Chapter 14: Mendel and the Gene Idea

Introduction to Mendelian Genetics

Gregor Mendel's experiments with pea plants established the foundational principles of inheritance, which are central to modern genetics. His quantitative approach allowed him to deduce patterns of heredity that had eluded previous researchers.

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

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

• Advantages of Pea Plants: Short generation time, large numbers of offspring, and controlled mating (self-pollination or cross-pollination).

Mendel's Experimental Design

Mendel tracked characters that appeared in two distinct forms and used true-breeding varieties to ensure consistency in his experiments.

• P Generation: True-breeding parents.

• F1 Generation: Hybrid offspring of the P generation.

• F2 Generation: Offspring from self-pollinated or cross-pollinated F1 hybrids.

• Hybridization: Mating of two contrasting, true-breeding varieties.

Mendel's Law of Segregation

Mendel observed that when crossing true-breeding purple and white-flowered pea plants, all F1 hybrids were purple. In the F2 generation, a 3:1 ratio of purple to white flowers appeared.

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

• Punnett Square: Used to predict possible combinations of sperm and egg.

• Alleles: Alternative versions of a gene located at a specific locus on a chromosome.

Genetic Vocabulary

• Homozygote: Organism with two identical alleles for a character (homozygous).

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

• Phenotype: Physical appearance.

• Genotype: Genetic makeup.

Testcross

A testcross is used to determine the genotype of an individual with the dominant phenotype by breeding it with a homozygous recessive individual.

• If any offspring display the recessive phenotype, the mystery parent must be heterozygous.

Law of Independent Assortment

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

• Dihybrid Cross: Cross between F1 dihybrids to study two characters simultaneously.

• Phenotypic Ratio: Approximately 9:3:3:1 in dihybrid crosses.

Probability in Genetics

• Multiplication Rule: Probability of two independent events occurring together is the product of their individual probabilities.

• Addition Rule: Probability that one of two mutually exclusive events will occur is the sum of their individual probabilities.

Example equations:

• Probability of YYRR genotype:

• Probability of YyRR genotype:

Complex Patterns of Inheritance

Inheritance of characters by a single gene may deviate from simple Mendelian patterns in several ways:

• Incomplete Dominance: Phenotype of F1 hybrids is intermediate between parental varieties.

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

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

• Pleiotropy: Genes have multiple phenotypic effects (e.g., cystic fibrosis, sickle-cell disease).

Epistasis and Polygenic Inheritance

• Epistasis: Expression of a gene at one locus affects the phenotypic expression of a gene at another locus (e.g., coat color in Labrador retrievers).

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

Environmental Impact on Phenotype

The phenotype for a character can depend on both genotype and environment. Traits influenced by multiple genes and environmental factors are called multifactorial.

• Example: Hydrangea flower color varies with soil pH.

Human Genetics and Pedigree Analysis

Humans are not ideal subjects for genetic research due to long generation times and ethical constraints. However, pedigree analysis helps trace inheritance patterns across generations.

• Pedigree: Family tree describing relationships and inheritance of traits.

• Multiplication and Addition Rules: Used to predict probabilities of specific phenotypes in offspring.

Recessively Inherited Disorders

• Albinism: Recessive condition characterized by lack of pigmentation.

• Cystic Fibrosis: Most common lethal genetic disease in the U.S.; caused by defective chloride transport channels.

• Sickle-Cell Disease: Caused by a single amino acid substitution in hemoglobin; heterozygotes have sickle-cell trait and are less susceptible to malaria.

Dominantly Inherited Disorders

• Achondroplasia: Form of dwarfism caused by a dominant allele.

• Huntington's Disease: Degenerative disease of the nervous system with late onset.

Multifactorial Disorders

• Many diseases (e.g., heart disease, cancer, alcoholism, mental illnesses) have both genetic and environmental components.

• Lifestyle choices can have a significant effect on phenotype, regardless of genotype.

Additional info: The notes are based on Campbell Biology, Chapter 14, and cover the essential principles of Mendelian genetics, including experimental design, laws of inheritance, genetic vocabulary, complex inheritance patterns, and human genetic disorders.