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3.2.13Unit 3: Practical Skills in Biology I

Polygenic Inheritance and Continuous Variation — IAL Edexcel Linear Revision Notes & Topical Questions

5 min read 5 parts Lesson 15 of 27 Unit quiz

📚 IAL Edexcel Linear Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Part 1 of 5Monogenic and Polygenic Traits

Many human traits are controlled by several genes and show a smooth range of phenotypes.

Monogenic
  • Monogenic A characteristic controlled by a single gene, such as ABO blood group, usually shows discontinuous variation.
Polygenic
  • Polygenic A characteristic controlled by several genes, such as height, mass or skin colour, usually shows continuous variation.
Discontinuous variation
  • Discontinuous variation Individuals fall into distinct categories with no intermediate values, which suits bar charts.
Continuous variation
  • Continuous variation A full range of values exists between two extremes, and the data usually give a bell-shaped distribution.
Polygenes
  • Polygenes Many genes each with a small effect on the same phenotype together determine the characteristic.
Environment
  • Environment Characteristics such as height are also affected by diet and other environmental factors.

Part 2 of 5The Additive Effect of Genes

1
Consider plant height controlled by two unlinked genes, H/h and T/t, with an additive effect.
2
Each recessive allele adds x cm to the height and each dominant allele adds 2x cm.
3
The genotype hhtt gives 4x, HhTt gives 6x, and HHTT gives 8x.
4
Different genotypes can give the same height, for example HHTt and HhTT both give 7x.
5
Questions may give different values for each gene, so check the information given rather than assuming equal effects.

Part 3 of 5Causes of Continuous Variation

Causes of Continuous Variation
Many genes
  • Many genes The more genes that contribute, the more phenotype classes there are and the more continuous the variation looks.
Multiple alleles
  • Multiple alleles Different alleles at one locus may each have a small effect on the phenotype.
Environment
  • Environment Environmental influences blur the boundaries between classes and make the variation more continuous.
Identifying it
  • Identifying it Data that show a range of values in a table or graph, rather than categories, suggest continuous variation.
FeatureDiscontinuousContinuous
GenesOne or fewMany (polygenic)
PhenotypesDistinct categoriesRange of values
Environment effectLittleLarge
ExampleBlood groupHeight

Part 4 of 5Worked Example

Question

  • Plant height is controlled by two unlinked genes with additive effects, where each recessive allele adds x cm and each dominant allele adds 2x cm, with x = 5 cm.
  • Two HhTt plants are crossed.
  • Give the heights and their probabilities.

  1. Height = 4x + (number of dominant alleles) × x, since each dominant allele adds one extra x compared with a recessive allele.
  2. In HhTt × HhTt, the number of dominant alleles k (0 to 4) occurs with probability 1, 4, 6, 4, 1 out of 16.
  3. With x = 5 cm the heights are 20, 25, 30, 35 and 40 cm.

Answer

  • 20 cm (1/16), 25 cm (4/16), 30 cm (6/16), 35 cm (4/16), 40 cm (1/16);
    • the distribution is bell-shaped, centred on 30 cm.

Part 5 of 5Exam Checklist & Tips

3.2.13 – Key Points for the Exam

  • Polygenic inheritance Inheritance of a characteristic controlled by several genes.
  • Continuous variation Variation with a full range of phenotypes between two extremes.
  • Discontinuous variation Variation with distinct phenotype categories.
  • Additive effect Contribution of each allele added together to determine the phenotype.

Common Mistakes

  • Saying polygenic traits are controlled by one gene with many alleles.
  • Assuming each gene always has an equal effect on the phenotype.
  • Drawing a bar chart for continuous variation instead of a histogram or line graph.
  • Thinking height depends only on genes and not on the environment.

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