Here is the definition from Wiki:
Introgression is a term used in genetics, particularly plant genetics, to describe the movement of a gene from one species into the gene pool of another by backcrossing an interspecific hybrid with one of its parents. Introgression of a transgene from a transgenic plant to a wild relative as the result of a successful hybridization is an example.
Illustration. You have a Eastern European mouse and a West European mouse. On the boundary between the two species West European females mate with East European males, and the F1 hybrid females mate only with East European males, while for whatever reasons the F1 hybrid males do not mate with anyone (this could be an example where hybrid breakdown occurs on the heterogametic sex, that is, the sex without the redundant chromosome, which in mammals is the male). What you have then is the introgression of West European mtDNA into the East European population, but no concurrent Y chromosomal transmission. Additionally, one could envisage a scenario where the West European mtDNA is selectively favored, perhaps because of better metabolic capacity (more efficient use of energetic resources). Over a few generations the mtDNA could spread throughout the population and fix, while the autosomal DNA remains predominantly East European! (the linkage between the mtDNA and other loci breaks down quickly over a few generations) In other words, even though the mtDNA is replaced the ancestry remains predominantly East European! (I use a European mouse example because something like this does occur in a species of rodent in Europe)
In any case, that's an illustration. But what about an analogy? Well, I got one: Introgression of alleles between populations is like horizontal gene transfer in bacteria! Though the bacterial lineages remain distinct, they swap selectively beneficial chunks of DNA promiscuously. I'll leave it to Chris to judge whether the analogy works....
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Here's a fun article about introgression. Excerpt:
I have often thought with envy about dog breeding at the turn of the Century. This must have been a truly exciting period. It was a time of construction when older breeds and varieties were melded together to make new and exciting ones, each developed for specific purposes. Today, crossbreeding is no longer a recognised option but for the first 50 or so years of this century, crossbreeding to allow the introduction of new or otherwise desirable characters into established breeds was permitted by the Kennel Club, and this too must have been fascinating. I believe that only three backcrosses were needed before registration was again permissable. Nowadays, however, with K.C. registered dogs, pure breeding is the only option. Crossbreeding for any purpose is not considered, and I'm sure that even the idea may be thought of as heresy by most present-day breeders.
During my 50 or so years in Boxers I have, like everyone else, pretty well accepted the status quo. The challenge has been to produce the top class specimen excelling in type, construction and temperament etc.. But, in Boxers, the need to dock tails has, for me, always been an irritant. Why does one have to cut something off to achieve the required overall appearance.
Why, therefore, not develop a Boxer with a naturally short tail - and maybe with erect ears too, if one prefers this image, instead of the cropped ear still largely accepted on the Continent or in American? How easy would it be in any case to introduce a gene from one breed into another?
What about viral and bacterial genetic transference between eukarytic cells?