...Why are some varieties called 'sports'? ...Is it simply that they're an unexpected result from a crossing that makes them a 'sport'?
Yes, you nailed it. A sport is an unexpected result from a crossing.
In the cast of most of these "Sports" they are just "throw backs" or "atavism" meaning that if you trace back their pedigree far enough you will find an ancestor that had the same trait and looked the same way meaning the trait is simply a recessive gene that has not been seen in many generations.
A true sport is NOT a throwback but rather the result of a genetic mutation meaning that when the trait pops out is it a new gene that was not previously in the gene pool.
In the case of the recessive genes that are "thrown backs" and in the case of genetic mutations, the term "Sport" is used to distinguish the bird as being from parent and grandparent stock that did not express the gene.
This could be sought after in new varieties because the gene pool may be very narrow and new blood from unrelated lines may be the key to keeping the variety from dying out.
Sports could be something to be avoided like the plague in breeds that already have well established and sustainable varieties because the established line has been refined over decades of breeding to weed out genes that are not desirable in the variety and bringing a sport into the gene pool that has not refined to that specific variety could ruin decades of breeding by bringing back in undesirable genes that are required in the variety the sport is from but not in the variety of the sport.
So, Sports are first-generation birds in their variety and don't have any parents or grandparents that share their variety. They are project birds and not an established line.
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