Trisomy, chromosomes question?

Question

Edward's Syndrome results in the affected individual having trisomy 18. The spindles attached to chromosome 18 break, resulting in one ovum receiving 2 copies of chromosome 18.

How many chromosomes would there be in an affected ovum, and what do you call such an event? How many chromosomes would an Edward's kid have?

and, why do all of the body cells of affected children have the same number of chromosomes?

thanks for helping

Answer 1

well there are normally 23 chromosomes in an ovum, so in this case there's one extra, 24. It is a meiotic non-disjunction of chromosome 18. Cells of affected children all have this extra chromosome as it is replicated as normal, through mitosis.  

Answer 2

Ok, during the production of gametes (egg and sperm) the number of chromosomes is reduced by half, mainly because we all have 2 copies of each type of chromosome, we received one from our mom and one from our dad.

In this case, while the chromosomes were dividing up, one didn't do so.  So instead of having one copy of every type of gene (humans normally have 46 chromosomes, so half of that would be 23 in eggs and sperm) for 23 chromosomes, they're going to have one extra copy of chromosome 18.. for a total of 24 chromosomes in the affected ovum.

Now when the sperm, with 23 chromosomes and the egg with the 24 chromosomes come together, they're going to have a total of 47 chromosomes instead of the normal 46.

The combination of egg and sperm is going to produce one cell that has 47 chromosomes.  When that reproduces to form all the cells of the new individual, it's going to copy all of its DNA.. giving each offspring cell 47 chromosomes.  It's an automatic process and the cell can't "know" better.

Answer 3

An affected ovum would have 24 chromosomes (22 + X or Y + extra 18)

It would be heteroploid (by definition).

In general, the chromosomal arrangement is maintained during mitosis because it does not involve the complicated pairing and crossing of meiosis: chromosomes are copied and the two daughter copies are pulled from the newly formed nuclei, endo of story. However, extra copies do tend to be lost from time to time - for example, many sufferers of Down's syndrome are a mosaic for the trisomy, which is why there is so much difference in the symptoms.