One of the practices in which G2 invests a significant amount of time is polyploid induction --- taking diploid genetics and creating tetraploids (and then potentially triploids or even higher ploidy levels).
Let's review some basic terminology. Every non-reproductive cell in a typical multicellular plant or animal has TWO sets of chromosomes. This condition is called diploid (di = two), and is the normal state for most living things. Cell division mechanisms work well when there are only two sets of chromosomes migrating at each division.
When reproductive cells form, a diploid cell (e.g., a pollen mother cell) goes through a unique series of divisions called meiosis, and produces up to 4 microspores which end up as pollen grains. Each microspore contains only ONE set of chromosomes. This is called the haploid state.
There is a standard abbreviation for the diploid condition: 2n. The abbreviation for haploid is 1n (or n --- lower case n, usually).
Breeders and geneticists frequently include the number of chromosomes when describing the ploidy of a species. For example, most seed-propagated Pelargonium xhortorum varieties are diploid, and have 18 chromosomes in each cell, or 2n=18. Pollen and egg cells in these varieties would be haploid, and then be n=9. Vegetatively-propagated zonal geraniums are tetraploid (tetra- meaning four, or four sets of chromosomes). For zonals then, 2n=36.
In those plant families where multiple ploidy levels may exist --- as with Pelargoniums --- it is standard practice to include a reference to the ploidy as you describe the chromosome number. Somatic cells (non-reproductive cells) are still defined as 2n (think of normal mitosis as working on a "diploid" level since cells divide in two), and the chromosome number is described using an upper case "X". For seed-propagated P. xhortorum then, 2n=2X=18, while for zonals, 2n=4X=36.
Why is any of this important? Look at the picture below: 
A pair of Pelargonium xhortorum (4X, 2X) flowers is on the left. A pair of P. peltatum (4X, 2X) flowers is on the right. Within each pair, the flower on the left is the tetraploid, while the flower on the right is the diploid. One of the classic "advantages" that polyploids exhibit is that they may have a larger flower size. The Pel flowers above are typical --- in each pair of flowers, the tetraploid on the left is visibly larger.
Tetraploids also tend to exhibit larger fruit, larger seed, larger pollen grains and sometimes larger plants. Please recognize that these potential benefits of polyploids do not always occur, and that variation for size may be equally possible within the range of diploid variation.
Often, the cells of a polyploid are larger than those of a diploid. Larger cells can frequently create leaves and flowers which are thicker and denser. The "feel" of a plant leaf or flower is referred to as "substance" and polyploids often have more substance. Leaves and flowers of polyploids will feel thicker.
To an ornamental breeder, flower color is always a consideration. Polyploids often display novel flower colors. The reasons for this are many but I find it useful to think about this as a gene dosage effect. In a diploid plant, there are two alleles (two genes) present for every biochemical step (and yes, I am oversimplifying here). If each allele contributes 10 units of flower color, a diploid plant might then make a total of 20 units. In a tetraploid, there are four alleles present. Theoretically, then, a tetraploid could make 40 units of flower color. If so, then tetraploids should have more color and perhaps be darker-colored than their diploid counterpart. Indeed, this is frequently what seems to occur.
But novel colors also occur in tetraploids. Keeping to my hypothetical model, if red=20 units of color, and deep red=40 units, what happens when you have 3 doses of red (30 units) and one dose (10 units) of some other color, perhaps blue. Red + blue = purple. 3 doses of red plus 1 dose of blue might give you a deep burgundy flower color. This color combination would probably NOT be possible in a diploid, since there would only be two doses available, and even though red+blue=purple might occur at the diploid level, it would only be purple (1 red + 1 blue). 3 red plus 1 blue would give you a very different shade of purple, and could possibly give you a rich burgundy color.
Other potential benefits to inducing polyploids include improved vase/shelf-life of flowers; improved cold-tolerance of plants; and improved pathogen-tolerance of plants or flowers. Most of these effects seem to be related to the change in cell size that occurs when a polyploid is induced, but not always. Most of us think first of BIG, but there are many other potential benefits possible when you create a polyploid.
Next time, I'll write about potential benefits to the breeder.
