Overview
This lecture covers the concept of overdominance in genetics, key models explaining hybrid vigor (heterosis), and the molecular and evolutionary consequences of these phenomena.
Basic Concepts of Dominance and Overdominance
- Homozygous individuals have two identical alleles at a gene locus; heterozygous individuals have two different alleles.
- Underdominance is when heterozygotes are less fit than both homozygotes.
- Overdominance is when heterozygotes have higher fitness than either homozygote, leading to balancing selection.
- Fitness refers to an organism's reproductive success across its life cycle.
Genetic Models Explaining Heterosis
- Dominance hypothesis: hybrid vigor results from masking deleterious recessive alleles.
- Overdominance hypothesis: heterozygote advantage arises from synergistic interactions at a locus.
- Pseudo-overdominance: linked recessive mutations in repulsion mimic overdominance.
- Epistasis: alleles at different loci interact to influence traits.
- Nonadditive gene expression and genetic linkage are additional explanatory models.
Examples and Applications
- Sickle-cell anemia in humans is a classic example of overdominance, maintaining both normal and sickle-cell alleles due to heterozygote advantage.
- In agriculture, heterozygous tomatoes for the rin allele result in desirable red-firm fruits due to overdominance.
- Overdominance can also arise from chromosomal inversions, coadaptation, and suppression of recombination.
Molecular Mechanisms and Tools
- Multiple allozymes in heterozygotes reduce harmful protein aggregates, boosting metabolic efficiency.
- Molecular biology tools like transcriptomics, proteomics, and metabolomics help dissect the mechanisms underlying heterosis and inbreeding depression.
Evolutionary and Ecological Consequences
- Hybrid vigor often benefits first-generation hybrids but declines with increased homozygosity in later generations.
- Hybrid gene combinations can lead to ecological adaptations and speciation.
- Transgressive segregation produces hybrid traits outside the parental range.
Key Terms & Definitions
- Homozygous — having two identical alleles at a gene locus.
- Heterozygous — having two different alleles at a gene locus.
- Overdominance — increased fitness of heterozygotes compared to both homozygotes.
- Underdominance — decreased fitness of heterozygotes.
- Balancing selection — maintains multiple alleles in a population.
- Heterosis (hybrid vigor) — hybrid phenotype superior to parents.
- Epistasis — interaction between genes at different loci.
- Pseudo-overdominance — apparent overdominance due to linkage of complementary alleles.
- Allozyme — enzyme variant encoded by different alleles.
- Transcriptomics/Proteomics/Metabolomics — large-scale studies of RNA, proteins, and metabolites.
Action Items / Next Steps
- Review examples of overdominance in human disease and agriculture.
- Understand the differences between dominance, overdominance, and pseudo-overdominance.
- Study the use of molecular tools to investigate inbreeding and heterosis.