Are Human Blood Groups Continuous or Discontinuous Variation
Types of variation
Key points
- Variation is the differences between individuals of the same species, caused by genetic and environmental factors.
- Surveys into variation give data that are continuous, which means to come in a range, or discontinuous, which means to come in groups.
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Variation
is all the differences that exist in a population of the same species. These differences are caused by:
- Genetic variation - these are differences between individuals that are inherited from parents, such as the colour of your eyes, hair and skin.
- Environmental variation - these are differences between individuals that are not inherited but caused by the environment that the organism lives in, including scars and tattoos.
- Genetic and environmental variation - differences between individuals that are caused by both genetic and environmental factors, such as height and weight.
Data on variation is collected by surveying the population. This data can be described as either continuous variation or discontinuous variation. It is important to remember that these are not the causes of variation, which are described above, but how we analyse the results of the surveys.
Continuous variation
Surveys of give us results that come in a range. Human height is an example of continuous variation. It ranges from that of the shortest person in the world to that of the tallest person. Any height is possible between these values, so this is continuous variation. For example, you can be 150 cm tall, 151 cm tall, or any height in between this - if you had a ruler that could measure small enough values.
So, a characteristic that changes gradually over a range of values shows continuous variation. Examples of such characteristics are:
- height
- arm span
- weight
Results from surveys of continuous variation are presented in line graphs or bar charts with a drawn through them. If you record the heights of a group of people and draw a graph of your results, it usually looks something like this:
Normal distribution
Surveys of continuous variation often give us results in a characteristic shape seen in the green line on the graph above. If there are fewer readings at either ends of the scale and far more in the middle, we see a of . The more people you measure and the smaller the categories you use, the closer the results will be to this shape.
Discontinuous variation
Surveys of give us values that come in groups rather than a range. Human blood groups are an example of discontinuous variation. In the ABO blood group system, only four blood groups are possible - A, B, AB or O. You cannot have a blood group in between these four groups, so this is discontinuous variation.
Here are some examples:
- blood group
- eye colour
Results from surveys of discontinuous variation are presented in charts. These is no line of best fit drawn because the values on the x-axis - blood groups in the graph below - could be placed in any order. If you record the blood groups of a group of people and draw a graph of your results, it usually looks something like this:
More examples
| Continuous variation | Discontinuous variation |
|---|---|
| Height | Blood group |
| Weight | Hand used to write with |
| Arm span | Eye colour |
| Head circumference at birth | Ability to roll tongue |
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Source: https://www.bbc.co.uk/bitesize/topics/zpffr82/articles/z6j66g8
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