Genetic Variation and Mutations

Raw Material

Heritable Variation Among Individuals

Students’ Learning Objectives

  • Homework due date for Chapter 4: September 24, 11:59 PM
  • Test Dates for Chapters 1, 2, & 3: September 23 & 24
  • Objectives include:
    • Understand the way mutations alter DNA
    • Differentiate between somatic mutations and germline mutations and their role in variation within a population
    • Explain the roles that independent assortment and genetic recombination play in evolution
    • Explain the types of phenotypic traits caused by genetic variation

Ways Mutations Alter DNA

Types of Mutations

  1. Point Mutation (Substitution)

    • A single base changes from one nucleotide to another.

  2. Insertion

    • A segment of DNA is inserted into the middle of an existing sequence.

  3. Deletion

    • A segment of DNA is deleted accidentally.

  4. Frameshift Mutation

    • Insertion or deletion of one or two bases shifts the reading frame of the entire sequence, changing the identity of the amino acid coded after the location of the mutation.

  5. Inversion

    • A segment of DNA is flipped around and inserted backward into its original position.

  6. Chromosome Fusion

    • Two chromosomes are joined together as one.

  7. Genome Duplication

    • Entire genomes are duplicated.

  8. Gene Duplication

    • A segment of DNA is copied a second time.

Point Mutations in Human Protein-Coding Genes

  • Point mutations in human proteins can lead to various phenotypic effects.
    • Example studies referenced include:
    • Oetting and King (1993)
    • Wang et al. (2007)
    • Leslie G. Biesecker (2008) on Greig cephalopolysyndactyly syndrome.
    • Other studies highlighting various mutations and their impacts (Sánchez-Martín et al., Shore et al., Eriksson et al.).

Types of Mutations and Their Impact on Inheritance

Somatic Mutations

  • Definition:
    • A mutation that affects cells in the body of an organism.
  • Effects:
    • They affect all daughter cells produced by the affected cell.
    • Can impact the phenotype of the individual.
    • Important Note: Somatic mutations are not passed down to offspring (non-heritable).

Germline Mutations

  • Definition:
    • A mutation that affects the gametes (eggs, sperm) of an individual.
  • Effects:
    • Can be transmitted from parents to offspring.
    • Create heritable genetic variation relevant to evolution.

Sexual Reproduction and Genetic Recombination

Importance of Genetic Recombination

  • Sexual Reproduction:
    • A biological process where a new organism is created by combining genetic materials from two parents.
  • Genetic Recombination:
    • The exchange of genetic material between paired chromosomes during meiosis.
    • This process can form new combinations of alleles, serving as a source of heritable variation.
  • Allele:
    • One of any number of alternative forms of the DNA sequence of the same locus.

Meiosis and Its Role in Variation

  • Meiosis:
    • A form of cell division that reduces the number of chromosomes by half.
    • Increased genetic diversity occurs through recombination, producing novel combinations of alleles.

Independent Assortment

Definition and Function

  • Independent Assortment:
    • The random mixing of maternal and paternal copies of each chromosome during meiosis.
  • Significance:
    • Results in the production of genetically unique gametes.
    • Increases genetic diversity in sex cells.

Mechanisms Driving Genetic Variation

Addition of Genetic Variation through Sexual Reproduction

  • During meiosis, genetic recombination and independent assortment work together to combine alleles, resulting in siblings inheriting different characteristics from their parents.

Mutation and Sexual Reproduction: Effects on Phenotypic Variation

  1. Mutations:
    • Cause phenotypic variations by introducing random changes in DNA.
    • Create new alleles that alter phenotypes.
  2. Sexual Reproduction:
    • Further drives phenotypic variation by combining alleles from two parents into unique combinations in each offspring.

Linking Genotype and Phenotype

Definitions

  • Genotype:
    • The genetic makeup of an individual.
  • Phenotype:
    • An observable, measurable characteristic of an organism.
  • Examples of Phenotype:
    • Morphological structure.
    • Performance traits.
    • Molecules produced by genes.

Types of Phenotypic Traits

Categories of Phenotypic Traits

  1. Genetic Polymorphism
    • The simultaneous occurrence of two or more discrete phenotypes within a population.
    • Phenotypic expression is directly attributable to alternative alleles at one or a few genes.
  2. Polyphenism
    • The phenomenon where discrete phenotypes can arise from a single genotype depending on environmental circumstances.
  3. Quantitative Traits
    • Traits influenced by multiple genes and the environment, generating a normal distribution.

Examples and Implications of Different Trait Types

Genetic Polymorphism Example

  • Alleles of a single gene control the shape of leaves in the ivy-leaf morning glory, resulting in discrete variation in leaf shape.

Polyphenism Example

  • In beetles, those developing in nutrient-rich environments attain larger sizes and grow horns, whereas those developing with insufficient nutrition fail to reach sizes necessary for horn growth.

Quantitative Traits Summary

  • Definition:
    • These traits reflect the simultaneous effects of many genes and environmental influences, making it misleading to speak of a gene representing a trait (e.g., there is no single “height gene”; height is influenced by multiple genes and environmental factors).

Conceptual Questions

Example Question

  • Question: The type of mutation in which one nucleotide is replaced by another is a(n):
    • deletion
    • frameshift
    • insertion
    • point mutation

Question on Environmental Influence

  • Question: Suppose that the sex of the offspring of a species of fish is determined by water temperature. In this case, sex is an example of a:
    • polyphenism
    • quantitative trait
    • genetic recombination
    • genetic polymorphism