Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

4.7.6Additional lessons

Continuous Variation: Skills — IB Biology SL Revision Notes & Topical Questions

5 min read 6 parts Lesson 128 of 142

📚 IB Biology SL Hub / Topical Revision
🩺
100% Curriculum Specification Match Lesson Doctor Verified
The only platform with 100% curriculum specification match and verified interactive question banks.
0%read

Part 1 of 6Two Kinds of Variation

4.7.6 Continuous Variation: Skills
Discrete variation
  • Individuals fall into clear categories with no intermediates, such as ABO blood groups.
Continuous variation
  • Characteristics show a range of values between two extremes, such as height or body mass.
Graphs
  • Discrete data are shown on bar charts, whereas continuous data give a bell-shaped normal distribution.
Genes versus environment
  • Discrete traits are usually controlled by one gene,
    • whereas continuous traits are controlled by many genes and affected by the environment.
FeatureDiscreteContinuous
CategoriesDistinctRange of values
GenesOne or fewMany (polygenic)
EnvironmentLittle influenceStrong influence
ExampleBlood groupHeight

Part 2 of 6Causes of Continuous Variation

Causes of Continuous Variation
Polygenic
  • Many genes each add a small effect to the phenotype; their effects add up.
Environment
  • Diet, exercise and sunlight change the final phenotype, so phenotype is genotype plus environment.
Skin colour
  • Several genes control melanin production, and exposure to UV light can darken the skin further.
Normal distribution
  • Most individuals have values near the mean, and extremes are rare.

Part 3 of 6Box-and-Whisker Plots

The box holds the middle 50% of the data, and each whisker covers about 25%.

1
Order the data and find the five values: lowest, first quartile, median, third quartile and highest.
2
Draw a scale and mark each value with a short vertical line.
3
Join the first and third quartile lines to form a box, with the median line inside.
4
Draw whiskers from the box to the lowest and highest values that are not outliers.
5
Plot outliers separately, as points beyond 1.5 × IQR from the box edges.

Part 4 of 6Interpreting Plots

Median
  • It is not always in the middle of the box; its position shows if data are skewed.
Interquartile range
  • The IQR is the third quartile minus the first quartile, and measures spread of the central half of the data.
Range
  • The overall range equals the highest minus the lowest value.
Comparison
  • Two plots on the same scale allow groups to be compared by their medians and spread.

Part 5 of 6Worked Example

Question

  • A box plot of plant heights shows lowest 20 cm, Q1 30 cm, median 36 cm, Q3 42 cm and highest 70 cm.
  • Calculate the IQR and decide if 70 cm is an outlier.

  1. IQR = Q3 − Q1 = 42 − 30 = 12 cm.
  2. 1.5 × IQR = 18 cm.
  3. Upper limit = Q3 + 18 = 60 cm; lower limit = Q1 − 18 = 12 cm.
  4. 70 cm is above the upper limit of 60 cm, while 20 cm lies above the lower limit of 12 cm.

Answer

  • IQR = 12 cm;
    • 70 cm is an outlier and 20 cm is not.

Part 6 of 6Exam Checklist & Tips

4.7.6 – Key Points for the Exam

  • Continuous variation Variation showing a range of values between extremes.
  • Polygenic Controlled by the combined effect of many genes.
  • Normal distribution Symmetrical bell-shaped distribution around the mean.
  • Interquartile range Difference between the third and first quartiles.
  • Outlier Value more than 1.5 × IQR beyond the quartiles.

Common Mistakes

  • Saying continuous traits are determined only by genes; the environment also contributes.
  • Treating the median line as the mean.
  • Drawing whiskers to outliers instead of plotting them as separate points.
  • Using a bar chart for continuous data without grouping.

Last updated:

\n