Percentages (AQA A-Level Biology): Revision Notes

Percentages

Percentages are a way of expressing fractions as decimals, making it easier to compare different proportions and communicate results clearly in biological studies.

Understanding percentages

A percentage represents a fraction where the denominator is always 100. To convert any fraction to a percentage, divide the numerator by the denominator to get a decimal, then multiply by 100.

The basic formula is:

$\text{Percentage} = \left(\frac{\text{part}}{\text{total}}\right) \times 100$

Percentages in biological surveys

Percentages allow scientists to compare data from different sample sizes effectively. When studying populations or conducting surveys, expressing results as percentages makes comparisons meaningful regardless of the total sample size.

Consider a survey examining flower types in different grassland areas. In one location, researchers found 323 pin-eyed primrose flowers out of 790 total plants surveyed.

Calculation:

• Fraction = $\frac{323}{790} = 0.41$ (as a decimal)
• Percentage = $0.41 \times 100 = 41$

In a second area with 988 plants total, 667 were pin-eyed flowers:

• Percentage = $\left(\frac{667}{988}\right) \times 100 = 67.5$

This shows that the second area had a much higher proportion of pin-eyed flowers, which would be difficult to see by comparing the raw numbers alone.

Percentages in genetics

In genetics, percentages express the probability of different offspring characteristics appearing. This is essential for predicting inheritance patterns and understanding genetic crosses.

For example, when two heterozygous parents carry the cystic fibrosis allele, each child has a 1 in 4 chance of being affected by the condition.

Calculation:

• Probability as fraction = $\frac{1}{4}$
• Probability as percentage = $\left(\frac{1}{4}\right) \times 100 = 25$

This means there is a 25% chance that any child from these parents will have cystic fibrosis.

Percentage change calculations

Percentage change is particularly useful in biological experiments where you need to measure how much a sample has gained or lost mass, such as in osmosis investigations.

The formula for percentage change is:

$\text{Percentage change} = \left(\frac{\text{mass change}}{\text{starting mass}}\right) \times 100$