6.2.10 - Discontinuous and Continuous Variation

• Discontinuous Variation:

  • Where phenotype classes are distinct and discrete, each clearly discernible from the others in a qualitative way.
  • There are no, or very few, intermediates between the different phenotypes.
• Characteristics that exhibit discontinuous variation are usually determined by the alleles of a single gene locus - they are monogenic.
• Sometimes, the alleles of two genes interact to govern a single characteristic:

  • Different alleles at a single gene locus have large effects on the phenotype
  • Different gene loci have quite different effects on the characteristic.
• In tomato plants, many genes determine features of a plant's leaves:

  • One gene locus codes for leaf shape, another determines whether the leaves have hairs, a third gene locus determines the presence or absence of chlorophyll.
• Genes at different loci may interact to influence one characteristic and produce discontinuous variation, as in epistasis.