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VariationIB MYP Biology: Revision notes

Section 1

What is variation?

Variation means the differences between individuals of the same species. No two people are identical, except for identical twins, and even they differ slightly. Variation is found in plants, animals and microorganisms.

Key termsvariation

Section 2

Continuous variation

In continuous variation a feature can take any value within a range, with many values in between. Examples: height, mass, foot length, leaf length. Results are measured, and when plotted, most individuals are near the middle and few at the extremes.

Continuous features are usually controlled by many genes and the environment together.

Key termscontinuous variation

Section 3

Discontinuous variation

In discontinuous variation a feature falls into separate categories with no values in between. Examples: blood group (A, B, AB or O), tongue rolling (can or cannot), and eye colour in simple cases. These features are usually controlled by one gene and are not affected much by the environment.

Key termsdiscontinuous variation
Exam tip

Ask: can I get a value in between? If yes, it is continuous. If no, it is discontinuous.

Section 4

Genetic and environmental causes

Variation can be caused by genes (inherited alleles), by the environment, or by both.

  • Genetic only: blood group, natural eye colour.
  • Environment only: a scar, or a language spoken.
  • Both: height (genes set the potential, nutrition decides how much is reached), mass, skin tone (sunlight), and plant height (light and water).

Identical twins show the effect of environment, because their genes are the same.

Key termsgenetic variationenvironmental variation
Common mistake

Most features, such as height, are influenced by both genes and the environment. Do not say it is only one or the other.

Section 5

Sources of genetic variation

Genetic variation comes from three sources:

  1. Mutation: a random change in DNA that can create a new allele.
  2. Meiosis: the chromosomes are shuffled, so each gamete carries a different mixture of alleles.
  3. Random fertilisation: any sperm can fuse with any egg, so each zygote has a different combination.

Variation helps a species to survive changes such as new diseases or a changing climate, because some individuals may be better suited to the new conditions.

Key termsmutationrandom fertilisation

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Exam questions on Variation

  1. A class in Auckland records several features of every student: height, blood group, eye colour, mass and whether the student can roll their tongue. The teacher uses the results to introduce the idea of variation within a species.
    Describe the difference between continuous and discontinuous variation.2 marks
  2. Researchers in Stockholm study identical twins who were separated at birth and raised in different countries. Identical twins have exactly the same genes because they come from the same fertilised egg.
    Explain why identical twins raised in different countries can have different heights as adults.2 marks
  3. A breeder in Vienna notices that the puppies in one litter, born to the same two parents, differ from each other and from their parents in coat colour and size. The smallest puppy had less access to food than its brothers and sisters.
    Outline three sources of genetic variation in the puppies.3 marks
See the full worksheet

Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).