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Variation and Selection - Variation

Grade 12IGCSEBiology

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Variation is defined as the differences that exist between individuals of the same species. These differences can be phenotypic (observable characteristics) or genotypic (genetic makeup).

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Continuous Variation results in a range of phenotypes between two extremes (e.g., height or mass). It is usually polygenic (controlled by many genes) and significantly influenced by environmental factors. When plotted, it typically forms a normal distribution curve.

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Discontinuous Variation results in distinct categories with no intermediates (e.g., ABOABO blood groups). It is usually controlled by a single gene (monogenic) and is little affected by the environment.

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Genetic Variation arises from three primary sources: Mutation (random changes in the DNADNA base sequence), Meiosis (through crossing over during prophase Iprophase\ I and independent assortment during metaphase Imetaphase\ I), and the random fusion of gametes during fertilization.

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Mutations are the ultimate source of all new alleles. Gene mutations involve changes in a single gene (e.g., sickle cell anemia caused by a substitution mutation in the β\beta-globin gene), while chromosome mutations involve changes in the number or structure of chromosomes (e.g., Trisomy 2121 in Down's Syndrome).

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Environmental Variation occurs when environmental factors such as light intensity, temperature, or nutrient availability modify the expression of the genotype. The relationship can be expressed as: Phenotype=Genotype+EnvironmentPhenotype = Genotype + Environment.

📐Formulae

p+q=1p + q = 1

p2+2pq+q2=1p^2 + 2pq + q^2 = 1

s=∑(x−xˉ)2n−1s = \sqrt{\frac{\sum (x - \bar{x})^2}{n - 1}}

VP=VG+VEV_P = V_G + V_E

💡Examples

Problem 1:

In a population of humans, the ability to taste the chemical PTCPTC is a dominant trait (TT). If 36%36\% of the population are non-tasters (tttt), calculate the frequency of the dominant allele (TT) and the frequency of heterozygous individuals (TtTt) using the Hardy-Weinberg principle.

Solution:

  1. Identify q2q^2: The frequency of the homozygous recessive genotype (tttt) is q2=0.36q^2 = 0.36.
  2. Find qq: q=0.36=0.6q = \sqrt{0.36} = 0.6.
  3. Find pp: Since p+q=1p + q = 1, then p=1−0.6=0.4p = 1 - 0.6 = 0.4. The frequency of allele TT is 0.40.4.
  4. Find 2pq2pq: Frequency of Tt=2×0.4×0.6=0.48Tt = 2 \times 0.4 \times 0.6 = 0.48.

Explanation:

We use the Hardy-Weinberg equations where pp represents the frequency of the dominant allele and qq the frequency of the recessive allele. The term q2q^2 represents the frequency of the recessive phenotype.

Problem 2:

Identify whether 'Human Skin Color' is an example of continuous or discontinuous variation and explain the genetic basis.

Solution:

Human skin color is an example of continuous variation.

Explanation:

It is controlled by multiple genes (polygenic inheritance). Each gene has an additive effect on the amount of melanin produced. Because many different gene combinations are possible, and environmental factors like UVUV radiation exposure also affect the trait, a wide spectrum of phenotypes is observed rather than distinct categories.

Variation - Revision Notes & Key Diagrams | IGCSE Grade 12 Biology