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Kerala Plus Two Biology (DHSE) study materials
Kerala Plus Two Biology (DHSE) study resources

Kerala Plus Two Biology (DHSE): Genetics and Evolution: Inheritance, Molecular Basis and Evolutionary Mechanisms

Kerala Plus Two Biology (DHSE) · Work through your exam, one topic at a time.

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Genetics and Evolution: Inheritance, Molecular Basis and Evolutionary Mechanisms

This domain tests Mendelian and non-Mendelian inheritance, the molecular machinery of replication, transcription and translation, and the population-level reasoning behind evolution. Kerala papers frequently combine a ratio calculation with a conceptual explanation.

Inheritance rules

  • The law of segregation is universal; independent assortment applies only to genes on different chromosomes.
  • Incomplete dominance gives a 1:2:1 F2 phenotypic ratio (snapdragon flower colour); codominance expresses both alleles, as in the AB blood group.
  • ABO blood groups involve multiple alleles (IA, IB, i) at one locus; IAi x IBi gives A, B, AB and O in 1:1:1:1.
  • Linked genes (Morgan, Drosophila) yield fewer recombinants than independent assortment predicts.

Molecular basis

  • DNA is antiparallel; A pairs with T by two hydrogen bonds, G with C by three.
  • Replication is semi-conservative (Meselson and Stahl, 15N), and DNA polymerase adds nucleotides 5' to 3'.
  • In eukaryotes RNA polymerase I makes rRNA, II makes mRNA, III makes tRNA.
  • The lac operon is inducible: lactose is converted to allolactose, which inactivates the repressor, allowing transcription of lacZ, lacY and lacA.

Evolution

  • Hardy-Weinberg equilibrium: p squared + 2pq + q squared = 1, maintained only with no mutation, no migration, no drift, no selection and random mating.
  • Natural selection may be stabilising, directional or disruptive; adaptive radiation and reproductive isolation drive speciation.

Worked example

Phenylketonuria appears in 1 in 10,000 people. Then q squared = 0.0001, so q = 0.01 and p = 0.99. Carrier frequency = 2pq = 2 x 0.99 x 0.01 = 0.0198, that is about 2 per cent of the population.

Traps that cost marks

  • Do not confuse a test cross with a back cross.
  • The 9:3:3:1 dihybrid ratio demands independent assortment; linked genes break it.
  • Transcription uses RNA polymerase; writing DNA polymerase loses the mark.
  • The lac operon is inducible and catabolic; the trp operon is repressible.