Practical skill 16: Continuous and discontinuous variation

Aim

To collect data to show that height in a sample of pupils shows continuous variation and that seed shape in peas shows discontinuous variation.

Background

Variation is the differences between individuals of the same species. There are two types. In continuous variation there is a range of phenotypes between two extremes, for example body length and body mass. In discontinuous variation there are a limited number of phenotypes with no intermediates, for example ABO blood groups.

Continuous variation is caused by both genes and the environment. For example, a person with genes for tallness will be shorter if their diet is poor. Discontinuous variation is usually caused by genes only. Seed shape in peas (round or wrinkled) is controlled by a single gene, so every seed falls into one of two clear groups. Ear lobe shape and tongue rolling are often used in class, but they are not clear examples: many ear lobes are in between, and some people can learn to roll their tongue.

To describe variation in a population you must use a sample that is representative and avoids bias. The sample should be large (at least 30 individuals), chosen at random from the same group, for example pupils of the same age. A random sample is one where every individual has an equal chance of being chosen, for example by using random numbers to pick names from a register, or by taking pea seeds from a well-mixed bag without looking.

Hypothesis

Height will show continuous variation, with most people near the middle of a range, and pea seed shape will show discontinuous variation, with only two categories (round and wrinkled) and no intermediates.

Equipment

• Class register (or list of names) and a calculator or random number table to choose a random sample of 40 pupils
• Metre rule fixed to a wall (graduated in mm)
• Set square or book to mark the top of the head against the metre rule
• A bag of about 200 dried pea seeds from a cross between round-seeded and wrinkled-seeded plants (from a biological supplier)
• Two Petri dishes or trays and a hand lens
• Results table (and scrap paper for a tally)
• Graph paper
• Sharp pencil and ruler

Risk assessment

Method

1. Choose a sample of 40 pupils from the same year group by random sampling. Give each pupil on the register a number, then use a random number table or calculator to choose 40 numbers.
2. Fix a metre rule vertically to a wall with the zero at floor level.
3. Ask the first pupil to stand with their back and heels against the wall, without shoes and with their feet flat on the floor.
4. Place a set square (or book) flat on the top of their head, touching the rule, and read the height at eye level to the nearest cm.
5. Record the height in a results table, using a number for each pupil rather than a name.
6. Repeat steps 3 to 5 for all 40 pupils.
7. Shake the bag of pea seeds to mix them. Without looking, take out seeds until you have a sample of 100.
8. Sort the seeds into two dishes: round (smooth) and wrinkled. Use a hand lens if you are not sure.
9. Group the heights into classes of 5 cm (for example 150–154 cm) and count the number of pupils in each class.
10. Count the number of round seeds and the number of wrinkled seeds.
11. Draw a histogram of the heights (bars touching) and a bar chart of the seed shapes (bars not touching).
12. Describe the shape of each graph and state which feature shows continuous variation and which shows discontinuous variation.

Results

Fill this table in as you go. Print the PDF for a copy to write on.

Drawing the graph

Two graphs. (1) Histogram of height: height / cm on the x-axis (classes 150–154 to 180–184, bars touching because height is continuous data) and number of pupils on the y-axis from 0 to 12 (for example 1 cm for 1 pupil). (2) Bar chart of pea seed shape: pea seed shape on the x-axis (round, wrinkled, bars of equal width with gaps between them because the data are in categories) and number of seeds on the y-axis from 0 to 80 (for example 1 cm for 10 seeds).