Trying to keep it simple:

Most forms of dwarfism are due to a recessive gene.

The one you mention, 'microtom', is the result of at least 3 recessive genes. These are referred to as "bird's nest" types. To get this type of habit requires the brachytic gene "br" and the presence and interaction of 2 alleles of the sun dwarf gene "sd" (specifically the alleles "sd-2" and "sd-3").

Unless one crosses it to another parent that already has at least one of these genes there is about a 1 in 64 chance (4x4x4) of finding one with that outcome in a F2 line. So if one wants to find at least 4 different ones to select from they'd need to look at about 256 plants.

The good thing is one wouldn't necessarily need to grow out that many to maturity as it is pretty easy to do some partial selection for that habit in the seedling stage by about 4-6 weeks (throw out the tall ones).

and yes, the sun dwarf gets that name because higher light intensities make for a more compact size when that gene (and/or its alleles) is/are present.

not to hijack the thread, but does that mean that if I crossed microtom (a preferably better tasting one, since I heard microtom doesn't taste very good) it will be more likely to produce a microtom-like dwarf (that tastes better)?

jwwc the answer is no

THIS is stuff they should have covered in first year bio...so much more useful!! and Why not? I want a tasty little tomato plant to grow indoors all year 'round!! =(

make it a large fruit ,small plant, excellent tasting ,heavy and fast producing one that can also babysit when you're gone

Hm...babysitting is rather unnecessary in my case, (no kids) but I'm sure someone's bound to appreciate that special function. =)

jwwc,

Try something like Legend or Siletz. 55-60 days, determinate, doesn't need as much light as others (developed for the Pacific Northwest), parthenocarpic, 6-8 ounce toms that have very few seeds and taste great!

Mike