Genetics Notes

Chapter Overview

  • Exploration of Genetics, focusing on key figures, principles, and genetic disorders.

Gregor Mendel

  • Historical Context: 1860’s monk instrumental in founding genetics.
  • Experimentation with Peas:
    • Selected peas due to variety in shapes and colors.
    • Peas are cheap and abundant.
    • Short generation times produce many offspring.

Flower Structure of Peas

  • Anatomy of Flower:
    • Stamen: Male reproductive part.
    • Anther: Produces pollen.
    • Filament: Supports the anther.
    • Carpel: Female reproductive structure,
    • Contains stigma, style, ovary (with ovules).
  • Pollination Process:
    • Cross-pollination by brushing pollen onto stigma from another plant.

Mendel's Key Contributions

  • Three Principal Laws
    • Law of Dominance: Dominant allele masks recessive allele.
    • Law of Segregation: Alleles segregate during gamete formation.
    • Law of Independent Assortment: Alleles for different traits assort independently.

Law of Dominance

  • Concept: Dominant traits expressed, recessive traits not.
  • Alleles:
    • Dominant: Represented as capital letters (e.g., T for tall).
    • Recessive: Represented as lowercase letters (e.g., t for dwarf).
  • Genetic Makeup:
    • Homozygous: Two identical alleles (e.g., TT or tt).
    • Heterozygous: Two different alleles (e.g., Tt).

Law of Segregation

  • Mechanism: Two alleles for each trait are found at the same location on homologous chromosomes.
  • Example: Bb genotype splits into gametes with B and b alleles during meiosis.

Genetic Problems

  1. Yellow vs. Green Seeds:

    • Yellow seed (E\text{Y}) dominant over green seed (E\text{g}).
    • Crossing two heterozygous plants: 3 yellow : 1 green.

  2. Freckles:

    • Freckles dominant over no freckles.
    • If parents both have freckles (Ff) but produce child without freckles (ff), each child has a 25% chance of inheriting no freckles.

Inheritance Patterns

  • Autosomal Dominant Disorders: Disorder present if AA or Aa.
  • Autosomal Recessive Disorders: Disorder present only if aa.
    • Example Disorders: Albinism, Cystic fibrosis, Tay-Sachs.

Inbreeding and Genetic Disorders

  • Inbreeding: Increases chance of homozygous recessive traits.
  • Specific impact on non-human examples: Dog breeds showing health issues due to common heritage.

Types of Genetic Disorders

  • Recessive:
    • e.g. Cystic fibrosis: Abnormal chloride ion transport leads to thick mucus.
  • Dominant:
    • e.g. Huntington's disease: Degenerative, appears in later life.

Beyond Mendelian Genetics

Incomplete Dominance

  • Description: Traits appear blended in heterozygous individuals.
    • Example: Snapdragon flowers - Red (RR) and white (rr) produce pink (Rr).

Multiple Allelic Traits

  • Concept: Traits controlled by multiple alleles (e.g., blood types).
    • Blood type example with genotypes:
    • O: OO
    • A: AO or AA
    • B: BO or BB
    • AB: AB

Sex-linked Traits

  • Definition: Genes located on X chromosome.
  • Examples: Color blindness, hemophilia.
  • Inheritance:
    • Males inherit traits more due to single X chromosome; females need two recessive alleles to express them.