Re: Differences in mental and physical abilities with Down Syndrome.

This is a very good question.  As you may know, Down syndrome is the result 
of trisomy 21, that is to say, that the affected person has 3 copies of 
chromosome 21 as opposed to the normal 2 copies.  Down syndrome caused by a 
14/21 translocation is a rare event, though it does occur.  You are 
partially correct when you suggest that the father of the affected 
individual (and other relatives) have the same translocation, however, they 
do not have the same exact genotype as the nephew because the nephew has an 
extra copy of chromosome 21.  Here is how it works.
The 14/21 translocation is what is known as a Robertsonian or centric 
translocation.  This occurs when acrocentric chromosomes fuse.  Acrocentric 
chromosomes are those chromosomes which have their centromere near their 
end instead of in their middle (there are 5 acrocentric chromosomes: 13, 
14, 15, 21, 22).  Thus a person who has a 14/21 translocation has one 
normal chromosome 14, one normal chromosome 21, and one fused chromosome 
14/21.  Since the fusion doesn’t result in any appreciable loss of genetic 
material, the translocation is termed "balanced" and it causes no 
phenotypic (i.e. outward appearance) abnormalities.
However, people who have these types of translocations often produce 
unbalanced gametes.  This is because, instead of having 4 total chromosomes 
containing the genetic material for 14 and 21, they only have 3 
chromosomes.  Therefore, their gametes (sperm and eggs, which only contain 
one set of chromosomes - remember, most cells of the body contain two sets 
of chromosomes) may contain several variations of these three chromosomes. 
 In a normal individual, each gamete would contain one chromosome 14 and 
one chromosome 21.  When gametes are formed in 14/21 translocation 
carriers, different possibilities exist, as illustrated in the accompanying 
figure.  One possibility is the formation of a gamete with one normal 
chromosome 21 and the fused 14/21 chromosome.  If this gamete is fertilized 
by a normal gamete, then trisomy 21 results.  As you can see from the 
diagram, approximately 1 out of 6 offspring of a person with a 14/21 
translocation will have trisomy 21.  Therefore, you can see how a 
grandmother and father are "normal", while the son has Down syndrome.