2n --> n
You start with a cell with a diploid set of chromosomes (2n). In S phase (a part of interphase), the chromosomes are doubled (4n). After meiosis, each cell has a haploid set of chromosomes.
n-->n happens in fungi, when haploid mycelia cells replicate but stay haploid. I'm a bit rusty, though, so I'm not too sure.
n --> 2n is fertilization
2n --> 2n is mitosis, the cell replication process for somatic cells.
