Aneuploidy and Mosaicism

[Procon1234]

Hi guys,

I was told by my teacher to research about Down Syndrome, and it seems that the more I study, the more questions I came up with . So any answers would help.

1. Why is that having more or having less chromosomes in a cell is bad. All of our chromosomes have a copy of each other right? And they code for the same genetic information. If one of the chromosomes mutate, then we would have another backup. Why is that having more or less chromosomes will lead an organism to unwanted results? Do chromosomes of the same type code for the same genetic information?

2. Mosaicism happens if nondisjunction happens during cell division of an embryo. If a normal disomic cell undergoes mitosis, and one of the outcome is a trisomic cell, then the other cell would be monosomic right? So there would be three population of cells?

3. If an embryo is diagnosed with Down Syndrome (Trisomy 21), and during early stages of the embryo, nondisjunction occurs to the cell, and one of the outcome is a normal cell, then the other outcome would be a cell with Tetrasomy 21? What will happen to that cell? Would apoptosis happen?

[Darby]

1. Making proteins depends on feedback between the codes and the production. That's difficult if you have extra or missing codes for proteins that depend on precise amounts to work properly - in Down's, some proteins coded on the 21 chromosome could quickly get overproduced and affect the chemistry of the cell - actually, all of the cells, since the extra chromosome is there from the beginning. There's more to it than that, but that's the simplest explanation.

2. There would be 3 cell populations.

3. Not at all sure. Certainly possible.

[Procon1234]

So if the organism happens to have monosomic 21, disomic 21, and trisomic 21 cells, would that organism show Down's Syndrome?

Thanks for your answer by the way

[kolean]

Apoptosis is what usually happens. For the organism to function properly, the extra proteins being made from the extra chromosome's genes would have to not be important for the development of the organism, nor would they have housekeeping genes (delicate balance when metabolism is concerned).

[Darby]

Since mosaicism involves a limited population of cells using limited groups of genes, the effect of chromosome 21 would be really variable.