Mendelian Genetics Revisited

Key Concepts

• Genotype and Phenotype: Basics of genetic inheritance relating to observable traits.
• Alleles: Variants of a gene; involve dominant and recessive types.
• Punnett Squares: A tool to predict the genotypes/phenotypes of offspring from parental alleles.

Beyond Simple Dominance

• Incomplete Dominance
  • Example: Snapdragon flowers, where red and white parents produce pink offspring.
  • Results in a third, intermediate phenotype.
• Codominance
  • Example: Cows with both white and brown coloring.
  • Simultaneous expression of both phenotypes.

Dominant and Recessive Alleles

• Dominance not due to suppression of another allele, but due to phenotype expression.
• Example: Seed Shape in Peas
  • Round vs. Wrinkled seeds due to enzyme activity.
  • Dominant allele codes for enzyme converting starch, affecting seed shape.

Multiple Alleles

• Blood Type in Humans
  • Alleles: IA, IB, i.
  • Different combinations yield blood types A, B, AB, O.

Extensions to Mendelian Genetics

Pleiotropy

• A single gene affects multiple phenotypic traits.
• Examples
  • Human diseases: Cystic fibrosis, sickle-cell disease.
  • Pea plants: Flower and seed coat color.

Epistasis

• Interaction of genes affects expression of a phenotype.
• Example: Labrador Retrievers
  • Black vs. brown pigment with deposition gene influencing fur color.

Polygenic Inheritance

• Traits influenced by multiple genes, existing along a continuum.
• Example: Human Skin Color
  • Involves multiple genes contributing to a range of skin tones.

Multifactorial Traits

• Phenotypes influenced by genetic and environmental factors.
• Example: Flower color varies with soil acidity.

Conclusion

• Expanded understanding from Mendel’s simple laws to complex inheritance patterns.
• Importance in modern genetics for understanding genetic disorders and treatment strategies.