Continuous and Discontinuous Variation in Humans

Aim

To investigate and classify examples of continuous variation, such as height, and discontinuous variation, such as the ability to roll the tongue, among a group of students, and to represent the data graphically.

Variables

• Manipulated variable: the characteristic or trait being measured or observed for each student in the sample group, such as height for continuous variation and tongue-rolling ability for discontinuous variation.
• Responding variable: the measured value, such as height in centimetres, for a continuous trait, or the category and frequency count, such as the number of students who can or cannot roll their tongue, for a discontinuous trait.
• Controlled variables: the same sample group of students used for both traits, the same measuring instrument or method for each trait, and consistent units throughout.

Materials and apparatus

• A measuring tape
• A record sheet or questionnaire for the discontinuous trait, such as tongue-rolling ability or earlobe attachment
• Graph paper
• A calculator

Procedure

1. Choose a continuous trait, such as height, and measure it for every student in the sample group using a measuring tape.
2. Record all the measurements in a table.
3. Group the height data into class intervals, such as 150 to 154 cm and 155 to 159 cm, and tabulate the number of students in each interval.
4. Plot a histogram or frequency curve of the height data, with the class intervals on the horizontal axis and the frequency on the vertical axis.
5. Choose a discontinuous trait, such as the ability to roll the tongue, and record which category each student falls into.
6. Tabulate the frequency of students in each category of the discontinuous trait.
7. Plot a bar chart of the discontinuous trait, with separate, evenly spaced bars for each category.

Expected results

The histogram of height is expected to show a continuous, gradually varying distribution with no clear gaps between the bars, often forming a shape close to a bell curve with most students clustered around an average height and fewer students at the very short and very tall ends.

The bar chart of tongue-rolling ability is expected to show two distinct, separate bars, one for students who can roll their tongue and one for those who cannot, with a clear gap between the bars and no intermediate category.

Conclusion

Continuous variation, such as height, shows a gradual range of values that usually produces a bell-shaped distribution, and it is typically influenced by the combined effect of multiple genes together with environmental factors such as diet. Discontinuous variation, such as tongue-rolling ability, shows distinct, separate categories with no intermediate forms, and it is typically controlled by a single gene with little or no environmental influence.

Paper 3-style questions

Context. A class measured the height of every student and recorded whether each student could roll the tongue. The heights ranged smoothly from 150 cm to 174 cm, while the tongue-rolling results gave two groups: 21 students who could roll their tongue and 9 who could not.

Question 1 (hypothesis). State a suitable hypothesis linking the type of gene control to the pattern of variation.

Model answer. A characteristic controlled by many genes together with the environment (such as height) shows continuous variation, while a characteristic controlled by a single gene (such as tongue-rolling) shows discontinuous variation.

Question 2 (variables). Identify the trait being studied and two conditions that should be kept the same to make the comparison fair.

Model answer. The trait studied is the characteristic being measured or observed, such as height or tongue-rolling ability. To keep the comparison fair, use the same sample group of students for both traits and the same instrument and method for each trait, for example measuring every student's height without shoes using the same tape.

Question 3 (tabulation and graph). Describe how each set of data should be tabulated and displayed.

Model answer. Group the height data into equal class intervals and record the frequency in each interval, then draw a histogram with bars touching to show a continuous scale. Record the tongue-rolling data as the frequency of each category, then draw a bar chart with separate bars and gaps between them to show distinct categories.

Question 4 (inference). The height data form a smooth bell-shaped curve and the tongue-rolling data form two separate bars. State the type of variation shown by each and what this suggests about how each trait is inherited.

Model answer. Height shows continuous variation, which suggests it is controlled by many genes and influenced by the environment. Tongue-rolling shows discontinuous variation, which suggests it is controlled by a single gene with little environmental influence.

Safety

• This investigation uses no chemicals, heat or electricity, so the main precautions are careful conduct and respect for privacy.
• Ask each student's permission before recording personal information such as height, and keep the record sheet confidential.
• Take height readings on a level floor and handle the measuring tape carefully so no one is tripped or scratched.
• Move sensibly around the room while collecting data so that apparatus and other students are not knocked over.

Common mistakes

Source:SRC-DSKP-EN

Frequently asked questions