here's a definition from most genetics type material
A type of genetic cross in which an organism is crossed to a strain from which it was not recently derived.
now of course the gnat frass problem is that thorny word is what does recently mean. since a lot of breeds are "inbred" since the 1750's or whatever start date, and we have only vague attempts to measure relatedness through molecular markers (which is currently insufficient, even though there are a couple of companies selling this), as oppossed to just pedigree's, two supposedly unrelated lines could be closely related, just independently derived, though statistically tougher to do than by two groups raised independently with the same pedigree. a group raised in isolation long enough are called land races, and were normally separated by geography. now we do it by design.
the variability of heterosis may be due to actual relatedness. so if two lines are bred in isolation, and they both "select" for the same genes and they are both homozygous for the same heterosis important factors, when they are combined, there may be little heterosis. this may be why it is important to not have everyone select for the the same kind of cattle, which we are doing for feedlot performance, which is cool that we have local growing conditions that force differences by things like different climate, maturity, fertility, grass, people selecting, etc.
here's a good explanation of heterosis, it's in plants mostly, but has some merit.
http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=540269
a couple of quotes from the paper
Furthermore, the quality of two inbred lines does not necessarily predict the amount of heterosis; this must be determined in a cross. this would account for cattle nicking well. it may not be prudent to obsess about outcrosses to get vigor in breeding cattle. if we continue to "get rid" of diseases like PHA, TH, etc, and there are no deleterous effects, but we just get to play with a better lego set, it might not be bad to get a little more inbred overall.
final comment, really. it is possible with what we are doing, to arrive at the same animal, just with different hair coats. one needs to ask the buyers at places like harris feed yards if the "optimum" animal has changed over time, which of course they won't tell you, because it allows them to make more profit. that's why it's cool to have a little tenderness power and retained ownership once the skin is off if you believe in your product.
