11_Mendelian Inheritance and Complex Inheritance

Mendel and the Gene Idea

Overview of Genetic Principles

  • Blending Hypothesis: Genetic material blends from parents (e.g., blue and yellow make green).

  • Particulate Hypothesis: Traits are passed as discrete units called genes.

  • Mendel's Experimentation: Documented a particulate mechanism using garden peas.

Mendel's Scientific Approach

  • Mendel's Experiments: Basic principles of heredity discovered through breeding garden peas.

  • Quantitative Approach: Used controlled experiments, focusing on distinct heritable features.

  • True-Breeding Varieties: Varieties that produce offspring identical to themselves.

Key Concepts

Law of Segregation

  • Crossing True-Breeding Plants: White and purple flowered pea plants resulted in all purple F1 plants; F2 generation had a ratio of 3:1 (purple to white).

  • Dominant and Recessive Traits: Purple flowers are dominant; white masks the trait but reappears in F2.

Mendel’s Genetic Model

  • Alleles: Different versions of a gene; inherit two alleles, one from each parent.

  • Dominance: In cases of differing alleles, dominant ones affect appearance.

  • Segregation: During gamete formation, alleles for heritable traits segregate into different gametes.

Genetic Vocabulary

  • Homozygous: Organism with identical alleles.

  • Heterozygous: Organism with different alleles.

  • Phenotype: Physical traits; Genotype: Genetic makeup.

Testcross and Inheritance

  • Testcross: Determines genotype of a dominant phenotype individual by crossing with recessive.

  • Law of Independent Assortment: Alleles for different traits segregate independently.

Extension of Mendelian Genetics

Non-Mendelian Inheritance Patterns

  • Complete Dominance: Dominant phenotype identical in homozygous and heterozygous.

  • Incomplete Dominance: Intermediate phenotype in heterozygotes.

  • Codominance: Distinguishable phenotypes from both alleles.

Multiple Alleles and Pleiotropy

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

  • Pleiotropy: Single gene influencing multiple traits (e.g., cystic fibrosis).

Complex Inheritance Patterns

  • Epistasis: One gene alters the expression of another (e.g., coat color in Labrador retrievers).

  • Polygenic Inheritance: Traits affected by multiple genes (e.g., skin color in humans).

Environmental Influence

  • Norm of Reaction: Range of phenotypes influenced by the environment.

  • Multifactorial Inheritance: Traits affected by both genetic and environmental factors.

Human Genetics and Pedigree Analysis

  • Human Traits: Mendelian patterns apply but complexities exist due to breeding constraints.

  • Pedigree: Tool for tracking inheritance across generations.

Genetic Disorders

Recessively Inherited Disorders

  • Cystic Fibrosis: Common lethal genetic disorder; symptoms include mucus buildup.

  • Sickle-Cell Disease: Caused by a single amino acid substitution; shows incomplete dominance.

Dominantly Inherited Disorders

  • Achondroplasia: A rare dominant disorder causing dwarfism.

  • Huntington’s Disease: Neurodegenerative condition appearing in adulthood.

Genetic Counseling

  • Genetic counselors provide assessment and information on heritable diseases.