A-Level Biology October/November 2024 Q2(b): The process of selective breeding changes allele frequencies in a population. Plant bre…

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  To produce a variety of maize that is both vigorous and of uniform height, breeders use a two-stage process.

  First, different groups of maize plants are inbred for several generations by repeated self-pollination. This process of inbreeding increases homozygosity within each line, producing several different pure-breeding lines that are uniform for certain traits, such as a specific height.

  Second, two different homozygous inbred lines are selected and cross-pollinated in a process called hybridisation (a form of outbreeding). The resulting F1 generation are all heterozygous. This increased heterozygosity results in hybrid vigour, making the plants vigorous, while their uniform heterozygous genotype ensures they are also of a uniform height.

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  How the marks are awarded

  • B1 — The answer describes the process of inbreeding by stating 'inbred for several generations by repeated self-pollination'.
  • B1 — The answer correctly states the genetic consequence of this process: 'inbreeding increases homozygosity'.
  • B1 — The answer describes the result of crossing the inbred lines, stating 'The resulting F1 generation are all heterozygous', which explains both the vigour and uniformity of the final crop.

  Common mistakes

  • Describing only simple selective breeding (e.g., 'breed the most vigorous and uniform plants together') without mentioning the specific two-stage process of inbreeding followed by hybridisation.
  • Confusing the roles of homozygosity and heterozygosity; for example, incorrectly stating that homozygosity leads to vigour.
  • Failing to explain how both desired traits (vigour and uniformity) are achieved; often an answer will explain one but not the other.
  • Using vague language like 'crossing good plants' instead of precise genetic terms such as inbreeding, outbreeding, homozygosity, and heterozygosity.

  Examiner tip: For process-based questions, break down the desired outcome into its components and explain the specific biological steps required to achieve each one in a logical sequence.

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