Lecture Notes: Linked Genes vs. Unlinked Genes

Key Concepts

• Linked Genes: Genes located on the same chromosome.
• Unlinked (Non-linked) Genes: Genes located on different chromosomes.
• Example:
  • Linked: Gene A and Gene B are on the same chromosome.
  • Unlinked: Gene A and Gene B are on different, non-homologous chromosomes.

Unlinked Genes Scenario

• Parent Cell Configuration:
  • Genes A and B are on separate chromosomes.
  • Each chromosome has a homolog with recessive versions.
• Meiosis Outcome:
  • Independent assortment leads to four gametes: AB, Ab, aB, ab.
  • Random chromosome arrangement during metaphase 1.
• Dihybrid Cross:
  • Male and female each produce four gametes with equal probability.
  • Assumption: AB genes follow Mendelian dominant-recessive pattern.
  • Resulting phenotype ratio: 9:3:3:1 for non-linked genes.

Linked Genes Without Crossing Over

• Assumption: No crossing over occurs between homologous chromosomes.
• Gamete Configuration:
  • Only two possible gametes: AB or ab.
• Dihybrid Cross:
  • Resulting phenotype ratio: 3:1 for linked genes.

Linked Genes With Crossing Over

• Crossing Over:
  • Random event causing exchange of gene segments between homologous chromosomes.
  • Rare occurrence; majority non-recombinant, some recombinant gametes.
• Gametes:
  • Predominantly AB or ab, occasionally Ab or aB.
• Dihybrid Cross:
  • Majority of offspring: double dominant (green) or double recessive (yellow).
  • Recombinant gametes produce offspring with mixed dominance phenotypes (pink, purple).
  • Phenotype ratio: Approximately 3:1 for double dominant to double recessive.
  • Low frequency for single dominant-recessive phenotypes.

Conclusion

• Frequency of recombinant gametes is low compared to non-recombinant.
• Observed phenotype ratio in linked genes with crossing over: 3:1:1 (with very low numbers for pink and purple phenotypes).

Important Points

• Linked genes tend to be inherited together unless crossing over occurs.
• Crossing over introduces genetic variation but occurs infrequently.

---

Note: Understanding the behavior of linked vs. unlinked genes is crucial in predicting genetic outcomes in various breeding analyses.