11_Mendelian+Inheritance+and+Complex+Inheritance

Overview of Mendel and the Gene Idea

  • Genetic principles explain trait inheritance from parents to offspring.

  • Blending Hypothesis: Traits from parents blend together (e.g., blue + yellow = green).

  • Particulate Hypothesis: Traits are inherited as discrete units (genes).

  • Mendel's Experiments: Conducted with garden peas to document hereditary principles.

Mendel's Experimental Approach

  • Chose peas due to distinct varieties with heritable features (traits like flower color).

  • Controlled mating through hybridization.

  • True-breeding parents produce offspring of the same variety when self-pollinating.

Law of Segregation

  • Criss-cross breeding of contrasting true-breeding plants (white and purple flowers).

  • All F1 hybrids exhibited the dominant purple trait.

  • In F2 generation: 3:1 ratio of purple to white flowers observed.

  • Dominant trait: purple; recessive trait: white.

Mendel’s Genetic Model

  1. Alternative Versions of Genes: Genes account for variations in traits (alleles).

  2. Two Alleles Inherited: Each organism inherits one allele from each parent.

  3. Dominance: The dominant allele determines appearance when two differ.

  4. Law of Segregation: Alleles segregate during gamete formation.

Genetic Vocabulary

  • Homozygous: Two identical alleles for a trait.

  • Heterozygous: Two different alleles for a trait (not true-breeding).

  • Phenotype: Physical appearance; Genotype: Genetic makeup.

Testcross

  • Determines unknown genotype of an individual exhibiting dominant phenotype.

  • Breeding with a homozygous recessive reveals genotype.

Law of Independent Assortment

  • Monohybrid Cross: F1 offspring heterozygous for one trait.

  • Dihybrid Cross: Compares inheritance of two traits simultaneously.

  • Mendel's experiments showed inheritance is independent for different traits.

Extended Mendelian Genetics

  • Inheritance may deviate from standard Mendelian patterns:

    • Incomplete dominance, codominance, multiple alleles.

    • Pleiotropy: One gene affects multiple traits.

    • Epistasis: One gene influences another's expression.

    • Polygenic Traits: Traits influenced by multiple genes.

Nature vs. Nurture

  • Phenotype is influenced by both genetics and environmental factors.

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

Human Genetics and Pedigree Analysis

  • Humans are complex subjects for genetic research.

  • Pedigrees trace inheritance patterns across generations.

  • Recessively Inherited Disorders: Present only in homozygous individuals (e.g. Cystic Fibrosis).

  • Dominantly Inherited Disorders: Caused by dominant alleles (e.g. Achondroplasia, Huntington's disease).

Multifactorial Disorders

  • Many disorders with genetic and environmental components (e.g. heart disease, diabetes).

  • Genetic counseling can assist families in understanding genetic risks.