Our genetic information, our DNA, is conveniently packaged
into little bundles called chromosomes.
Humans have 23 pairs of them, 46 altogether.
They contain the instructions that the body uses to build
itself, the recipe for a human, if you will.
Accidentally duplicating copies of the DNA instructions
can cause problems later on.
That's what happens in Down's syndrome, a genetic condition
that causes developmental and physical disabilities.
It's a result of accidentally inheriting additional copies
of some of the DNA instructions.
Down's patients typically have an extra chromosome 21.
So instead of 46 chromosomes, they have 47.
Where does this extra chromosome come from?
It happens during the formation of sperm and eggs, a
process called meiosis.
During meiosis, your 46 chromosomes are shared out so
that only half of them, 23, go into each sperm or egg cell.
I'll show you using just one of the sets of chromosomes,
although actually it's happening to all
23 at the same time.
Now originally, one of the pair will have
come from your father.
Let's say it's this blue one.
And the other one will have originally
come from your mother.
Let's say it's this red one.
First of all, each chromosome makes a copy of itself.
It duplicates, making four altogether, two stuck together
from the father and two stuck together from the mother.
These then move to the middle of the cell, where they match
up and almost wrap around each other.
It's then that something called crossing over occurs,
where genetic material is exchanged between chromosomes.
This mixing process can generate new
combinations of genes.
It's one of the most vital features of sexual
reproduction.
Now the chromosomes are pulled in opposite directions by
fibres that come from the sides of the cells.
They're pulled apart once and then once again.
And the four separate chromosomes go into four
separate sex cells.
Remember, this is actually happening with 23
pairs, not just 1.
And so each sex cell ends up with 23 chromosomes.
In males, those all go on to become sperm cells.
In females, it's slightly different.
Only one of them goes on to become an egg cell.
During fertilisation, you get 23 from the male sperm and 23
from the female egg, making 46 in total again, 23 pairs.
Down's syndrome occurs when during this pulling apart
process of meiosis something goes wrong with
chromosome number 21.
Instead of the four chromosomes separating and
being pulled apart, at some point two of them go together.
And both go into one sex cell.
During fertilisation that sex cell fuses with another one to
form an embryo.
It means that there are three copies of chromosome 21,
rather than the usual two.
In total, the embryo has 47 chromosomes.
Chromosome 21 being the shortest human chromosome, the
foetus can normally survive.
If this sort of thing happens to other chromosomes with more
genetic information, the outcome is usually fatal.
The intricate mechanics of how our sex cells are created can
make the difference between life or death.