Genetic and Protein Polymorphism

Genetic and Protein Polymorphism

Definition of Polymorphism

  • Polymorphism: The condition of having more than one allele at a locus.

    • Distinction is made between polymorphic (more than one allele) and monomorphic (only one allele) traits at the population level.

    • Questions to consider:

    • Does the gene locus code for one or multiple allelic products within a population or species?

    • If natural selection is the driving force of evolution, why wouldn’t only the best allele be present at a locus?

Heterozygosity

  • Heterozygosity: In diploid organisms, this condition refers to having two alternative alleles present at a gene locus.

    • Implications of heterozygosity:

    • Individuals are able to produce two different types of gametes regarding this gene locus.

    • Measurement of heterozygosity can be approached in two ways:

    1. Estimating the average number of loci that are heterozygous within a population.

    2. Assessing the average number of individuals that are heterozygous for a specific gene locus.

    • These metrics help in studying population dynamics.

Polymorphism and Heterozygosity: Levels Observed in Plants and Animals

  • Plants:

    • Based on protein electrophoretic studies, plants are thought to be polymorphic at approximately 50% of their loci, with a mean of 1.96 alleles per locus.

    • On average, about 34% of loci in populations within a given species are polymorphic.

    • Comparison of Polymorphism Levels:

    • Gymnosperms: Higher levels of polymorphism than monocot angiosperms and dicot angiosperms.

    • Explanation for gymnosperms' higher polymorphism:

      • Long lifespan

      • Wide geographic range

      • Outcrossing due to wind pollination

    • Each factor contributes significantly, making it difficult to determine which is most influential.

Geographic Variation in Genetic Polymorphism

  • Geographic Range Effects:

    • Endemic species (narrow ranges): Average 40% polymorphic loci.

    • Widespread species: About 59% of loci are polymorphic.

  • Comparative Polymorphism in Animals:

    • Generally, animals exhibit lower polymorphism levels than plants across studied loci.

    • Vertebrates:

    • Mean polymorphism of 17.3%

    • Mean heterozygosity of 4.9%

    • Invertebrates: Generally higher, with marine invertebrates approaching the highest polymorphism levels observed in plants.

    • Reasoning: Release of gametes into water mimics wind pollination in gymnosperms, which may explain higher polymorphism.

Functional Implications of Polymorphism

  • How does variability in enzyme substrates relate to polymorphism?

    • Homozygotes at a locus: Produce only one protein form.

    • Heterozygotes at a locus: Produce two protein forms.

    • Possessing two forms of a protein (e.g., enzyme) may allow for:

    • Catalyzing a wider array of substrates.

    • Adapting enzymatic function across temperature ranges (e.g., summer/winter).

    • Potential Selective Advantage: Heterozygous individuals could have an advantage in fluctuating environmental conditions.

    • However, it is crucial to note that much observed genetic variation in animals is selectively neutral:

    • Some enzyme forms function similarly, displaying little to no significant advantages.

    • A considerable amount of variation may not confer any advantage.