self-pollinated.

• 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.

Mendel's Crosses and the Law of Segregation

Mendel's experiments disproved the blending theory of inheritance. When crossing true-breeding purple and white flowered plants, all F1 hybrids were purple. Self-pollination of F1 hybrids produced both purple and white flowers in the F2 generation, revealing a 3:1 ratio.

• Law of Segregation: Two alleles for a gene segregate during gamete formation and reunite at random during fertilization.

• Physical Basis: The movement of chromosomes during meiosis explains allele segregation.

Mendel's Model of Inheritance

Mendel proposed a model to explain the 3:1 ratio observed in the F2 generation, based on four concepts:

1. Alternative versions of genes (alleles) account for variations in inherited characters.

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

3. If alleles differ, the dominant allele determines appearance; the recessive allele has no noticeable effect.

4. The two alleles segregate during gamete formation.

Useful Genetic Vocabulary

• Homozygous: Two identical alleles for a gene.

• Heterozygous: Two different alleles for a gene.

• Phenotype: Physical appearance.

• Genotype: Genetic makeup.

Concept 14.2: The Law of Independent Assortment

Monohybrid and Dihybrid Crosses

Mendel derived the law of segregation by following a single character (monohybrid cross). He identified the law of independent assortment by following two characters (dihybrid cross), showing that genes are inherited independently.

• Monohybrid Cross: Heterozygous for one character.

• Dihybrid Cross: Heterozygous for two characters.

• Law of Independent Assortment: Genes for different traits can segregate independently during gamete formation.

Concept 14.3: Extensions of Mendelian Genetics

Degrees of Dominance

• Complete Dominance: Heterozygote and dominant homozygote have identical phenotypes.

• Incomplete Dominance: F1 hybrids have intermediate phenotypes.

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

Multiple Alleles and Pleiotropy

• Multiple Alleles: Most genes exist in populations in more than two allelic forms (e.g., ABO blood group).

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

Epistasis and Polygenic Inheritance

• Epistasis: One gene affects the phenotype of another gene.

• Polygenic Inheritance: Multiple genes independently affect a single trait.

Environmental Impact on Phenotype

• Phenotypic Plasticity: Ability to produce different phenotypes under varying environmental conditions.

• Multifactorial Traits: Traits influenced by multiple genes and environmental factors.

Concept 14.4: Human Genetics and Pedigree Analysis

Pedigrees and Mendelian Patterns

Pedigrees are family trees that describe the inheritance of traits across generations. They are used to predict the probability of specific phenotypes in future offspring.

• Pedigree: Diagram showing the inheritance of a trait.

• Testcross: Breeding a mystery individual with a homozygous recessive individual to determine genotype.

Recessively and Dominantly Inherited Disorders

• Recessive Disorders: Show up only in individuals homozygous for the allele (e.g., albinism, cystic fibrosis).

• Dominant Disorders: Caused by dominant alleles (e.g., achondroplasia, Huntington's disease).

• Carriers: Heterozygous individuals who carry the recessive allele but are phenotypically normal.

Genetic Testing and Counseling

• Carrier Identification: Tests to identify carriers of genetic disorders.

• Fetal Testing: Techniques such as amniocentesis and chorionic villus sampling to detect genetic disorders in fetuses.

Summary Table: Mendel's Pea Plant Characters

Key Equations

• Punnett Square for Monohybrid Cross:

• Phenotypic Ratio: 3:1 (dominant:recessive)

• Genotypic Ratio: 1:2:1 (homozygous dominant : heterozygous : homozygous recessive)

Summary

Mendel's principles of heredity—segregation and independent assortment—form the basis of classical genetics. These principles explain how traits are inherited and provide the foundation for understanding more complex patterns of inheritance, including those influenced by multiple genes and environmental factors.