Unit 2: Linkage, Mutation & Population Genetics

Syllabus Reference: Linkage, Crossing over; Recombination frequency, Interference/coincidence; Gene mapping; Sex Linkage. Deletion, Duplication, Inversion, Translocation, Position effect. Euploidy/Aneuploidy. Mutagens; Transposons. [cite_start]Hardy-Weinberg Law [cite: 507-510].

1. Linkage and Crossing Over

• Linkage: Tendency of genes located on the same chromosome to stay together during inheritance.
  Complete Linkage: Genes never separate (rare).
  Incomplete Linkage: New combinations form due to crossing over.
• Crossing Over: Exchange of genetic material between non-sister chromatids of homologous chromosomes during Pachytene of Meiosis I.
• Recombination Frequency (RF): Used to map genes.
  RF = (Number of Recombinants / Total Offspring) × 100.
  1% Recombination = 1 Map Unit (cM or centimorgan).
• Interference & Coincidence:
  Interference: One crossover inhibits the occurrence of another nearby.
  Coefficient of Coincidence = Observed DCO / Expected DCO.

2. Chromosomal Aberrations (Structural)

Changes in the structure of chromosomes.

1. Deletion (Deficiency): Loss of a chromosome segment. (e.g., Cri-du-chat syndrome in humans).
2. Duplication: A segment is repeated. (e.g., Bar eye in Drosophila).
3. Inversion: A segment rotates 180°.
   Paracentric: Centromere not included.
   Pericentric: Centromere included.
4. Translocation: Exchange of segments between non-homologous chromosomes (Reciprocal Translocation).
5. Position Effect: Change in phenotype due to change in the position of a gene (often caused by inversion/translocation).

3. Chromosomal Aberrations (Numerical)

• Euploidy: Change in whole sets of chromosomes.
  Monoploidy (x): One set.
  Polyploidy: >2 sets (Triploid 3n, Tetraploid 4n). Common in plants (Wheat is hexaploid).
• Aneuploidy: Loss or gain of individual chromosomes (2n ± x).
  • Nullisomy: 2n - 2 (Loss of a pair).
  • Monosomy: 2n - 1 (Loss of one). Turner's Syndrome (XO).
  • Trisomy: 2n + 1 (Addition of one). Down Syndrome (Trisomy 21).

4. Gene Mutations

Sudden, heritable change in DNA sequence.

• Point Mutations: Change in a single base pair.
  Transition: Purine ↔ Purine (A ↔ G).
  Transversion: Purine ↔ Pyrimidine (A ↔ T).
• Mutagens: Agents causing mutation.
  • Physical: X-rays, UV rays (form Thymine dimers).
  • Chemical: Base analogs (5-Bromouracil), Alkylating agents (EMS), Intercalating agents (Acridine orange).
• Transposons: "Jumping Genes" capable of moving positions within the genome (Discovered by Barbara McClintock in Maize).

5. Population Genetics

Hardy-Weinberg Law: In a large, random-mating population, allele and genotype frequencies remain constant generation after generation in the absence of evolutionary influences.

Where:
p = Frequency of dominant allele (A)
q = Frequency of recessive allele (a)
p² = Frequency of homozygous dominant (AA)
q² = Frequency of homozygous recessive (aa)
2pq = Frequency of heterozygous (Aa)