Principles of Inheritance and Variation

• Introduction

  • Genetics: Study of heredity and variations
  • Heredity: Transmission of characters from parents to offspring
  • Variation: Degree by which progeny differ from parents

• Mendel's Laws of Inheritance

  • Gregor Mendel: Father of Genetics
  • Hybridization Experiments: Garden pea (Pisum sativum)
  • Why Pea Plant?:
    • Contrast characters
    • Easy cultivation
    • True-breeding lines
    • Self-pollination and cross-pollination
  • Mendel's 7 Characters:
    • Stem height (Tall/Dwarf)
    • Flower color (Violet/White)
    • Flower position (Axial/Terminal)
    • Pod shape (Inflated/Constricted)
    • Pod color (Green/Yellow)
    • Seed shape (Round/Wrinkled)
    • Seed color (Yellow/Green)
  • Monohybrid Cross (Inheritance of one gene)
    • Law of Dominance: Alleles express hierarchically ($F_1$ generation)
    • Law of Segregation: Alleles segregate during gamete formation (pure gametes)
    • Test Cross: Determining genotype of $F_1$/$F_2$ dominant phenotype
  • Dihybrid Cross (Inheritance of two genes)
    • Law of Independent Assortment: Genes assort independently of each other
    • Phenotypic Ratio: $9:3:3:1$ in $F_2$ generation

• Post-Mendelian Discoveries (Exceptions)

  • Incomplete Dominance:
    • Intermediate phenotype in $F_1$
    • Example: Flower color in Mirabilis jalapa (Four o'clock plant), Snapdragon
  • Co-dominance:
    • Both alleles express equally
    • Example: ABO blood grouping in humans ($I^A$, $I^B$, $i$)
  • Multiple Alleles:
    • More than two alleles governing a character (e.g., ABO blood groups)
  • Polygenic Inheritance:
    • Character controlled by multiple genes (e.g., Human skin color, height)
  • Pleiotropy:
    • Single gene influencing multiple traits
    • Example: Phenylketonuria (PKU) in humans, starch grain size in peas

• Chromosomal Theory of Inheritance

  • Proposed by Sutton and Boveri (1902)
  • Parallels between chromosome behavior and gene behavior
  • Linkage and Recombination
    • Discovered by T.H. Morgan using Drosophila melanogaster
    • Linkage: Physical association of genes on a chromosome
    • Recombination: Generation of non-parental gene combinations due to crossing over
  • Sex Determination
    • XX-XY type: Humans, Drosophila (Male heterogamety)
    • XX-XO type: Grasshopper (Male heterogamety)
    • ZZ-ZW type: Birds, Butterflies (Female heterogamety)
    • Haplodiploidy system: Honeybees, ants, wasps

• Mutation

  • Alteration of DNA sequence resulting in changed phenotype
  • Point Mutation: Change in a single base pair (e.g., Sickle-cell anaemia)
  • Chromosomal Aberrations: Deletion, duplication, inversion, translocation
  • Mutagens: Physical or chemical agents causing mutations (e.g., UV rays, chemicals)

• Genetic Disorders

  • Pedigree Analysis: Study of family history for inheritance patterns of a trait
  • Mendelian Disorders (Single gene mutation)
    • Haemophilia: Sex-linked recessive disease (blood clotting defect)
    • Sickle-cell Anaemia: Autosomal recessive trait (glutamic acid to valine substitution)
    • Phenylketonuria (PKU): Inborn error of metabolism (mental retardation)
    • Thalassemia: Autosomal recessive blood disease (abnormal globin chain synthesis)
  • Chromosomal Disorders (Presence/absence/abnormal arrangement of chromosomes)
    • Down's Syndrome: Trisomy of chromosome 21 (mental retardation, slanted eyes)
    • Klinefelter's Syndrome: Karyotype $47, XXY$ (sterile males with feminine development)
    • Turner's Syndrome: Karyotype $45, XO$ (sterile females with rudimentary ovaries)