Final answer:
Synapsis is the association of homologous pairs guided by heterochromatin sequences. It occurs during prophase I of meiosis and involves the tight pairing of homologous chromosomes. This process allows for crossing over, which leads to genetic variation.
Step-by-step explanation:
In biology, the association of homologous pairs guided by heterochromatin sequences is called synapsis. During prophase I of meiosis, the homologous chromosomes come together and form a tight pairing, or synapse, facilitated by the synaptonemal complex. This process allows for the exchange of chromosomal segments between homologous NON-sister chromatids, known as crossing over.