Chapter 14: Mendel and the Gene Idea

Introduction

Gregor Mendel’s experiments with pea plants established the fundamental principles of heredity, forming the basis of classical 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 P generation).

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

The Law of Segregation

Mendel’s crosses between true-breeding purple and white flowered pea plants produced all purple F1 hybrids, contradicting the blending hypothesis. When F1 hybrids were self-pollinated, the F2 generation showed a 3:1 ratio of purple to white flowers, indicating that traits are inherited as discrete units.

• Law of Segregation: The two alleles for a heritable character separate during gamete formation and end up in different gametes.

• This law is explained by the movement of chromosomes during meiosis.

Mendel’s Model of Inheritance

Mendel’s model explains the 3:1 inheritance pattern 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 or observable traits.

Punnett Squares and Monohybrid Crosses

Punnett squares are used to predict the results of genetic crosses. In a monohybrid cross (one gene), the F2 generation shows a 3:1 phenotypic ratio and a 1:2:1 genotypic ratio.

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 unknown parent is heterozygous.

The Law of Independent Assortment

Dihybrid Crosses

Mendel’s second law, the law of independent assortment, was derived from experiments following two characters at once (dihybrid crosses). This law states that each pair of alleles segregates independently during gamete formation, leading to new combinations of traits.

• Monohybrid cross: Involves one gene; produces a 3:1 ratio.

• Dihybrid cross: Involves two genes; produces a 9:3:3:1 ratio.

Extensions of Mendelian Genetics

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 separate, distinguishable ways (e.g., AB blood type).

Multiple Alleles

Some genes have more than two alleles in the population. For example, the ABO blood group in humans is determined by three alleles: IA, IB, and i.

Pleiotropy

Most genes have multiple phenotypic effects, a phenomenon known as pleiotropy. For example, the gene responsible for sickle-cell disease affects multiple systems in the body.

Epistasis and Polygenic Inheritance

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

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

Environmental Impact on Phenotype

The environment can influence the expression of genes, leading to phenotypic plasticity. Traits influenced by both genes and environment are called multifactorial.

Human Genetics and Pedigree Analysis

Pedigrees

A pedigree is a family tree that describes the inheritance of a trait across generations. Pedigree analysis helps predict the probability of inheriting certain traits.

Recessively Inherited Disorders

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

• 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. Heterozygotes (carriers) are less susceptible to malaria, illustrating a heterozygote advantage in certain environments.

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 counseling and testing can help prospective parents assess the risk of inherited disorders. Techniques include carrier testing, amniocentesis, and chorionic villus sampling.

Summary Table: Mendel’s Seven Pea Plant Characters

Key Terms and Definitions

• Allele: Alternative versions of a gene.

• Locus: Specific location of a gene on a chromosome.

• Punnett square: Diagram used to predict the outcome of a genetic cross.

• Genotype: Genetic constitution of an organism.

• Phenotype: Observable traits of an organism.