Can a Down syndrome patient produce gametes ????

 

And which step of the Meiosis they fail to undergo ??

yes, they can. Down syndrome occurs when there is nondisjunction of the 21st chromosome during meiosis. And that's in the formation of the gametes of the parents of the Down syndrome patient, not in the person themself.

are you shure? trisomy at chromosome 21 is a cause of downs syndrome, in which case I would have thought that meiosis would fail at a ceckpoint due to the inability of an odd-number of chromosomes to segregate evenly?

Meiosis would tend to fail. However, Down syndrome patients are apparently fertile, though at a much greater risk of passing down the trisomy than the population at large (40% vs <1%).

 

I've always kind of wondered (though I would never dare to ask) if people with Down syndrome are aware that they are missing anything in terms of the normal human experience (if indeed they are missing anything). I have a cousin with Down syndrome and he is one of the most consistently happy people I have ever known.

but the point is there is 47 chromosome in the cell of the down syndrome patient ... during prophase I in meiosis ,

 

1 of the chromosome cant pair up ... the chromosome is not homologous !!!

 

 

I think that meiosis cant proceed and no gamete is formed !!!!

but the point is there is 47 chromosome in the cell of the down syndrome patient

 

Note that, afaik, trisomy-21 isnt the only cause of downs syndrome. The actual cause is likely to be over-expression of a/some gene(s) on chromosome 21. trisomy at that chromosome would obviously cause this, but variouse other things to do with the promoter for these genes would also cause it.

 

In other words, not everyone with downs syndrom is going to have trisomy, so even if trisomy does screw up meiosis, that doesnt mean that all downs syndrome people are going to be infertile.

Down syndrome is characterized by a specific phenotype including subfertility or sterility and hypogonadism in males. In contrast, several females with Down syndrome have borne offspring. Here, a male with trisomy 21 fathering an infant is described. This observation is verified by serological markers, DNA fingerprinting using different DNA micro- or minisatellites and andrological investigations.

 

http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=7834902&query_hl=5&itool=pubmed_docsum

Trisomy greatly reduces fertility but does not necessarily make an individual sterile. Whether or not the individual is fertile is very much due to a genetic disposition.

 

For example: In watermelons seedless varieties are created by crossing a diploid with a tetraploid line (colchicine induced). The resulting triploid is mostly sterile so the seeds never form. However have you ever gotten a black seed in a supposedly "seedless" watermelon. This means that somehow the plant was able to overcome the trisomy of all chromosomes (11 pair) to produce a viable seed (they use a diploid pollen source).

 

Mammals are much more strongly affected by polyploidy and most are sterile. However once in a while there is an individual that is fertile (mules, Beefalo's, and other interspecific hybrids)

 

Basically what happens during meiosis is that the majority of gametes fail and die early on because the chromosomes fail to line up. But once in a while in some individuals one side will recieve one chromosome and the other will recieve two. This means that the offspring has roughly a 50:50 chance of having trisomy with no other factors.