Autopolyploidy occurs when there are additional sets of chromosomes in a cell. The chromosomes in the parent individual with 3 sets of homologous chromosomes (2n=6) go through a meiotic error which results in an unreduced gamete with 6 chromosomes (rather than 3) that self fertilize and create a tetraploid zygote. However, the polyploid offspring is unable to reproduce sexually. Normally, the 2n offspring would mate with another 2n offspring. Then the 1n gametes produced combine with the other and produces another 2n offspring in the parent generation. However, what reproductively isolates this example from its diploid parent species is the fact that through the meiotic error, 4n cells in the the adult occurs.
They are then unable to produce fertile offspring with the parent species. When the 3n offspring results, (or any uneven numbered offspring), mitosis would work just fine because the homologous chromosomes don’t have to match up, BUT, meiosis is faltered because the 3n offspring trying to produce gametes through this genetic mechanism will fail because the cell will not be able to divide. The result is impaired meiosis of unpaired chromosomes. This is a postzygotic isolating barrier.