Meiosis and genetic recombination: During which meiotic substage does crossing over primarily occur?

Introduction / Context:
Meiosis generates genetic diversity through recombination. Knowing the precise substage of prophase I where crossing over occurs is critical for cytogenetics and genetics.

Given Data / Assumptions:

• Prophase I substages: leptotene, zygotene, pachytene, diplotene, diakinesis.
• Synaptonemal complex formation is necessary for recombination.
• Chiasmata become microscopically evident later than the actual exchange event.

Concept / Approach:
Homologous chromosomes pair during zygotene and achieve full synapsis by pachytene. The synaptonemal complex facilitates homologous recombination; the actual exchange of genetic material (crossing over) occurs during pachytene. Chiasmata are most clearly visible in diplotene as synapsis dissolves.

Step-by-Step Solution:

Verification / Alternative check:
Electron microscopy of synaptonemal complexes shows recombination nodules during pachytene; cytology confirms chiasmata subsequently in diplotene.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing the visibility of chiasmata (diplotene) with the timing of crossing over (pachytene).

Final Answer:
Pachytene stage (full synapsis and recombination).