Unit 1: Mendelian Genetics & Extra Chromosomal Inheritance

Syllabus Reference: Mendelism; Principles of inheritance; Probability/pedigree; Incomplete/Co-dominance; Multiple alleles, Lethal alleles, Epistasis, Pleiotropy, Penetrance, Expressivity; Polygenic inheritance. Classical vs molecular concepts; Phage T4, rII Locus. Chloroplast/Mitochondrial mutations; Kappa particles; [cite_start]Plastid inheritance [cite: 503-506].

1. Mendelism & Principles of Inheritance

Gregor Mendel, the "Father of Genetics," formulated the basic laws of inheritance using pea plants (Pisum sativum).

• Law of Segregation (Purity of Gametes): During gamete formation, the two alleles for a character segregate (separate) such that each gamete receives only one allele.
• Law of Independent Assortment: In a dihybrid cross, the segregation of one pair of characters is independent of the other pair. (Phenotypic Ratio: 9:3:3:1).

2. Deviations from Mendelism

Not all traits follow simple dominant/recessive rules.

• Incomplete Dominance: The heterozygote shows an intermediate phenotype.
  Example: Mirabilis jalapa (4 O'clock plant). Red (RR) x White (rr) = Pink (Rr).
  Ratio: 1:2:1 (Phenotype matches Genotype).
• Co-dominance: Both alleles express themselves fully in the heterozygote.
  Example: ABO Blood groups (I^A and I^B are co-dominant in AB blood).
  Ratio: 1:2:1.
• Multiple Alleles: More than two alleles exist for a single gene in a population.
  Example: Human ABO blood groups (I^A, I^B, i).
• Lethal Alleles: Alleles that cause death when present in a specific genotype (usually homozygous).
  Example: Coat color in mice (YY is lethal). Ratio becomes 2:1 instead of 3:1.

3. Gene Interactions (Epistasis)

Epistasis is the phenomenon where one gene masks or modifies the expression of another non-allelic gene.

4. Key Genetic Terms

• Pleiotropy: A single gene influences multiple unrelated phenotypic traits. (e.g., Sickle cell anemia affects blood, heart, and liver).
• Penetrance: The percentage of individuals with a specific genotype that actually express the expected phenotype. (Complete vs Incomplete).
• Expressivity: The degree/intensity to which a phenotype is expressed in an individual.
• Polygenic Inheritance: Traits controlled by multiple genes (quantitative traits).
  Example: Human skin color or Kernel color in Wheat. Shows a bell-curve distribution.

5. Classical vs Molecular Concepts of Gene

• Classical Concept: Gene is the smallest unit of recombination, mutation, and function (Bead-on-a-string model).
• Molecular Concept (Benzer):
  • Cistron: Unit of function (codes for one polypeptide).
  • Recon: Unit of recombination (smallest distance between exchangeable bases).
  • Muton: Unit of mutation (smallest change, e.g., single base pair).
• rII Locus in T4 Phage: Seymour Benzer used the rII locus of T4 bacteriophage to prove that a gene is divisible (fine structure of gene).

6. Extra Chromosomal Inheritance

Inheritance of traits via DNA located outside the nucleus (in Chloroplasts or Mitochondria). Also called Cytoplasmic Inheritance or Maternal Inheritance (since cytoplasm comes mostly from the egg).

Examples:

1. Plastid Inheritance: In Mirabilis jalapa, leaf color depends solely on the branch from which the female flower (egg) came. Pollen source does not matter.
2. Mitochondrial Mutations (Yeast): 'Petite' mutants in Yeast are smaller due to defective mitochondrial DNA affecting respiration.
3. Kappa Particles (Paramecium): Paramecium aurelia strains ("Killers") contain cytoplasmic "Kappa particles" (symbiotic bacteria) that release paramecin (toxin). This trait is inherited through the cytoplasm.