Lecture Notes: Genetic Linkage and Recombination

Introduction

• Topic: Genetic linkage and recombination patterns of inheritance.
• Analogy: Genes can be compared to friends moving together, reflecting genetic linkage.

Genetic Linkage

• Definition: Genes tend to be inherited together with adjacent genes due to their close proximity on a chromosome.
• Example: Like going out with a group of friends.

Recombination

• Definition: Genes may recombine, similar to pairing up with different classmates.
• Chromosomes: Humans have 23 pairs with genes arranged along these chromosomes.
• Genes vs. Chromosomes: More genes than chromosomes; approximately 19,000 genes on 23 chromosomes.

Mendel’s Law of Independent Assortment

• Concept: Genes are independently inherited, not influenced by other genes.
• Illustration: Mendel's pea plant experiments resulting in four different offspring.

Importance of Gene Placement

• Recombination:
  • Occurs during crossing over between non-sister chromatids of homologous chromosomes.
  • Greater between distant genes, less likely for adjacent genes.
• Genetic Linkage:
  • Adjacent genes, with fewer recombination sites, are inherited together.
  • These are called 'linked genes'.

Gene Mapping and Recombination Frequency

• Determining Distance: Distance between genes can be determined using recombination frequency.
• Recombination Frequency (RF):
  • Expressed as a percentage.
  • 1% RF equals 1 centimorgan or 1 map unit.
  • RF > 50% indicates genes are 'unlinked' and independently assorted.

Recombination in Closely Located Genes

• Low Likelihood: Though low, recombination can occur between closely located genes.
• Example Calculation:
  • 45 recombinants among 1000 offspring results in RF of 4.5%.
  • Indicates genes are 4.5 centimorgans apart.

Summary

• Key Takeaway: Genes that are closely located tend to be inherited together due to linkage.
• Crossing Over: Recombination occurs due to crossing-over in homologous chromosomes.

Conclusion

• Distant genes follow Mendel’s law of independent assortment, while closely located genes exhibit linkage.
• Recombination patterns help in mapping genomes through understanding gene distance and frequency.