Genetics and Inheritance Notes

Types of Cells

  • Somatic cells: Body cells
  • Gametes: Sex cells

Chromosome Sets

  • Diploid cells: Cells with 2 sets of chromosomes (2n), such as somatic cells.
  • Humans have 46 chromosomes: 44 autosomal chromosomes (non-sex) and 2 sex chromosomes.

Definitions

  • Alleles: Alternative forms of a gene, representing variations of a trait.
  • Trait: Any inherited characteristic.
  • Gene: A segment of DNA that codes for a trait.
  • Homologous Chromosomes: Chromosomes that contain genes for the same types of proteins, one inherited from each parent.

Eukaryotic Cell Division

  • Mitosis: Cell division for unicellular organisms or somatic cells in multicellular organisms.
  • Meiosis: Reduces chromosome number by half to form gametes. Consists of Meiosis I and Meiosis II.

Meiosis

  • Produces 4 genetically different haploid cells from 1 diploid cell.
  • Diploid (2n): 2 sets of chromosomes (46 in humans).
  • Haploid (n): 1 set of chromosomes (23 in humans).
  • Meiosis I:
    • Prophase I: Homologous chromosomes pair and form a Tetrad; crossing over occurs.
    • Metaphase I: Homologous pairs line up at the cell center.
    • Anaphase I: Homologous chromosomes separate to opposite sides of the cell.
    • Telophase I: Nuclear membrane reappears; spindle fibers disappear.
    • Results: Two haploid cells (genetically different).
  • Meiosis II:
    • Similar to mitosis but does not pair up homologous chromosomes.
    • Produces four genetically different haploid cells.

Nondisjunction

  • Failure of chromosomes to separate properly during cell division, leading to disorders like Klinefelter's Syndrome or Down's Syndrome.

Mitosis vs Meiosis

  • Mitosis: 1 cycle, produces 2 identical diploid cells, no crossing over, occurs in somatic cells.
  • Meiosis: 2 cycles, produces 4 genetically different haploid cells, crossing over occurs, happens in gametes.

Mendel's Principles

Gregor Mendel

  • Austrian monk, known as the "Father of Genetics" for his work with pea plants.
  • Utilized cross-pollination to study true-breeding plants.

Traits

  • Defined as specific genes that vary.
  • Genotype: Combination of alleles.
  • Phenotype: Observable trait.

Mendel’s Experiments

  • Studied seven pea plant traits: seed shape, color, pod shape, color, flower color, position, and stem length.
  • Crossed tall and short plants to create hybrids.
  • F1 generation was all tall.
  • F2 generation revealed a 3:1 ratio of tall to short.

Mendel's Principles of Inheritance

  1. Principle of Dominance: One allele may mask the effect of another.
  2. Principle of Segregation: Alleles separate into different gametes.
  3. Principle of Independent Assortment: Different traits are inherited independently.

Monohybrid and Dihybrid Crosses

  • Monohybrid: One trait, e.g., fur color.
  • Dihybrid: Two traits, e.g., fur color and texture.

Genetic Disorders Inherited Patterns

  • Autosomal diseases: Passed down through autosomes (e.g., cystic fibrosis, sickle cell anemia, Huntington's disease).
  • Sex-linked diseases: Passed down via sex chromosomes (e.g., hemophilia, color blindness).

Intermediate Inheritance

  • Codominance: Both alleles expressed equally (e.g., roan cattle).
  • Incomplete dominance: Blend phenotypes (e.g., pink flowers from red and white parents).

Blood Types and Antibodies

Blood Grouping

  • Antigens on RBCs determine blood types (A, B, AB, O).
  • Type O: No antigens, forms Anti-A and Anti-B antibodies.
  • Type AB: Both antigens, no antibodies.
  • Rh Factor: Determines positive/negative blood type (dominant vs. recessive inheritance).

Sex-Linked Traits

  • Traits linked to X chromosome (e.g., hemophilia, color blindness). Mendel's work essential for understanding inheritance.

Common Examples of Sex-Linked Traits

  • Color blindness: A recessive trait affecting color perception.
  • Hemophilia: Affects blood clotting.

Conclusion

  • Genetic inheritance is complex, involving multiple patterns and exceptions to basic Mendelian genetics. Understanding these principles is crucial for studies in biology and medicine.