Down is the result of the non disjunction in 21st chromosome, so how can it be that some cells have it and others don't?

My 2 year old has downs trisomy 21 and when we were talking about the fact that she had it the doctor said there are different stages.Mosaic is where they have downs but the only way to explain it is that they don't have it in the same way?If it were possible to have downs only partly thats what it could be. They are higher functioning is how one person described it.I am not a doctor that is just what I have heard.

From my understanding, most cases of DS occur from a nondisjunction that occurs in either the sperm or ovum BEFORE fertilization. Mosaicism in Down Syndrome happens when the nondisjunction occurs AFTER fertilization, in one of the first cell divisions. Some studies seem to show that people with mosaic DS function at higher levels than those with the classic Trisomy, but other studies show no difference. For more information, go to www.ds-health.com/mosaic.htm (http://www.ds-health.com/mosaic.htm) where you will find a very good article.

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Actually when the egg is fertilized is when it happens, there is an extra chromosome. That is how they get trisomy 21! I have spent many hours researching this since my daughter was born and I have met with medical genetics at the hospital 3 times since then and this is how they explained it . basically in anycase it can either be genetic from the parents(very rare) or it happens during fertilization.