Percentage = $\frac{\mathrm{Number}}{\mathrm{Total}}$ x 100%

Number of rollers = 15

Total number of students = 45

Percentage of rollers = $\frac{15}{45}$ x 100% = 33.3%

Group O can donate red blood cells to anybody. It's the universal donor.

There are 2 out of 4 (half) Aa , 1 out of 4 of AA and 1 out of 4 of aa

Variation is the differences between individuals of the same species, caused by genetic and environmental factors.

A characteristic that changes gradually over a range of values shows continuous variation

Examples of continuous variation includes: height, arm span, weight, head circumference at birth

Discontinuous variation 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.

Examples of discontinuous variation includes: blood group, hand used to write with, eye colour, ability to roll tongue

You cannot have a blood group in between these four groups, so this is discontinuous variation.