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CIE AS/A Level Biology -17.1 Variation- Study Notes

CIE AS/A Level Biology -17.1 Variation- Study Notes- New Syllabus

CIE AS/A Level Biology -17.1 Variation- Study Notes- New Syllabus

Ace A level Biology Exam with CIE AS/A Level Biology -17.1 Variation- Study Notes- New Syllabus 

Key Concepts:

  • explain, with examples, that phenotypic variation is due to genetic factors or environmental factors or a combination of genetic and environmental factors
  • explain what is meant by discontinuous variation and continuous variation
  • explain the genetic basis of discontinuous variation and continuous variation
  • use the t-test to compare the means of two different samples (the formula for the t-test will be provided, as shown in the Mathematical requirements)

CIE AS/A Level Biology 9700-Study Notes- All Topics

Phenotypic Variation

🧬What is Phenotypic Variation?

  • Phenotype = observable characteristics of an organism.
  • Variation = differences in phenotype between individuals.
  • Caused by:
    • Genetic factors
    • Environmental factors
    • Combination of both

1. Genetic Factors

  • Controlled by differences in genes/alleles.
  • Passed from parent to offspring.
  • Examples:
    • Flower colour in pea plants → controlled by a single gene.
    • Blood group in humans (A, B, AB, O) → determined by multiple alleles of the ABO gene.
    • Sickle-cell anaemia → caused by mutation in the HBB gene.

2. Environmental Factors

  • Caused by conditions in the organism’s surroundings, not genes.
  • Not inherited.
  • Examples:
    • Soil pH affecting flower colour in hydrangeas (blue in acidic soil, pink in alkaline soil).
    • Sunlight exposure affecting skin colour (tanning).
    • Diet affecting body mass.

3. Combination of Genetic and Environmental Factors

  • Most characteristics are influenced by both genes and environment.
  • Examples:
    • Height in humans → genetic potential, but nutrition affects final height.
    • Skin colour → genetic base, but sunlight can darken skin (tanning).
    • Milk yield in cows → genetic makeup sets potential, but diet and management affect actual yield.
✅ Summary:
Phenotypic variation can be:
  • Genetic only (e.g., blood group),
  • Environmental only (e.g., tanning), or
  • Combination of both (e.g., height, milk yield).

Types of Variation

Discontinuous Variation 

  • Definition: Variation where individuals fall into distinct categories with no intermediates.
  • Controlled by: Usually a single gene (monogenic).
  • Not influenced much by environment.
  • Data type: Qualitative (non-numerical).
  • Examples:
    • Blood group in humans (A, B, AB, O)
    • Sex in humans (male/female)
    • Flower colour in pea plants (purple or white)

Continuous Variation

  • Definition: Variation where individuals show a range of values between two extremes, with intermediates.
  • Controlled by: Many genes (polygenic inheritance).
  • Strongly influenced by environment.
  • Data type: Quantitative (measured in numbers).
  • Examples:
    • Human height
    • Human body mass
    • Milk yield in cows
    • Grain size in wheat

✅ Key Difference:

  • Discontinuous = either/or categories, controlled by one gene, little environmental effect.
  • Continuous = measurable range, controlled by many genes, strongly affected by environment.

Genetic Basis of Variation

🌸 Discontinuous Variation 

  • Genetic Control: Usually controlled by a single gene (monogenic).
  • Alleles: Different alleles of a gene produce distinct phenotypes.
  • Effect of Environment: Minimal; phenotype is largely determined by genotype.
  • Examples:
    • Blood group in humans (A, B, AB, O)
    • Flower colour in pea plants (purple or white)
    • Presence or absence of a particular trait (e.g., earlobe attachment)

🌿 Continuous Variation

  • Genetic Control: Controlled by many genes (polygenic inheritance).
  • Alleles: Multiple genes contribute additively to the trait.
  • Effect of Environment: Strong influence; environmental factors modify the phenotype.
  • Examples:
    • Human height and weight
    • Skin colour in humans
    • Milk yield in cows

📊 Key Points

  • Discontinuous variation: single gene → distinct categories → minimal environmental influence.
  • Continuous variation: multiple genes → wide range of phenotypes → strongly affected by environment.

T-Test for Comparing Two Means

🌱 Purpose of the t-test

  • Used to determine if the difference between the means of two samples is statistically significant.
  • Helps decide whether observed differences are due to chance or a real effect.

Formula

  • t = \( t = \frac{\bar{X}_1 – \bar{X}_2}{\sqrt{\frac{s_1^2}{n_1} + \frac{s_2^2}{n_2}}} \)
  • Where:
    • \(\bar{X}_1, \bar{X}_2\) = means of the two samples
    • \(s_1^2, s_2^2\) = variances of the two samples
    • \(n_1, n_2\) = sample sizes

🌿 Steps to Perform a t-test

  • State hypotheses:
    • Null hypothesis (H₀): No significant difference between sample means.
    • Alternative hypothesis (H₁): Significant difference exists.
  • Calculate t-value using the formula above.
  • Determine degrees of freedom (df):
    • \( df = n_1 + n_2 – 2 \)
  • Compare t-value with critical t from tables at chosen significance level (usually p = 0.05).

🌱 Decision Rule

  • If t < critical value → Accept H₀ → difference is not significant.
  • If t ≥ critical value → Reject H₀ → difference is significant.

Example:

  • Sample 1: Mean height = 150 cm, n = 10
  • Sample 2: Mean height = 155 cm, n = 10
  • Calculate t using the formula → compare with critical t (df = 10+10–2 = 18)
  • Decide if the difference in mean height is significant or due to chance.
✅ Key Points:
  • t-test compares means, not individual values.
  • Requires sample mean, variance, and size.
  • Helps test hypotheses in experiments and biology data.
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