Chapter 14: Mendel and the Gene Idea

Big Idea

Gregor Mendel’s experiments with pea plants established the fundamental principles of heredity, forming the basis of modern genetics. His work revealed how traits are inherited from one generation to the next through discrete units called genes.

Concept 14.1: Mendel’s Laws of Inheritance

Mendel’s Experimental Approach

Mendel used a scientific and quantitative approach to study inheritance by breeding garden peas. He selected pea plants because they had many distinct varieties, a short generation time, and allowed controlled mating (self- or cross-pollination).

• 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 of two contrasting, true-breeding varieties.

• P generation: Parental generation (true-breeding parents).

• F1 generation: First filial generation (hybrid offspring of the P generation).

• F2 generation: Second filial generation (offspring of F1 individuals).

Mendel’s Key Experiments

Mendel tracked characters that occurred in two distinct forms and started with true-breeding varieties. He performed hybridization and observed the inheritance patterns in subsequent generations.

The Law of Segregation

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

• Law of Segregation: The 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:

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

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

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

4. The two alleles segregate during gamete formation (law of segregation).

Genetic Vocabulary

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

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

• Genotype: Genetic makeup of an organism.

• Phenotype: Physical appearance of an organism.

The 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 any offspring display the recessive phenotype, the mystery parent is heterozygous.

The Law of Independent Assortment

Mendel’s second law states that genes for different traits assort independently during gamete formation. This was demonstrated by following two characters at once (dihybrid cross), resulting in a 9:3:3:1 phenotypic ratio in the F2 generation.

• Monohybrid cross: Cross between individuals heterozygous for one character (3:1 ratio).

• Dihybrid cross: Cross between individuals heterozygous for two characters (9:3:3:1 ratio).

Concept 14.3: Extensions of Mendelian Genetics

Complex Patterns of Inheritance

Inheritance patterns can be more complex than simple dominance and recessiveness. Key extensions include:

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

• Incomplete dominance: Heterozygote phenotype is intermediate between the two homozygotes.

• Codominance: Both alleles are fully expressed in the phenotype (e.g., IA and IB in human blood types).

• Multiple alleles: More than two alleles exist for a gene (e.g., ABO blood group).

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

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

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

Example: ABO Blood System

The ABO blood group in humans is determined by three alleles (IA, IB, and i). IA and IB are codominant, while i is recessive.

Concept 14.4: Human Genetics and Pedigree Analysis

Pedigrees

A pedigree is a family tree that describes the inheritance of a trait across generations. Pedigrees are used to analyze human inheritance patterns and predict the probability of traits appearing in future generations.

Recessively Inherited Disorders

• Recessive disorders: Only appear in individuals homozygous for the recessive allele (e.g., cystic fibrosis, sickle-cell disease).

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

Dominantly Inherited Disorders

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

• Dominant lethal alleles are rare because they often result in death before reproductive age.

Genetic Testing and Counseling

Genetic counselors help families assess risks for inherited disorders. Fetal testing methods include amniocentesis and chorionic villus sampling (CVS), which can detect genetic abnormalities before birth.

Summary Table: Mendel’s Seven Pea Plant Characters

Key Equations and Concepts

• Punnett Square: Used to predict the genotypic and phenotypic ratios of offspring from a genetic cross.

• Genotypic ratio (monohybrid cross):

• Phenotypic ratio (monohybrid cross):

• Dihybrid cross phenotypic ratio:

Summary

Mendel’s discoveries laid the groundwork for understanding genetic inheritance. His laws of segregation and independent assortment explain how traits are passed from parents to offspring, and modern genetics has expanded on these principles to include more complex patterns of inheritance.