Re: why the young ones produce by hybridization could not produce a young one?

Dear Dimple

You ask a very interesting question, but the answer depends on exactly 
what kind of hybridization has been used.

The most common form of hybridization does not involve two different 
species. Rather, it is a cross of two different varieties, or lines, of 
one species. This is commonly used in plant breeding to produce so-called 
F1 hybrids (F1 because they are the 1st Filial generation). Usually, the 
parent lines of an F1 hybrid are themselves inbred and uniform. They are 
homozygous for all genes, meaning that they have two copies of the same 
allele for every gene. When the two lines are crossed, their offspring 
inherit one allele from one parent and one allele from the other, for 
every gene. The offspring are usually very vigorous and very uniform, 
which are traits that intensive farmers and growers want. F1 hybrids often 
do produce offspring of their own, but these offspring will contain a 
mixture of alleles from their parents, and so they will not be uniform and 
some may not be as vigorous as the parents. So if the farmer or grower is 
looking for the same qualities as he or she had in the F1 hybrids, those 
qualities will not be there. That is why people are told they cannot save 
the seed of F1 hybrids. The seeds may be perfectly good, but they will not 
retain all the qualities of their parents.

Other kinds of hybridization, which do involve two distinct species, may 
or may not permit the production of fertile offspring. Often, when the two 
species are animals, the male may be fertile and the female sterile. This 
is often the case with mules, for example, which are a cross between a 
horse and a donkey. In many plants, interspecies hybrids are perfectly 
fertile. Other crosses are not fertile, or not very fertile. Usually, the 
problem in these cases is that the two sets of chromosomes, one from each 
parent, are too different to allow them to align during meiosis. This 
means that no gametes are formed.

I realize that this is a complex answer, but as I said, you posed a 
question that is actually rather difficult without knowing more about the 
specific cross you have in mind.

Best wishes and good luck

Jeremy Cherfas