Allele Variation Bio1090

Mendelian Genetics: Beyond Simple Inheritance

  • The simplicity observed in Mendel's experiments doesn't always apply in nature.

  • Traits often involve multiple alleles and various expressions that complicate expected outcomes.

More than Two Alleles

  • Diploid Organisms: Individuals like humans carry two alleles but populations can have multiple allele variants.

  • Example: Eye Color in Drosophila

    • Variation includes alleles for red, brown, scarlet, and white eyes.

    • Demonstrates that traits can exhibit a range rather than binary options (like yellow vs. green).

  • Mutations: Multiple mutations within the same gene can lead to diverse phenotypes.

Understanding Alleles

  • Wild Type Allele: Most common in a population; typically considered the baseline phenotype.

  • Superscript Notation:

    • More complex allele notation includes superscripts instead of utilizing additional letters, simplifying classification (e.g., A+, A1, A2).

  • Polymorphism: Variants that occur in a population with at least 1% frequency.

Examples of Multiple Alleles

  • Rabbit Coat Color:

    • Variations on the C gene (e.g., Himalayan, Chinchilla, and Null) illustrate multiple alleles at a single locus acting recessively:

      • C+: Wild type.

      • Ch: Himalayan (differently pigmented extremities).

      • c: Null alleles (no pigmentation).

Effects of Mutations

  • Types of Mutations:

    • Missense Mutation: One amino acid is replaced, which may alter protein function.

    • Nonsense Mutation: Results in a truncated protein.

    • Insertions/Deletions: Changes the downstream amino acid sequence.

  • Recessive Alleles

    • Often manifest with loss of protein function, either wholly (null allele) or partially (hypomorphic allele).

Dominance and Its Variants

  • Recessive Alleles:

    • A single dominant allele can generally produce a wild type phenotype in heterozygotes.

    • Homozygous Dominant = Heterozygous phenotype.

  • Types of Dominance:

    • Complete Dominance: Phenotypically indistinguishable between homozygous dominant and heterozygous.

    • Incomplete Dominance: Results in intermediate phenotypes (e.g., red and white flowers produce pink).

    • Co-dominance: Both alleles manifest (e.g., checkered chicken).

Blood Types as Examples

  • ABO Blood Group System:

    • A (I^A), B (I^B), O (i).

    • Universal Donor: Type O (I^i) has no antigens.

    • Universal Recipient: Type AB (I^A I^B) can accept any type due to both A and B antigens.

Penetrance and Expressivity

  • Penetrance:

    • The percentage of individuals with a given genotype that express the associated phenotype.

    • Example: Dominant mutation that is not expressed in all carriers (incomplete penetrance).

  • Expressivity:

    • Variability in the degree of phenotype expression among individuals who have the same genotype (e.g., Ichabod mutation in zebrafish).

Environmental Effects on Phenotype

  • Temperature-sensitive Alleles: Certain alleles may express differently at various temperatures (e.g., Himalayan rabbits and fruit flies).

  • Maternal Age: Older maternal age can influence the severity and expression of some phenotypes in offspring.

Conclusion

  • Mendelian principles remain foundational in understanding genetics, but real-world scenarios often illustrate complexity beyond simple inheritance patterns.