πŸ’— Mendelian Genetics Notes πŸ’—

Mendelian Genetics Notes

Gregor Mendel

  • Austrian monk in the 1800s.
  • Studied garden peas to observe traits (e.g., color, height, seed type).
  • Discovered true-breeding plants (identical offspring).
  • Recognized as the Father of Genetics due to his foundational experiments in heredity.

Genetics Overview

  • Genetics: Study of heredity and how traits are passed from one generation to the next.
  • Began with true-breeding plants:
    • P (Parental): Starting plants having different traits (e.g., flower color).
    • F1 Generation: Offspring resembling one parent.
  • Mendel's work predates knowledge of genes, DNA, and chromosomes.

Genes and Alleles

  • Meiosis: Process forming gametes (egg and sperm) with half the chromosomes.
  • Each offspring receives Β½ genetic material from each parent.
  • Genes: Sections of DNA representing traits.
    • Alleles: Variations of a gene (e.g., blue vs. brown eyes).

Mendel’s Law of Dominance

  • Dominant genes overshadow recessive ones.
  • The recessive gene only manifests when no dominant allele is present.

Genotype & Phenotype

  • Genotype: Allelic combination for a trait (e.g., BB, Bb, bb).
    • Homozygous: Same alleles (BB or bb).
    • Heterozygous: Different alleles (Bb).
  • Phenotype: Physical manifestation of traits (e.g., hair color, flower color).

Law of Segregation

  • During gamete formation, gene alleles separate so each gamete carries one allele.
  • They do not mix to create new traits.

Monohybrid Cross

  • Analysis involving one trait from parents:
    • Example: Flower color (e.g., Pp x Pp).
  • Punnett Square: Tool to predict genotypic and phenotypic ratios of offspring.
    • Ratios:
    • Genotypic: 1 PP : 2 Pp : 1 pp
    • Phenotypic: 3 purple : 1 white

Testcross

  • Cross between a dominant phenotype with a recessive organism to determine the genotype of the dominant.
  • Can reveal if dominant trait is homozygous or heterozygous.

Autosomal Genetic Diseases

  • Autosomes: All chromosomes except sex chromosomes (X & Y).
  • Autosomal Disorders: Genetic diseases from genes on autosomes.
  • Use Punnett squares to assess probabilities of inheritance.
  • Examples of Autosomal Recessive Disorders:
    • Cystic fibrosis: Thick mucus, severe infections, no cure.
    • Phenylketonuria (PKU): Missing enzyme affects amino acid metabolism, brain damage risk.
    • Tay-Sachs: Fatty accumulation in brain cells, early death, no cure.

Autosomal Dominant Disorders

  • Caused by a dominant gene; less common in offspring.
  • Example: Huntington’s disease - symptoms appear in mid-life, causes degeneration of brain cells.

Types of Genetic Disorders

  1. Missing or extra chromosome diseases.
  2. Autosomal gene diseases.
  3. Sex-linked disorders.

Dihybrid Crosses

  • Examines inheritance of two traits.
  • Law of Independent Assortment: Traits are inherited independently.
  • Dihybrid Cross: Results in 16-box Punnett Square for combinations based on FOIL method:
    • Phenotypic ratio is usually 9:3:3:1.

Practice Problems

  • Examples include using Punnett squares for diseases like PKU to determine chances of inheritance.
  • Important to comprehend Mendel’s laws in the context of genetic crosses and probability calculations.