Patau syndrome (Trisomy 13)

Patau syndrome is a chromosomal disorder where a person inherits an extra copy of chromosome 13, or a part of it.

So instead of having two, they have three copies of chromosome 13, and so Patau syndrome is also known as trisomy 13.

Patau syndrome is named after Dr. Klaus Patau, who first described the chromosomal nature of the syndrome.

All right, our DNA is like this humongous blueprint of information on how to make a human.

Usually this massive document is packaged up nicely into storage bins called chromosomes.

Actually, we usually 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 trisomies, in other words, having three copies of a particular chromosome.

Trisomy 13, or Patau syndrome, is the least common, but most severe trisomy in live births.

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 then splits in two, and pulls apart the paired chromosomes, so in each of these cells, you now have 23 chromosomes.

Now the two copies of the chromosome get pulled apart, and the cells split again, which means four cells, each cell still with 23 chromosomes.

Now these are ready to pair up with a sperm cell from dad that has 23 chromosomes as well, totaling 46 chromosomes, and voila–nine months down the road you’ve got yourself a baby.

OK, let’s now see how someone can end up with three copies of chromosome 13 instead of two.

Well, with Patau syndrome, or trisomy 13, a process called nondisjunction accounts for most of the cases.

Non-disjunction means the chromosomes don’t split apart.

If the chromosomes in this first step don’t split apart, then one cell ends up with both chromosomes, and the other gets none.

Then the final result is 2 cells with an extra chromosome, and two cells missing a chromosome.

Nondisjunction can also happen in the second step, so the first steps have no problems, and both cells have a chromosome, but if they don’t split apart in the second step, then the final result is one cell with an extra chromosome, one cell missing a chromosome, and two with the correct number of chromosomes.

Now, if a sperm cell combines with any of these eggs that have a duplicate of chromosome 13, then the combined cell will have three chromosome 13s, or trisomy 13.

Apart from nondisjunction, Robertsonian translocation accounts for a small percentage of trisomy 13 cases.

Translocation means that a part of one chromosome switches places with a part from another chromosome.

One of these chromosomes needs to be chromosome 13, but the other can be any other chromosome.

In this example, the long arm of chromosome 13 translocates over to chromosome 14, and you end up with one hybrid that has both long arms, and one hybrid with both short arms.

This guy with the short arms carries nonessential genes, or genes that are not essential to survive and is typically lost by the end of meiosis.

All right, let’s say these cells replicate and split into one with both normal chromosomes, and one with a chromosome with only long arms and one with only short arms, in which case after splitting again you’d have two normal cells, and two cells with a big guy, since we lost the little guy along the way.

So after contributing the other parent’s DNA, you’ve got two normal cases.

These two cases are “balanced carriers”, and we say they’re balanced because they’ve got both long arms, and so most of the genes are still here.

So balanced carriers usually don’t have any symptoms.

OK, now let’s say the normal chromosome 14 ends up with the short arm, and normal chromosome 13 with the long arm.

Now you get two cells with the normal and long-arm, and two cells with the normal and short arm, which remember is usually lost.