As you’re probably well-aware, our DNA is like this humongous blueprint of information on how to make a human.
Usually this massive document is packaged up nicely into a storage bin called a chromosome.
Actually, usually we have 46 chromosomes that we use to neatly organize all our information, depending on how you define organize.
Each of the 46 chromosomes is actually part of a pair of chromosomes, since you get one from each parent, so 23 pairs.
If you wanted to make another human, first you’d have to find someone that feels the same way, and then you both contribute half of your chromosomes, so one from each pair, right? Fifty-fifty.
Now, what if someone contributes one too many? Say Dad contributes 23 and Mom contributes 24, is that possible?
Yes, and it’s the basis of one of the most common chromosomal disorders—Down Syndrome.
Someone with Down syndrome has 47 chromosomes instead of 46, specifically they have an extra copy of chromosome 21, so instead of two, they have three, so Down Syndrome’s also known as trisomy 21, in other words, “three chromosome 21s”.
Alright, so in order to package up half the chromosomes into either a sperm cell or an egg cell, you actually start with a single cell that has 46 chromosomes, let’s just say we’re making an egg cell for the mother, I’m just going to show one pair of chromosomes, but remember that all 23 pairs do this.
So the process of meiosis starts, which is what produces our sex cells, and the chromosomes replicate, and so now they’re sort of shaped like an ‘X’—even though there are two copies of DNA here, we still say it’s one chromosome since they’re hooked together in the middle by this thing called a centromere.
OK then the cell splits in two, and pulls apart the paired chromosomes, so in each of these cells you’ve now got 23 chromosomes.
Now the two copies of the chromosome get pulled apart, and the cells split again, which means four cells, each still with 23 chromosomes.
Now these are ready to pair up with a sperm cell from dad that has 23 chromosomes as well, totaling to 46 chromosomes, and voila–nine months down the road you’ve got yourself a baby.
Now, with Down Syndrome or trisomy 21, a process called nondisjunction accounts for about 95% of cases.
Non-disjunction means the chromosomes don’t split apart.