Non-Mendelian Genetics Lecture Notes

Introduction

• Non-Mendelian Genetics: Different from Mendelian genetics which deals with complete dominance.
  • Mendelian genetics: Complete dominance (purple x white = purple or white).
  • Non-Mendelian genetics: Includes cases like gene linkage, incomplete dominance, etc.

Key Concepts in Non-Mendelian Genetics

Gene Linkage

• Definition: Two genes located close together on the same chromosome.
• Independent Assortment: Describes genes on different pairs of homologous chromosomes.
  • Example: Dihybrid cross results in a 9:3:3:1 phenotypic ratio.
• Linked Genes: Genes are linked if they are on the same chromosome and do not assort independently.
  • Non-crossing over is common when genes are very close.
  • Linked genes do not follow a 9:3:3:1 ratio.

Incomplete Dominance

• Example: Red and white flowers producing pink offspring.
• Intermediate Expression: Heterozygote displays an intermediate phenotype (e.g., pink from red and white).
• Not Blending: Unlike blending, original parental phenotypes can reappear in subsequent generations.
• Examples: Hair texture (straight, wavy, curly); Hypercholesterolemia - mild disease is an intermediate condition.

Multiple Alleles and Codominance

• Multiple Alleles: More than two alleles in a population (e.g., blood groups).
• Blood Typing
  • Alleles: I^A, I^B, i (O).
  • Complete Dominance: A is dominant over O; B is dominant over O.
  • Codominance: Example is AB blood type where both A and B are expressed.
• Sickle Cell Anemia: Example of codominance
  • Heterozygotes (HbA HbS) have higher resistance to malaria.
  • Homozygous recessive (HbS HbS) causes sickle cell disease.

Polygenic Inheritance

• Definition: The additive effect of two or more genes on a single trait.
• Examples: Skin color, height, weight, IQ.
• Continuum: Traits do not fall into discrete categories but exist along a continuum.
• Environmental Impact: Nutrition and environment affect the phenotypic expression of polygenic traits.

X-Linked and Sex-Linked Traits

• X-Linked Traits: Genes located on the X chromosome.
  • More genes on X than Y chromosome.
  • Examples include color blindness and hemophilia.
• Sex Determination: XX (female), XY (male).
  • Y chromosome carries the SRY gene responsible for male characteristics.

Environmental Impacts on Gene Expression

• Example: Hydrangea flower color varies with soil pH.
• Temperature Effects: Siamese cats and Himalayan rabbits have fur color influenced by temperature.
• Lifestyle and Environment: Conditions such as diet and physical activity can influence genetic expression.

Conclusion

• Non-Mendelian genetics encompasses a variety of genetic inheritance patterns that do not follow simple Mendelian laws.
• Understanding these patterns is crucial for explaining genetic diversity and expression in populations.

Additional Resources

• For further understanding, refer to class examples, textbook exercises, and seek clarification from additional sources if needed.

Fun Video Recap

• A humorous song highlighting the role of maternal contributions beyond DNA, emphasizing mitochondria and other factors from the mother.

This concludes the lecture on non-Mendelian genetics. Review these notes and examples to prepare for your upcoming examination.