Given the task definition and input, reply with output. Given a scientific passage and an answer, generate a question for the given answer.

Passage: When homologous chromosomes form pairs during prophase I of meiosis I, crossing-over can occur. Crossing-over is the exchange of genetic material between non-sister chromatids of homologous chromosomes. It results in new combinations of genes on each chromosome. Answer: meiosis
When do homologous chromosomes form pairs?