Lecture Notes: Mendelian and Non-Mendelian Genetics

Introduction

• Discussion on Marfan syndrome, an autosomal dominant disease.
• Observations of variable symptoms in patients with the same genetic disorder.

Mendelian Genetics

Mendel's Laws

1. Law of Dominance
   • Dominant alleles suppress the expression of recessive alleles.
2. Law of Segregation
   • Organism's gametes randomly receive one of the parent's two alleles.
3. Law of Independent Assortment
   • Inheritance of one gene does not affect the probability of inheriting another.

Genetic Concepts

Genotype and Phenotype

• Genotype: Collection of alleles in an individual.
• Phenotype: Observable characteristics.
• Homozygous: Two of the same alleles.
• Heterozygous: Two different alleles.

Non-Mendelian Genetics

Linkage

• Genes close to each other on a chromosome tend to be inherited together.
• Exception to the Law of Independent Assortment.

Penetrance and Expressivity

• Penetrance: Proportion of individuals with a genotype that express the phenotype.
  • Example: BRCA1 mutations and breast cancer.
• Expressivity: Severity of phenotype expression.
  • Example: Neurofibromatosis 1 variability.

Incomplete Dominance and Codominance

• Incomplete Dominance: Offspring phenotype is between parents.
  • Example: Snapdragon color (red, white, pink).
• Codominance: Both alleles are simultaneously expressed.
  • Example: ABO blood type (both A and B antigens).

Mosaicism

• Somatic Mosaicism: Mutation in non-germline cells.
• Germline Mosaicism: Mutation in germline cells.
  • Affects offspring if involved in fertilization.

Epistasis

• One gene affects expression of another gene.
• Example: Gene for baldness affecting hair color expression.

Allelic and Locus Heterogeneity

• Allelic Heterogeneity: Different mutations at the same locus causing similar phenotype.
• Locus Heterogeneity: Same disorder caused by mutations in different loci.

Genetic Imprinting

• Expression dependent on whether allele is maternal or paternal.
• High methylation can prevent allele expression.
  • Example: Angelman syndrome and UBE3A gene.

Conclusion

• Genetic disorders exhibit variable expressivity.
• Importance of recognizing the spectrum of genetic disease manifestations.

Reflection Questions

• Why do some individuals with the same genetic disorder exhibit different symptoms?
• How do linkage and mosaicism differ in terms of genetic inheritance?
• What is the difference between allelic and locus heterogeneity?
• What role does genetic imprinting play in inheritance?

Key Takeaways

• Mendel's laws provide a foundation for understanding inheritance, but exceptions exist.
• Non-Mendelian genetics involve complex patterns like linkage, penetrance, expressivity, and more.
• Understanding these concepts is crucial for diagnosing and understanding genetic disorders.