17.02 Continuous and Discontinuous Variation

1. Discontinuous Variation

• Definition: Discontinuous variation refers to phenotypic differences where individuals fall into distinct, non-overlapping categories. There are no intermediate forms between these categories.

Key Characteristics:

• Distinct Categories: Traits are clearly separated without blending (e.g., blood types).
• Genetic Determination: Primarily controlled by one or a few genes with large effects.
• Minimal Environmental Influence: Phenotypes remain consistent regardless of environmental conditions.

Causes:

• Single Gene or Few Genes: Traits are determined by specific alleles at one or a few loci.
• Epistasis: Interaction between different genes at distinct loci can create distinct phenotypic categories.

Examples:

• Human Blood Groups: A, B, AB, and O types determined by the ABO gene.
• Genetic Disorders:
  • Sickle Cell Anemia (HBB gene)
  • Hemophilia (F8 gene)
• Plant and Animal Traits:
  • Flower color in Salvia
  • Stem color in tomato plants
  • Feather color in chickens

Visual Representation:

• Pie Charts: The distribution of blood types in a population.

2. Continuous Variation

• Definition: Continuous variation describes phenotypic differences that fall along a spectrum, with no clear-cut categories. Traits exhibit a range of values between two extremes.

Key Characteristics:

• Spectrum of Traits: Individuals display a variety of phenotypes that blend into each other (e.g., height, weight).
• Polygenic Inheritance: Controlled by multiple genes, each contributing small, additive effects.
• Significant Environmental Influence: Environmental factors can greatly modify the phenotype.

Causes:

• Polygenes: Multiple genes located at different loci contribute to a single trait.
• Environmental Factors: Conditions such as nutrition, climate, and lifestyle impact the expression of the trait.

Examples:

• Human Traits:
  • Height
  • Weight
  • Skin color
• Plant Traits:
  • Cob length in maize (as seen in Emerson and East’s study)
• Animal Traits:
  • Fur length in rabbits

Genetic Mechanisms:

• Additive Effects: Each allele at a gene locus adds to the trait in a cumulative manner.
  • Example: For height, each dominant allele (A or B) might add 2 cm, while each recessive allele (a or b) adds 1 cm.
• Polygenic Interaction: Many genes interact to produce a continuous range of phenotypes.

Visual Representation:

• Bell Curves/Histograms: The smooth distribution of traits across a population (e.g., distribution of heights).

3. Genetic Basis of Variation

4. Genetic Mechanisms in Detail

Discontinuous Variation:

• Single Gene Locus:
  • Example: ABO Blood Group
    • Alleles: IA, IB, IO
    • Genotypes and Phenotypes:
      • IAIA or IAIO → Type A
      • IBIB or IBIO → Type B
      • IAIB → Type AB
      • IOIO → Type O
• Epistasis:
  • Interaction between genes can create distinct phenotypic outcomes.
  • Example: One gene may mask or modify the expression of another, leading to clear categories.

Continuous Variation:

• Polygenic Inheritance:
  • Definition: Traits are influenced by multiple genes, each contributing a small effect.
  • Example: Human height is influenced by numerous genes, each adding a small amount to the overall height.
• Additive Effects:
  • Each allele’s effect accumulates to produce the final phenotype.
  • Hypothetical Example:
    • Gene A: A (adds 2 cm), a (adds 1 cm)
    • Gene B: B (adds 2 cm), b (adds 1 cm)
    • Genotypes:
      • AABB → 8 cm
      • AAbb or aaBB → 6 cm
      • aabb → 4 cm
    • Result: A range of heights from 4 cm to 8 cm, with intermediates.

Polygenes:

• Definition: Several genes at different loci contribute to a single trait.
• Example: Maize Cob Length
  • Case Study: Emerson and East’s experiment showed that crossing two homozygous maize varieties resulted in F1 with uniform cob length, while F2 displayed a normal distribution due to polygenic inheritance.

5. Case Study: Continuous Variation in Maize Cob Length

Experiment by Emerson and East:

• Parental Generation:
  • Black Mexican and Tom Thumb maize varieties were homozygous at most loci.
• F1 Generation:
  • Crossed to produce offspring with uniform cob lengths due to genetic recombination.
• F2 Generation:
  • Displayed a normal distribution of cob lengths, illustrating polygenic inheritance.

Analysis:

• F1 Generation: Limited variation as parental traits were homozygous.
• F2 Generation: Increased variation due to multiple genes segregating and recombining, alongside possible environmental influences.

6. Analyzing Variation

Statistical Tests:

• t-test:
  • Purpose: Compare the means of traits between different populations.
  • Application: Determines if observed differences are statistically significant or due to random chance.
  • Reference: Detailed in Chapter P2 for comprehensive understanding.

7. Key Terms

• Genetic Variation: Differences in DNA sequences among individuals within a species.
• Phenotypic Variation: Observable differences in traits among individuals of a species.
• Discontinuous Variation: Traits with distinct categories and no intermediates (e.g., blood types).
• Continuous Variation: Traits that exhibit a range of values without clear categories (e.g., height, weight).
• Epistasis: Interaction between genes at different loci that affects a single trait.
• Polygenes: Multiple genes at different loci that collectively contribute to a single phenotype.

8. Visual Representations

Diagram Suggestions:

• Punnett Squares:
  • Single-gene inheritance (e.g., ABO blood groups).

• Polygenic Trait Models:
  • Multiple genes contributing to a single trait with additive effects.

9. Discussion Questions

• Explain the difference between continuous and discontinuous variation with examples.
  • Continuous variation involves traits that show a range of values (e.g., height), while discontinuous variation involves traits that fall into distinct categories without intermediates (e.g., blood types).
• How does polygenic inheritance contribute to continuous variation?
  • Multiple genes, each with small additive effects, combine to produce a wide range of phenotypes, resulting in continuous variation.
• Why is environmental influence more significant in continuous variation compared to discontinuous variation?
  • Continuous traits are influenced by multiple genes and environmental factors, allowing for a spectrum of phenotypes, whereas discontinuous traits are primarily controlled by specific genes with little to no environmental impact.
• Describe how epistasis can result in discontinuous variation.
  • Epistasis involves interactions between different genes, where one gene can mask or modify the expression of another, leading to distinct phenotypic categories.
• Using the case study of maize cob length, explain how polygenic inheritance can produce a normal distribution of traits in the F2 generation.
  • In the F2 generation, multiple genes segregate and recombine, each contributing small effects to cob length. The cumulative effect of these polygenes results in a continuous range of cob lengths, forming a normal distribution.