Mendelian Genetics

🧬 Chapter 10-1: Mendel’s Laws of Heredity

Gregor Mendel – The Father of Genetics

  • Mendel was an Austrian monk who studied inheritance in pea plants.

  • He discovered basic laws of heredity by observing how traits are passed from parents to offspring.

Key Terms

  • Trait: A specific characteristic (e.g., flower color).

  • Gene: A segment of DNA that codes for a trait.

  • Allele: Different versions of a gene (e.g., tall or short).

  • Homozygous: Two identical alleles (TT or tt).

  • Heterozygous: Two different alleles (Tt).

  • Genotype: The genetic makeup (e.g., TT, Tt, tt).

  • Phenotype: The physical expression of the trait (e.g., tall or short).

  • Dominant allele: Masks the effect of the recessive allele (represented by capital letter).

  • Recessive allele: Only expressed if two copies are present (lowercase letter).

Mendel’s Experiments

  • P generation: Parental generation (true-breeding).

  • F1 generation: First filial generation – showed only dominant trait.

  • F2 generation: Second generation – 3:1 ratio (dominant:recessive).

Mendel’s Laws

  1. Law of Dominance: One trait may mask another.

  2. Law of Segregation: Alleles separate during gamete formation.

  3. Law of Independent Assortment: Genes for different traits separate independently.

🧬 Chapter 12-1: Mendelian Inheritance in Humans

Mendel’s Laws in Humans

  • Mendel’s principles apply to human traits too (e.g., attached earlobes, tongue rolling, dimples).

Single-Gene Traits

  • Controlled by one gene with two alleles.

  • Examples: Widow’s peak – dominat and Hitchhiker’s thumb – recessive

Pedigrees

  • A pedigree is a diagram that shows how traits are inherited over generations.

    • Circles = females, Squares = males

    • Shaded = has the trait

    • Half-shaded = carrier (for recessive traits)

Carrier

  • A person who carries one recessive allele but does not show the trait.

Recessive Genetic Disorders

  • Examples: Cystic fibrosis, Tay-Sachs disease – must inherit two recessive alleles.

Dominant Disorders

  • Less common than recessive.

  • Only one copy of the dominant allele is needed.

  • Examples: Huntington’s disease, dwarfism (achondroplasia).