Inheritance & Selection — A-Level Biology Revision

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What is Inheritance & Selection?

Inheritance is the process by which genetic information is passed on from parent to offspring. A-Level Biology explores Mendelian inheritance, including monohybrid and dihybrid crosses, codominance, and sex linkage. Natural selection, the driving force of evolution, acts on the variation within a population, with individuals possessing advantageous alleles being more likely to survive, reproduce, and pass on these alleles.

Board notes: All A-Level Biology boards (AQA, Edexcel, OCR) cover Mendelian genetics, natural selection, and speciation. The use of the chi-squared test to analyse genetic cross data is a key statistical skill required by all boards. Specific examples of selection and speciation may vary.

Worked example

Common mistakes

• 1Confusing genotype and phenotype. Genotype is the genetic makeup of an organism (the combination of alleles), while phenotype is the observable characteristics resulting from the genotype and environmental factors.
• 2Incorrectly setting up Punnett squares for dihybrid crosses. A common error is not correctly determining all possible gamete combinations from the parental genotypes.
• 3Misunderstanding the different types of selection. Directional selection favours one extreme phenotype, stabilising selection favours the mean phenotype, and disruptive selection favours both extreme phenotypes.

Inheritance & Selection exam questions

Exam-style questions for Inheritance & Selection with mark-scheme style solutions and timing practice. Aligned to AQA, Edexcel and OCR specifications.

Frequently asked questions

• What is the difference between codominance and incomplete dominance?

  In codominance, both alleles are fully expressed in the phenotype (e.g., AB blood type). In incomplete dominance, the heterozygous phenotype is an intermediate blend of the two homozygous phenotypes (e.g., a pink flower from red and white parents).

• What is speciation?

  Speciation is the evolutionary process by which new biological species arise. It often occurs when a population becomes reproductively isolated, leading to the accumulation of genetic differences over time through natural selection and genetic drift.