Categories: uncategorized
Date: 19 June 2008 22:21:21
I've had to explain this to so many people lately I thought I'd just stick it here and then I can refer them as necessary!
So. Imagine DNA as written information on paper. This information is organised into cardboard files called genes. The files are kept in big metal filing cabinets called chromosomes.
We have 46 chromosomes, arranged in pairs. Imagine two long rows of 23 filing cabinets each, arranged back to back in pairs.
Now. Lots of things can go wrong with information written on the paper in the files & filing cabinets, in lots of different ways.
The papers within the cabinets might be all wrong, but you wouldn't be able to see that by just looking at the filing cabinets, if the cabinets themselves looked okay. This is why amniocentesis, which looks only at the chromosomes (cabinets) but can't see inside them to the genes (files) will pick up chromosome defects but will not see genetic conditions such as cystic fibrosis.*
There are several ways that things can go wrong with the filing cabinets. Sometimes there is an extra cabinet where there shouldn't be - this is called Trisomy. Imagine your two lines of 23 - if, at the 21st pair, there were three cabinets instead of two, that would be Trisomy 21 - Down's Syndrome.
Sometimes a filing cabinet is missing a drawer. Sometimes a drawer is missing from one cabinet and has somehow attached itself to another.
Because the filing cabinets are quite big and clunky, they are reasonably easy to see. They are in the amniotic fluid which surrounds the baby and that's how we can look at them before birth, by taking a sample of the fluid.
And this is how you explain the above to the three and a half year old. In case you ever need to, which I hope you don't:
It has been previously established that for baby Ginger, being inside mummy is like being in a lovely warm bath, which is just the right temperature and never goes cold, and she can splash and kick about in it, and still breathe because she has a special tube. Because we can see that some bits of baby Ginger aren't growing quite right inside mummy, the doctors need to have a look at her bathwater and check there is nothing nasty in it which is making Ginger poorly. So mummy has to go to hospital and have something like a big injection, and the doctors will take some of Ginger's bathwater and put it in a special machine to look at it.
I thank you.
*bear in mind also that although our genes obviously are to do with passing things on to our children, 'genetic' in this context simply means something to do with the genes and not necessarily an inherited condition.