Genetic Variation - continuous & discontinuous

Continuous & discontinuous

Some of the features of the different organisms in a species show continuous variation, and some features show discontinuous variation.

Continuous variation

Human height is an example of continuous variation. Height ranges from that of the shortest person in the world to that of the tallest person. Any height is possible between these values. So it is continuous variation.

For any species a characteristic that changes gradually over a range of values shows continuous variation. Individuals cannot be grouped into distinct & discrete phenotypes. Character is easily influenced by environment and can be measured or graded. 

Examples of such characteristics are:

• height
• weight
• foot length.

If you record the heights of a group of people and draw a graph of your results, it usually looks something like this:

The more people you measure, and the smaller the categories you use, the closer the results will be to the curved line. This shape of graph is typical of a feature with continuous variation. Weight and foot length would give graphs similar in shape to this.

Discontinuous variation

Human blood group is an example of discontinuous variation. There are only 4types of blood group. There are no other possibilities and there are no values in between. So this is discontinuous variation.

A characteristic of any species with only a limited number of possible values shows discontinuous variation. Individuals can be grouped into distinct & discrete phenotypes. Character cannot be measured or graded and is not influenced by environment. Here are some examples:

• gender (male or female)
• blood group (A, B, AB or O)
• eye colour.
• tongue rolling 

Inherited & environmental

Some variation within a species is inherited, and some variation is due to the environment.

Inherited causes of variation

Variation in a characteristic that is a result of genetic inheritance from the parents is called inherited variation.

Children usually look a little like their father, and a little like their mother, but they will not be identical to either of their parents. This is because they get half of their inherited features from each parent.

Each egg cell and each sperm cell contains half of the genetic information needed for an individual. When these join at fertilisation a new cell is formed with all the genetic information needed for an individual.

Here are some examples of inherited variation in humans:

• eye colour
• hair colour
• skin colour
• lobed or lobeless ears.

Gender is inherited variation too, because whether you are male or female is a result of the genes you inherited from your parents.

Environmental causes of variation

Characteristics of animal and plant species can be affected by factors such as climate, diet, accidents, culture and lifestyle. For example, if you eat too much you will become heavier, and if you eat too little you will become lighter. A plant in the shade of a big tree will will grow taller as it tries to reach more light.

Variation caused by the surroundings is called environmental variation. Here are some other examples of features that show environmental variation:

• your language and religion
• flower colour in hydrangeas - these plants produce blue flowers in acidic soil and pink flowers in alkaline soil.

Both types together

Some features vary because of a mixture of inherited causes and environmental causes. For example, identical twins inherit exactly the same features from their parents. But if you take a pair of twins, and twin 'A' is given more to eat than twin 'B', twin 'A' is likely to end up heavier.

Representing the data

Continuous Variation is a quantitative trait that can be measure so it can are represented using a histogram with can group and range the data in order for it to be properly analysed.

Discontinuous Variation is a qualitative trait that cannot be measure so a bar chart is more suited to represent this type of data.