Apes have 48 Chromosomes, humans 46.  So wouldn't the first human (with 46) have significant trouble reproducing?  Could he or she reproduce with an ape mate?  And if so, would the human count be transferred to the child?

How frequent are chromosome count reductions?  Is there any reasonable probability for finding a mate that randomly was born with 46 compatible chromosomes?  Or is the probability far greater that the first human wouldn't reproduce at all or that his offspring would revert to 48 chromosomes?

Cell reproduction begins at the centromere, right?  So in reproducing the new, long human chromosome (what is it?  chromosome 2?) with an ape mate, wouldn't the child get only one or the other of the ape chromosomes (what?  ape chromosome 13 or 14?)?  And if so, wouldn't this child have a very severe, probably lethal, birth defect?

What causes infirtility in hybrids?  Wouldn't the first humans be infirfile?

Wow,  I count 14 question marks in one post.  Is that a record?

I'll try to hit as many of the question marks as possible.

1. So wouldn't the first human (with 46) have significant trouble reproducing?  Not necessarily.  Research has been performed to investigate just this question and found that fertility is effected only minimally.  Essentially, it turns out that some centromeres are 'better' at attracting the kinetochore machinery than others and thus outcompete the neighboring centromere for resources [a].  Thus, even in the case of a fusion, only one centromere will remain active.

2.  Could he or she reproduce with an ape mate?  No.  Nor would they likely be interested in doing so - any more than you are interested in mating with a gorilla.  When a chromosomal fusion occurs in one individual human, they would look no different than any other human.  All the same genes are still there, being expressed the same way.  When the fusion of chromosomes 12 and 13 occurred in the human lineage, nobody would have noticed any major difference in the individual in which it occurred.  Also keep in mind that this fusion occured AFTER the divergence from the common ancestor with chimpanzees.  If it didn't chimpanzees would probably also have the fusion.  Humans had already embarked on their own evolutionary trajectory.

3. And if so, would the human count be transferred to the child?  Give the answer above, this question is no longer relevant.

4. How frequent are chromosome count reductions? I don't know the answer to this right off hand but one paper I found suggests that it is rather often, "The Robertsonian (Rb) fusion, a chromosome rearrangement involving centric fusion of two acro-(telo)centric chromosomes to form a single metacentric, is one of the most frequent events in mammalian karyotype evolution."  [c]

5. Is there any reasonable probability for finding a mate that randomly was born with 46 compatible chromosomes?  That depends on the population size in which one was looking for a date.  If it's very large, the probability is small and vice versa.  However, given the answer to question one, it doesn't matter because there would be little trouble finding someone with whom to successfully reproduce.

6. Or is the probability far greater that the first human wouldn't reproduce at all or that his offspring would revert to 48 chromosomes? See answers to question 1 and 5.

7. Cell reproduction [I assume you mean DNA repliction] begins at the centromere, right?  No.  In eukaryotes including mammals and humans, each chromosome has multiple origins of replication and they are not in the centromeres.  DNA replication begins at many, many places on the chromosome.  If you didn't mean to say 'DNA replication'  this question doesn't make any sense.

8, 9, 10, 11. So in reproducing the new, long human chromosome (what is it?  chromosome 2?) with an ape mate, wouldn't the child get only one or the other of the ape chromosomes (what?  ape chromosome 13 or 14?)?  Again, based on earlier answers, these aren't valid questions.  There is no major decrease in fertility due to a Robertsonian fusion.  The offspring of a person with the fusion (one chromosome is fused, the other isn't) and one without would have a 50% chance of inheriting the fusion.  Assuming they did, they would have a 50% chance of passing it on.  Eventually many in the population would have it and it could spread throughout the population and become the norm.  As is apparently the case, this is what happened in the ancient human population.   And yes, it's chromosome 2.

12. And if so, wouldn't this child have a very severe, probably lethal, birth defect? No, see answers to 1.

13. What causes infirtility in hybrids?  This is a separate question but ok.  Hybridization occurs when two populations that have become genetically distinct encounter one another and attempt to produce offspring.  The key there is that they have become genetically distinct.  They have adapted to different lifestyles and their genomes have changed accordingly.  When those two populations produce offspring, the offspring will be trying to mix and match genes from two different pools that may or may not work together.  Hence, in some cases, you'll get infertility.  In other cases you actually get offspring with greater fitness (hybrid vigor).  

14. Wouldn't the first humans be infirfile?  It wasn't the chromosomal fusion that made us human.  We were already on a separate evolutionary path from the rest of the apes when the fusion occured.  

You seem to be under the impression that a single human-appearing human was born into the middle of some ancient population of apes and that this person was human because he/she had a chromosomal fusion.  This is simply not what evolutionary theory suggests.  First, as I explained above, it isn't the fact that we have one less pair of chromosomes than apes that makes us human.  We just happen to have 23 pairs of chromosomes.  We would still be human even if the fusion hadn't happened because its the content of the chromosomes that is being selected for, not the number of chromosomes themselves.  What the theory does suggest is that there was a common ancestral population of ape-like creatures that existed before there were humans and chimpanzees.  That population likely became divided for some reason - migration, climatic change, geographical change, whatever.  When those now separate populations adapted to their now distinct environments, they were subjected to different pressures that shaped their genomes in different ways.  It appears that one population became adapted a lifestyle on the savannah and eventually developed into humans over many, many generations.  The other population became adapted to a lifestyle in the forests and eventually developed into the two current species of chimpanzees.  At some point in the history of the population that would go on to become us, somebody was born with a chromosomal fusion that had minimal impact on their ability to reproduce.  They looked no different from anybody else.  They reproduced and about half of their kids had the fusion as well.  Those kids had kids and the fusion spread throughout the population as they did so.  Eventually, the fusion became the norm for humans.  

I hope this helps.  References are below.

a. Sullivan BA, DJ Wolff, and S Schwartz (1994). Analysis of centromeric activity in Robertsonian translocations: implications for a functional acrocentric hierarchy. Chromosoma 103(7):459-67
b. Ijdo JW, Baldini A, Ward DC, Reeders ST, Wells RA, Origin of human chromosome 2: an ancestral telomere-telomere fusion. Proc Natl Acad Sci U S A 1991 Oct 15;88(20):9051-5
c. Slijepcevic, P (1998) Telomeres and mechanisms of Robertsonian fusion. Chromosoma 107:136-140

Good thing I hit the 'preview' button! I had a reply written up, but David wrote a longer and better one than mine before I had a chance to post it.

I would add just one statistic that I found via Wikipedia that relates to question 4 (How frequent are chromosome count reductions?): "About one in a thousand newborns has a Robertsonian translocation" (E. Therman, B. Susman and C. Denniston. The nonrandom participation of
human acrocentric chromosomes in Robertsonian translocations. Annals
of Human Genetics
1989;53:49-65.)

Also, I found a great article that explains it again but with diagrams:
http://www.thetech.org/genetics/news.php?id=124

That link is awesome.  It helps to makes the point I was trying the get across crystal clear.  That is, just because someone has a fused chromosome, it doesn't make them substantially different from anyone else, physically.  He hasn't passed from being human into being something else, he "just has [his genes] packaged a bit differently."

Thanks.

"So wouldn't the first human (with 46) have significant trouble
reproducing?  Could he or she reproduce with an ape mate? "

Just to add a bit more, this kinda implies that you go from an ape to a human in a single generation. Aside from the stuff disucssed above, the chromosonal changes would be one of a great many. Gorialls don't give birth to humans. There would be a great many changes in dozens or even hundreds of geners over hundreds or evene thosands of generatiosn to get from something more like a great ape to more like a modern human (let's not forget things like Australopethicus and neanderthals etc.). There's a whole lienages and changes over time to get from one species to another and it affects whole populations - dramatically different new 'species' don't appear from one odd generational change.

(Note. Actually this does kinda happen in plants sometimes, but not in mammals).