Lecture Notes: Non-Mendelian Patterns of Heredity

Introduction

• Speaker: Mr. Yeoman
• Topic: Non-Mendelian heredity patterns
• Overview: Gregor Mendel discovered dominant and recessive inheritance patterns, but not all genes follow this model.
  • Only 10-20% of genes follow Mendelian inheritance.
  • Examples: Eye color, height, psychiatric diseases involve multiple genes/environmental influences.

Incomplete Dominance

• Definition: One allele does not completely mask the other allele.
• Example: Flowers with two alleles (red and white).
  • Red flower produces red pigment.
  • White flower produces no pigment.
  • Pink flower has intermediate red pigment from heterozygous alleles.
• Punnett Square Interpretation:
  • Red (RR), Pink (Rr), White (rr) ratio is 1:2:1.

Codominance

• Definition: Both alleles in a heterozygote are fully expressed.
• Example: Appaloosa horses (brown and white hair).
  • Use of notation: Capital letter with superscript (e.g., C^B, C^W).
  • Ratio: Similar to incomplete dominance (25% brown, 25% white, 50% mixed).
• Blood Types: Codominance example with surface proteins on red blood cells.

Sex-Linked Traits

• Example: Red-green colorblindness (X-linked trait).
• Punnett Square: Include X and Y chromosomes.
  • Males have one X chromosome; females have two.
  • Example cross: Colorblind male and carrier female results in 1:1:1:1 ratio of offspring with different vision outcomes.

Multiple Alleles

• Example: Blood types (A, B, O alleles).
  • Represented by I^A, I^B, and i.
  • A and B are codominant; O is recessive.
  • Different genotype combinations lead to A, B, AB, O phenotypes.

Polygenic Inheritance

• Definition: Trait controlled by multiple genes, e.g., height, skin color.
• Complexity: Difficult to predict due to many gene interactions.
• Example: Skin color controlled by at least 34 genes.

Pleiotropy

• Definition: Single gene affects multiple traits.
• Example: Ehlers-Danlos Syndrome affects connective tissue, heart valves.

Epistasis

• Definition: One gene influences/prevents expression of another.
• Example: Albinism, where melanin production is inhibited.

Epigenetics

• Concept: Environmental factors can alter gene expression without changing DNA sequence.
• Methylation: Stress-induced changes to DNA packaging can silence genes.

Environmental Influence

• Examples:
  • Hydrangeas: Flower color changes with soil pH.
  • Siamese cats: Melanin production influenced by temperature.

Conclusion

• Complexity: Biological systems are intricate and gene expression is influenced by multiple factors.
• Encouragement: Stay curious and seek truth.