I think Stray covered most of what you asked, but I wanted to clear something up just a bit. Let's take humans as an example, our n (called a haploid number) equals 23. This means that there are a total of 23 unique chromosomes in a human gamete
. Normal, human, non-reproductive cells (anything but sperm and ovum) are 2n, or diploids. This means each of the 23 unique chromosomes has a duplicate attached to it. This is the whole reason why reproduction is possible. The cells responsible for making sperm or ovum, which are 2n (46 chromosomes, or 23 matching pairs), split into haploid cells (gametes), each containing one of the 23 chromosomes. When the male and female gametes meet, the haploid chromosomes combine and you get a fertilized ovum with the full diploid number.
Tetraploids are possible be treating small, germinating seeds with chemicals that inhibit mitotic spindles. During a certain phase in cell division, all the chromosomes make a duplicate of themselves, so for a moment it is in a sense a tetraploid, as there is four copies of each chromosome. Normally, shortly after the doubling, little threads in the cells start pulling the chromosomes away from eachother, which results in two daugher cells with a diploid chromosome number. What these chemicals, such as colchicine or oryzalin, do is in some form dissolve those little spindles that pull the chromosomes apart. This action causes the cell to stop, and reset the process, leaving one cell with a tetraploid number of chromosomes, which then restarts the process and eventually splits into two, tetraploid daughter cells.
Now I'm positive that any alteration to the human chromosomes is usally fatal, and I think its the same case in the animal world, but for some reason plants can handle it much better. In fact, most of the times tetraploids are more vigorous than their diploid counterparts, but in a good percentage of the cases, triploids may actually be the most vigorous of all. The huge downfall of triploids is that they are very difficult to get to breed. The reason is that during the point where the cells producing gametes start dividing into their haploid numbers, you have three pairs of chromosomes to go into two cells, which often case doesn't go nice and evenly and you have plants that are genetic messes. In some very rare
cases, certain gametes of a triploid may luckily end up with all three pairs of chromosomes, giving you a triploid, or 3n, gamete. This gamete, when paired with a haploid (1n, or normally called n) gamete, gives you a tetraploid (4n) cell, which will then be able to breed in the future. I do really want to stress that the chances of getting even tetraploids out of a 3n x 2n cross are very small, and in the Phalaenopsis world, there is only one triploid hybrid that is known to make triploid gametes more often than the nearly never of most other triploids, although there are still many cases where you get aneuploid (not being a whole number) seeds from it if you get anything at all.
If you didn't notice by the end of the first paragraph, I absolutely LOVE
anything to do with chromosomes, genetics, or genes!
I really hope this helped, and if you have any further questions, ask away!