It should be noted that it is well regarded and recognised that hybrid vigour or heterosis effect is 'strongest' or 'highest' in the first generation of progeny produced from the crossing of the two purebred breeds, such as in Labradoodles. That is, when the two parents are purebred (eg Labrador Retriever and Poodle) and are crossbred, their progeny or offspring have the most hybrid vigour or heterosis effect. And this first generation of progeny or offspring are commonly referred to as the F1. And the next generation F2, etc, etc.
If, for example, you were to then breed an F1 progeny to another F1 progeny, then the hybrid vigour or heterosis in the progency or offpsring produced would be significantly lower than in the F1 generation. And the further down the generations you continue (eg F2, F3, etc), the hybrid vigour or heterosis continues to diminish to the extent that it is insignificant.
Some breeders, for example, of mixed dog breeds such as Labradoodles, even further minimise hybrid vigour or heterosis by 'back breeding'. For example, they may breed an F1, F2, etc back to one of the original purebred breeds (eg Poodle). And may even deliberately continue this practice, in attempts to 'strengthen' a desired characteristic in the offspring or progeny (eg the soft curly coat of the Poodle).
However, not only would this significantly reduce and/or remove the hybrid vigour or heterosis (which is one of the main advantages of mixed dog breeds such as Labradoodles), but it would be producing offpsring or progeny which are basically just more and more Poodle, and less and less Labrador Retriever. And if that is what is truly desired, then why not just breed Poodles in the first instance? Rather than creating a mixed breed (eg Labradoodles), which initially has significant benefits from the hybrid vigour or heterosis, but which is gradually reduced or removed through the back-breeding etc?
I have provided various extracts, links etc below that relate to hybrids, and to hybrid vigour or heterosis. And hope that these may assist with the understanding etc of hybrid vigour or heterosis.
Firstly, in a New Zealand government publication(1):
"What is a hybrid?
A hybrid is an organism resulting from a cross between genetically different parents. Hybrids can arise from crosses between closely related species (interspecific hybrids) or by crosses between different types (subspecies, varieties, cultivars) within a species (intraspecific hybrids).
Hybridisation occurs naturally, but it is also widely used in selective breeding programmes for both plants and animals. The mule is an interspecific hybrid between the horse and the donkey, bred to combine some of the favourable characteristics of each parent. Intraspecific hybrids often show "hybrid vigour" (heterosis), growing more vigorously and yielding more than in-bredlines."
Hence, mixed dog breeds such as Labradoodles are 'intraspecific hybrids' (ie are within species breeding) and do show hybrid vigor or heterosis.
In an article titled 'Heterosis Effect, Hybrid dogs',(2) it states:
"Heterosis effect results in a healthier, more vigorous dog with a reduced chance of genetic disease. It is well known in all domestic animal breedings, hybrids 50%-50% mixes of two different breeds will raise the chances of having less genetic diseases because all doubling of detrimental effects will stop in the first generation. The genetic term for this is HETEROSIS EFFECT. This effect often gives non-related individuals stronger descendants than inbreeds."
"Once one goes beyond first generation purebred to purebred, you loose the heterosis effect, which is the goal for most hybrid breeders."
A noteworthy extract from 'Principles of Genetics I', D.H. "Denny" Crews, Jr., Ph.D., P.A.S.; Breeding and Genetics Section, LSU Department of Animal Science, Baton Rouge(3), states:
"Mating of animals which are not alike in their pedigree or that are not related are those which will generally exhibit higher levels of hybrid vigor. Closely related animals do not exhibit high levels of heterosis. "
Also noteworthy is the extract from 'Heterosis and Outbreeding Depression in Interpopulation Crosses Spanning a Wide Range of Divergence',
Suzanne Edmands, Department of Biology, University of Oregon, Eugene, Oregon, (4), which states:
"Relative to parentals, F1 hybrids showed a trend toward increased fitness, with no correspondence with population divergence, and a decrease in variance, which in some cases correlated with population divergence. In sharp contrast, F2 hybrids had a decrease in fitness and an increase in variance that both corresponded to population divergence."
From a United States Department of Agriculture, University of Arkansas, and County Governments Cooperating:
"The highest level of hybrid vigor is obtained from F1's, the first cross of unrelated populations."
"Hybrid Vigor - An increase in the performance of crossbred animals over that of purebreds, also known as heterosis."
From a transcript of the Senate of Australia(5):
"veterinarians recognize the concept of - hybrid vigor," especially in first generation hybrids"
In New Zealand, it was reported that keynote speakers at a series of Lincoln University Animal Industry workshops(6) indicated that:
"Another important but often forgotten principle is what happens when you interbreed the first cross (F1) progeny. In essence if you breed (F1) sires and dams together, there will be less hybrid vigour in the progeny. Like wise hybrid vigour also drops if you breed back to either of the parent breeds."
From a report by the Australian Government's CSIRO(7):
"The benefits of hybrid vigor only appear in crossbred animals that have parents of completely different breeds."
From MSN encarta online:
improved traits resulting from crossbreeding: the increased growth, disease resistance, or fertility seen in hybrid species.'
From Encylopedia Brittanica online:
'Increase in such characteristics as size, growth rate, fertility, and yield of a hybrid organism over those of its parents.'
'Plant and animal breeders exploit heterosis by mating two different purebred lines that have desirable traits. The first-generation offspring generally show, in greater measure, the desired characteristics of both parents. Since this vigour may decrease if the hybrids are actually mated together, the parental lines must be maintained and crossed for each new crop or group desired.'
From Merriam-Webster Online Dictionary:
the marked vigor or capacity for growth often exhibited by crossbred animals or plants -- called also hybrid vigor'
From Britannica online (8):
"also called hybrid vigour, the increase in such characteristics as size, growth rate, fertility, and yield of a hybrid organism over those of its parents. Plant and animal breeders exploit heterosis by mating two different pure-bred lines that have certain desirable traits. The first-generation offspring generally show, in greater measure, the desired characteristics of both parents."