Chapter 14: Mendel and the Gene Idea

Introduction to Mendelian Genetics

Gregor Mendel's experiments with pea plants established the foundational principles of heredity. His quantitative approach and careful selection of traits allowed him to deduce laws that govern genetic inheritance, which remain central to modern biology.

Mendel’s Experimental Approach

Mendel chose pea plants for their distinct traits, short generation time, and ability to control mating. He tracked characters with two alternative forms and used true-breeding varieties to ensure reliable results.

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

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

• 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 flowers produced only purple F1 hybrids, contradicting the blending hypothesis. Self-pollination of F1 hybrids yielded a 3:1 ratio of purple to white flowers in the F2 generation.

• Dominant Trait: The trait that appears in the F1 generation (purple).

• Recessive Trait: The trait that reappears in the F2 generation (white).

• Gene: Mendel's "heritable factor" is now known as a gene.

Table: Mendel’s Crosses for Seven Characters

This table summarizes the results of Mendel's crosses for seven pea plant characters, showing dominant and recessive traits and their ratios.

Mendel’s Model of Inheritance

Mendel proposed four concepts to explain the 3:1 ratio observed in F2 offspring:

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 alleles differ, the dominant allele determines appearance; the recessive allele is masked.

4. Law of Segregation: The two alleles for a character segregate during gamete formation, ending up in different gametes.

Genetic Vocabulary

• Homozygote: An organism with two identical alleles for a gene (homozygous).

• Heterozygote: An organism with two different alleles for a gene (heterozygous).

• Phenotype: Physical appearance.

• Genotype: Genetic makeup.

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 mystery parent is heterozygous.

The Law of Independent Assortment

Mendel’s second law states that each pair of alleles segregates independently during gamete formation. This law applies to genes on different chromosomes or those far apart on the same chromosome.

• Monohybrid: Heterozygous for one character.

• Dihybrid: Heterozygous for two characters.

Probability Laws in Genetics

Mendelian inheritance follows the rules of probability. The multiplication rule calculates the probability of independent events occurring together, while the addition rule calculates the probability of mutually exclusive events.

• Multiplication Rule:

• Addition Rule:

Complex Patterns of Inheritance

Inheritance patterns can be more complex than simple Mendelian genetics. These include incomplete dominance, codominance, multiple alleles, pleiotropy, epistasis, and polygenic inheritance.

Degrees of Dominance

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

• Incomplete Dominance: Heterozygote phenotype is intermediate between two homozygotes.

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

Multiple Alleles

Most genes exist in populations in more than two allelic forms. The ABO blood group in humans is determined by three alleles.

Pleiotropy

Pleiotropy occurs when one gene affects multiple phenotypic characters. For example, sickle-cell disease and cystic fibrosis are caused by pleiotropic alleles.

Epistasis

Epistasis is when the expression of one gene affects the expression of another gene. In Labrador retrievers, coat color depends on two genes, resulting in modified phenotypic ratios.

Polygenic Inheritance

Polygenic inheritance occurs when multiple genes independently affect a single trait, such as height or skin pigmentation.

Environmental Impact on Phenotype

Phenotype can be influenced by environmental factors as well as genotype. Phenotypic plasticity describes the ability to produce different phenotypes under varying conditions. Multifactorial traits depend on both genetic and environmental influences.

Human Mendelian Genetics

Many human traits follow Mendelian patterns, though humans are not ideal subjects for genetic research due to long generation times and ethical constraints. Pedigree analysis is used to study inheritance in families.

Recessively Inherited Disorders

Recessive disorders appear only in individuals homozygous for the allele. Carriers are heterozygous and phenotypically normal. Albinism is an example of a recessive condition.

Cystic Fibrosis

Cystic fibrosis is caused by a recessive allele resulting in defective chloride transport channels. Symptoms include mucus buildup and abnormal nutrient absorption.

Sickle-Cell Disease

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

Dominantly Inherited Disorders

Some disorders are caused by dominant alleles, such as achondroplasia (dwarfism) and Huntington’s disease. Dominant lethal alleles are rare and often arise by mutation.

Multifactorial Disorders

Many diseases, including heart disease and cancer, have both genetic and environmental components. Lifestyle choices significantly affect phenotype.

Genetic Testing and Counseling

Genetic counselors provide information to prospective parents. Fetal and newborn testing can reveal genetic disorders, aiding in early intervention and informed decision-making.

Summary Tables: Extensions to Mendelian Genetics

Key concepts and relationships among alleles and genes are summarized in the following tables:

Additional info: These notes expand on Mendel’s principles with modern genetic terminology and examples, providing a comprehensive overview suitable for exam preparation in General Biology.