In certain cases we can like with Down syndrome but there is always a disability associated with it. Why is this the case? What is wrong with having an extra set of instructions? If we can have two copies of a gene, why can’t we have three or four and still function normally?
There is not always some disability associated with it. Depends on the gene/chromosome. There are some genes that you can have multiple copies of and it might increase your chances of certain disorders, but it isn’t guaranteed. And there are lots of people who are XYY who never even know it.
Sex chromosomes are more the exception than the rule. Most other trisomies are fatal in utero. Downs is another rare example of a non fatal trisomy.
Extra copies of genes increase the so-called gene dosage. The simplest result is that having 4 copies of gene x means you’ll have 4 times as protein y, the protein gene x codes for. For many genes, this is no big deal. But for some genes, particularly genes involved in development, this is a big problem. It can throw off the timing and pattern of development resulting in an embryo that just doesn’t grow right. On most chromosomes you’re bound to have at least one of these ‘gene dosage sensitive’ genes.
The table at the bottom of the wiki page gives a good account of the chances of survival for having various trisomies in humans:
Yes, but the OP didn’t make that distinction.
Yes, and note that Downs Syndrome is trisomy 21. Chromosomes are numbered (roughly) largest to smallest, and 21 is one of the smallest*. The larger the Chromosome, the more chance of problems. The Y chromosome is generally considered to be the smallest.
*perhaps the smallest, depending on how you measure size. But one of the smallest no matter how you measure it.
This is fascinating, and touches on a question that I’ve always been interested in for genetics. I know the fairly simple description for how proteins are put together based on chromosomal DNA, but not that much about how a particular strand of DNA off a particular chromosome is… is selected? (wrong word?)… to get proteins made off it at a particular time. There has to be something more than random selection to that, considering junk DNA, cellular specialization, and the idea that in a system as complicated as a living creature, certain enzyme catalysts will be required in greater quantity than others. But I’m not clear on the mechanism, or where to look for more information about it.
chrisk: You might also be interested in reading about Epigenitcs.
chrisk: What you’re describing is part of the vast world of signal cascades. Cells take information from both the outside world and their internal state, and through various mechanisms, transduce it to concentrations of activator and repressor molecules. These bind to regulatory sequences on the DNA, and enhance or repress the transcription of various genes.
On top of that, you have chromatin in eukaryotic cells; this is the scaffolding around which the DNA is wound. Individual nucleosomes have various dynamic post-translational modification marks, which alter how the nucleosomes interact with the assembling transcription complex, the rest of the chromatin structure, and various other systems. This falls under the rubric of ‘epigenetics’ as well.
Also, that ‘junk’ DNA now looks like it’s largely RNA-coding. We’re only now beginning to understand how the endogenous RNA-regulatory system works, but it looks like many genes can be regulated post-transcription but pre-translation via the action of small RNAs. It’s all rather complex.
Or, more generally, gene regulation. This is a HUGE HUGE topic, and one not worth trying to describe in a post. You could spend an entire career on it and still not have the whole thing worked out. Seriously, this is a very large area of research, and one that impacts pretty much every question in molecular and cellular biology.
Forget whole chromosomes, just a little extra genetic material in some places can be quite bad. Look at Huntington’s disease. If you have at most 36 copies of a certain 3 base dna sequence you’re okay. 40 or more and you’re in trouble.
There are a lot of places in cells where the balance between production and destruction of some chemicals is very finely balanced. If the sequence for the production part is on a different chromosome from the destructor part, then an extra of either one is going to be a Bad Thing.
She said it was a good size!
There’s the phrase I was looking for - flew completely out of my head.
Good example for this: one of the symptoms of Alzheimer’s Disease is the deposition of beta-amyloid plaques in the brain. Amyloid beta is a peptide that’s cut from a protein called Amyloid Precursor Protein (APP). The gene that codes for APP is on chromosome 21. It is unsurprisng then, that most individuals with Down’s syndrome (trisomy-21) will develop symptoms that are almost indistinguishable from Alzheimer’s later in life.
(Caveat: We’re not really sure if the beta-amyloid plaques are a cause, just a symptom, or a compensatory measure for Alzheimer’s. Research is ongoing.)
So maybe someone can answer this for me. How come polyploidal plants are so big and healthy yet the same doesn’t work for animals.
Well, yes, but that’s not really the same thing. That’s not about having too many copies, that’s more about one gene’s expression being screwed up. It happens to be that it’s screwed up because of too much DNA, but it could have happened by other mechanisms.
Saint Cad - that’s a good question, and I don’t think we can say we understand it fully. The best we can do is sort of a hand-waving explanation that plants seem to be less complex than we are, so they’re better able to handle these sorts of imbalances than we are. In fact, polyploidy is quite useful in plants in nature as well as in artificial breeding. Seedless watermelons have, IIRC, seven copies of each chromosome, and bananas are triploid. Lots of new plant species seem to have arisen via hybridization of two related species, followed by a chromosome doubling event. Interesting stuff.