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.

The Law of Segregation

Mendel observed that crossing true-breeding purple and white flowered pea plants resulted in all purple F1 hybrids, contradicting the blending hypothesis. When F1 hybrids were crossed, the F2 generation showed a 3:1 ratio of purple to white flowers.

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

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

Mendel's Model of Inheritance

Mendel's model explains the 3:1 ratio observed in the F2 generation and consists of four concepts:

1. Alleles: Alternative versions of genes account for variations in inherited characters.

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

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

4. Segregation: The two alleles for a heritable character separate 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.

The Testcross

A testcross is used to determine the genotype of an individual with a dominant phenotype by breeding it with a homozygous recessive individual. If any offspring display the recessive phenotype, the mystery parent must be heterozygous.

The Law of Independent Assortment

Mendel's second law states that genes for different traits can segregate independently during the formation of gametes. This was demonstrated by dihybrid crosses, which produced a 9:3:3:1 ratio in the F2 generation.

Concept 14.3: Extensions of Mendelian Genetics

Complex Patterns of Inheritance

Inheritance patterns can be affected by various genetic phenomena:

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

• Incomplete Dominance: F1 hybrids have phenotypes intermediate between parental varieties.

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

• Multiple Alleles: More than two allelic forms exist in a population (e.g., ABO blood group).

• Pleiotropy: One gene affects multiple phenotypic traits.

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

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

Nature and Nurture: Environmental Impact on Phenotype

Phenotypic plasticity describes the ability of an individual to produce different phenotypes under varying environmental conditions. Multifactorial traits depend on multiple genes and environmental influences.

Concept 14.4: Human Genetics and Pedigree Analysis

Pedigree Analysis

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 and to deduce genotypes of family members.

Recessively Inherited Disorders

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

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

Sickle-Cell Disease

Sickle-cell disease is caused by a single amino acid substitution in hemoglobin. Homozygous individuals have abnormal hemoglobin, while heterozygotes (sickle-cell trait) are usually healthy and have increased resistance to malaria.

Dominantly Inherited Disorders

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

• Mutation: Dominant alleles causing lethal diseases are rare and arise by mutation.

Genetic Testing and Counseling

Genetic counselors provide information to prospective parents. Tests for identifying carriers and fetal testing (e.g., amniocentesis, chorionic villus sampling) help detect genetic disorders and inform reproductive decisions.

Summary Table: Mendel's Pea Plant Characters

Key Equations

• Punnett Square: Used to predict genotypic and phenotypic ratios in crosses.

• Genotypic Ratio (Monohybrid Cross):

• Phenotypic Ratio (Monohybrid Cross):

• Dihybrid Cross Ratio:

Summary

Mendel's principles of heredity—segregation and independent assortment—form the basis of classical genetics. Extensions to Mendelian genetics include complex inheritance patterns, environmental influences, and the study of human genetics through pedigree analysis and genetic testing.