Sometimes tested B3.3

Asexual and Sexual Reproduction

This topic contrasts asexual and sexual reproduction, focusing on the number of parents involved and the resulting genetic variation in offspring. Understanding this is crucial for questions on inheritance, population dynamics, and evolution.

Part of the ESAT Biology syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.

Key points

  • Asexual reproduction requires only one parent and produces genetically identical offspring, often called clones.
  • The cellular process underlying most asexual reproduction is mitosis.
  • Sexual reproduction involves the fusion of gametes from two parents, creating genetically unique offspring.
  • The genetic shuffling during meiosis and the combination of two parents' genes in sexual reproduction are major sources of variation within a species.
  • Increased genetic variation is advantageous for a population's survival, particularly in changing environments, as it is the raw material for natural selection.
  • Asexual reproduction is typically faster and more energy-efficient, allowing for rapid population growth in stable conditions.

Formulae

Final Population = Initial Population × 2n

To calculate the size of a population undergoing asexual reproduction (like binary fission in bacteria) after 'n' rounds of division, assuming no deaths.

Definitions

Asexual Reproduction
A mode of reproduction where offspring arise from a single parent, inheriting the genes of that parent only. It does not involve the fusion of gametes.
Sexual Reproduction
A mode of reproduction involving two parents who contribute genetic information to produce unique offspring. It involves the fusion of specialised sex cells (gametes).
Variation
The differences in genetic and physical traits between individuals within a population.
Clone
An organism or group of organisms that are genetically identical to each other and their parent, produced through asexual reproduction.

Worked example

A single bacterial cell is placed in a liquid culture. It divides by binary fission (a form of asexual reproduction) every 30 minutes. How many bacterial cells will be present after 3 hours, and what can be said about their genetic relationship?

  1. 1

    First, convert the total time into minutes:

    3 hours × 60 minutes/hour = 180 minutes.

  2. 2

    Next, determine the number of division cycles (n) by dividing the total time by the time per division:

    n = 180 minutes / 30 minutes/division = 6 divisions
  3. 3

    The population doubles with each division.

    Starting with 1 cell, the final population is 1 × 2n = 26.

  4. 4

    Calculate 26:

    2*2=4, 4*2=8, 8*2=16, 16*2=32, 32*2=64
  5. 5

    Since the reproduction is asexual, all resulting cells are genetically identical to the original parent cell, assuming no mutations have occurred.

Answer: There will be 64 bacterial cells. They are all genetically identical clones.

Common mistakes

  • ×Forgetting that asexual reproduction leads to exponential (2n), not linear, growth. A common error is to multiply the number of divisions by 2, instead of calculating 2 to the power of the number of divisions (e.g., calculating 6 × 2 = 12 instead of 26 = 64).
  • ×Making arithmetic errors under pressure when calculating powers of 2. Double-check your multiplication.
  • ×Confusing the outcomes: stating that asexual reproduction leads to variation or that sexual reproduction produces clones.

No-calculator tips

  • ✓To calculate powers of 2, like 26, break it down: 26 = (23) × (23) = 8 × 8 = 64. This is often faster and less error-prone than repeated doubling.
  • ✓Memorise the first few powers of 2 (21=2, 22=4, 23=8, 24=16, 25=32, 26=64, 210=1024) as they appear frequently in biology and computing questions.
  • ✓When converting units like hours to minutes, write the calculation down (e.g., 3 x 60) to avoid simple mental slips.

Read this topic in the official UAT-UK ESAT guide →

All Biology topics