37.2

37.2 Genotype and Phenotype

  • Definitions

    • Genotype: The genetic makeup of an organism (specific DNA sequence).

    • Phenotype: Observable characteristics of an organism (e.g., height, weight, eye color).

1. Describing Mutations

  • Two Perspectives:

    • DNA Level Changes: Types of mutations include:

      • Point Mutations: Change in a single nucleotide.

      • Insertions/Deletions: Small changes in DNA sequence.

      • Chromosome Level: Larger mutations affecting entire chromosomes.

    • Effect on Traits: How mutations influence the traits of cells/organisms:

      • Silent Mutations: Do not affect protein function.

      • Missense Mutations: Change one amino acid, can impact function.

      • Nonsense Mutations: Introduce premature stop codons, negatively impact proteins.

      • Frameshift Mutations: Disrupt reading frame, altering entire polypeptide.

2. Relationship Between Genotype and Phenotype

  • Mutations can vary in effect:

    • Noncoding Mutations: Often lack observable effects.

    • Chromosomal Changes: Changes in chromosome number usually lead to observable conditions (e.g., Down syndrome).

    • Homozygous vs. Heterozygous:

      • Homozygous: Same allele from both parents (AA or aa).

      • Heterozygous: Different alleles from parents (Aa).

    • Example - Pea Plants:

      • Dominant A allele (yellow) vs. recessive a allele (green).

      • Homozygous AA or heterozygous Aa = yellow peas; homozygous aa = green peas.

3. Molecular Basis of Traits

  • Enzyme Function:

    • Yellow color in peas results from an enzyme breaking down chlorophyll to reveal yellow pigments.

    • Green seeds arise from mutations that inactivate this enzyme.

4. Effects of Mutations

  • Some mutations result in traits that:

    • Harmful: Increase disease susceptibility (e.g., coronary artery disease).

    • Neutral: No observable effect, often in noncoding regions.

    • Beneficial: Enhance survival/reproductive success (e.g., certain alleles provide malaria resistance in heterozygotes).

5. Example of β-Globin Variants

  • Alleles: A (normal) and S (mutated).

    • A allele: Leads to normal hemoglobin (Glu).

    • S allele: Point mutation changes Glu to Val, causing sickle-cell anemia when homozygous (SS) but offering malaria protection when heterozygous (AS).

6. Environmental Influence

  • Mutation Effects:

    • Change in effect based on environment (e.g., S allele beneficial in malaria regions but harmful otherwise).

    • Distinguishing harmful, neutral, or beneficial mutations may depend on context.

7. Exam Preparation Tips

  • Be prepared to explain types of mutations:

    • Point, silent, missense, nonsense, insertion, deletion, and frameshift mutations.

    • Understand effects of transposable elements and nondisjunction.

  • Prepare for conceptual questions on genotype and phenotype relationships regarding health and disease.