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Genetics and Inheritance

Year 1 / ASYear 2 / A-Level All Boards (AQA, Edexcel, OCR, WJEC, CCEA) AQA

A-Level Biology revision: Genetics and Inheritance. Learning objectives, key points, worked examples and practice questions across AQA, Edexcel, OCR, WJEC and CCEA.

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📌 Key Points

Key Fact: DNA: antiparallel strands, complementary base pairing (A-T, G-C), sugar-phosphate backbone; Replication: helicase, DNA polymerase, Okazaki fragments, ligase
Key Fact: Transcription: RNA pol, promoter, mRNA; Translation: tRNA, ribosome, codon-anticodon, start/stop codons; Eukaryotes: splicing (introns/exons), 5' cap, poly-A tail
Key Fact: Mendel: segregation, independent assortment; Punnett squares; Test cross: unknown x homozygous recessive
Key Fact: Linkage: genes on same chromosome; recombination frequency ∝ distance; Epistasis: one gene masks another (e.g. 9:3:4, 12:3:1, 9:7 ratios)
Key Fact: Chi-squared: χ^2 = Σ(O-E)^2/E; df = n-1; p>0.05 = accept null (observed = expected)
Key Fact: Mutations: substitution (silent/missense/nonsense), insertion/deletion (frameshift); Chromosomal: deletion, duplication, inversion, translocation, non-disjunction

🎯 Learning Objectives

  • Describe DNA structure, replication (semi-conservative), and the genetic code
  • Explain transcription, translation, and post-transcriptional modification
  • Apply Mendelian genetics: monohybrid, dihybrid, test crosses
  • Understand gene linkage, crossing over, and epistasis
  • Apply chi-squared test to genetic ratios
  • Describe mutations: gene (point, frameshift) and chromosome (structural, numerical)

💡 Worked Example

Exam-Style Question

Question: In sweet peas, flower colour is controlled by two genes (C and P). Both dominant alleles needed for purple flowers. Cross CcPp x CcPp. Find phenotypic ratio

Model Answer:

Both genes needed for purple. C_P_ = purple (9/16); ccP_ = white (3/16); C_pp = white (3/16); ccpp = white (1/16). Ratio = 9 purple : 7 white (complementary epistasis)

❓ Practice Questions

Questions:

  • Describe semi-conservative replication
  • A cross gives 9:3:3:1. What does this indicate?
  • Chi-squared test: observed 52:18, expected 3:1 ratio. Calculate χ^2
  • Explain how a frameshift mutation affects protein

🎬 Video Resources

📄 Past Papers & Exam Resources

🔗 Further Reading & Resources