Introduction to Genetics

No prob. Come back to it when you feel you're ready.

Hello Percy. I think I'm getting it about chromosomes, that numbers are shared between different species so can't be used to identify a species without further investigation. Apparently I had a very oversimplified notion about that.You are saying that all individuals of a species have the same number of chromosomes, but didn't someone here say that even within a species some individuals may have a different number? Ignore this if I got that wrong. I didn't review the whole thread. PaulK's link gives the biggest number to a fern called "adders tongue" at a staggering 1440 chromosomes.Now I'm getting more curious about chromosomes. What on earth IS a chromosome anyway? It SEEMS like it's just arbitrary segments of DNA that COULD have been simply one long string instead. Is there any point to its being broken up into separate chromosomes? Do the different chromosomes have some identifiers that give them particular functions? Not sure what I'm asking, it's hard to get the questions clear.it's very interesting that a particular gene is always located on a particular chromosome. Is this true across species in some overall general way, that genes for particular traits that are shared among different species show up on the same chromosome sort of more or less? Meaning for instance eye color which is always the one that comes to mind for some reason, is that gene or set of genes always located on a chromosome that is the same in all species where variations in eye color are a factor?Are my questions making any sense?

Hi NWR,
Yes, I think it's becoming clear that breeds COULD be identifed by DNA but that it would take the building up of a library or database of the identifying sequences for each. It's more a technical problem than a conceptual probleml, as you say.

Hi Faith, glad you are back.Yes, even within a species it is possible for an individual to have more or less chromosomes. See The 44 Chromosome man.It's unusual though.Another great example is that one of the things that makes humans different than our cousins the other primates is that at sometime in the past in an even earlier primate two chromosomes fused together to make one chromosome so that humans only have 23 Chromosomes while our cousins the chimps, orangutans, bonobos and gorillas have 24.Since each individual has two copies that means the normal human has 46, our cousins have 48 and the person in the link above has only 44.You can read more on the fused chromosome in the article on human chromosome 2 at Wikipedia.

Yes, someone did say this, and I have no idea why. While it is possible, it's potential for confusion makes it something that shouldn't be mentioned when merely introducing genetic concepts. I agree. I think you're asking a great question: what is the advantage of dividing a cell's DNA into separate pieces we call chromosomes. Presumably it came about because it provides some advantage (error reduction possibly?) Maybe someone out there knows the answer. It is genes that code for the proteins that produce different functions. Each gene is comprised of a part of the length of the DNA strand that makes up a chromosome. Usually the coiled DNA strand that is a chromosome contains many genes. Checking with Wikipedia, there are many genes that play a role in determination of eye color. Choosing just one as an example, OCA2, for humans it always resides on chromosome 15, and almost always in the same place on chromosome 15. I wasn't able to find any information about where it is in other species. Rather a lot.--Percy

Maybe you answered it and I didn't recognize it, but the question I had in mind was whether the chromosomes might be identified by the particular genes that reside there? They are numbered so that you run across such statements as "such and such a gene on Chromosome 15," so the question is whether this sort of locatability perhaps gives a chromosome its peculiar identity, even a raison d'etre.

If you're asking how you look at a set of chromosomes and tell that this one is #12 and that one is #17, I don't know.--Percy

Faith! Glad to see you back, my favorite adversary. You seem to have a lot of questions concerning chromosomes at the moment. May I offer the humble suggestion you read this book, it would help with some of the answers: Genome: The Autobiography of a Species in 23 Chapters by Matt Ridley.You don't have to buy it, any decent library should have it available for loan for free.One of my top ten, but what would I know about books? P.S. - As a knowledge junkie, I have returned to grad school, most likely on a permanent basis, so when the time comes, I probably won't be around to argue with you. Sorry .

The particular set of genes on a chromosome give it its identity, but there (mostly) isn't anything special about which genes are where. Big chunks of genes can be swapped between chromosomes without it having any real effect on the organism, or example.

Yes, I think that is the question I'm trying to ask. Maybe someone else will have the answer.

This is the sort of frustrating information I've thought might be the case. So you are saying that genes on say Chromosome 12 can trade places with genes on Chromosome 8, I assume during meiosis -- I also assume it would be a comparable chunk size-wise? -- so that pretty much does in any idea that the chromosome can be definitively identified by its set of genes. Or is there nevertheless some rule to this sort of event, rather than the randomness it sounds like?But let me ask: Is this swapping a rare occurrence or fairly common?{ABE} Also, I'm assuming the DNA strand doesn't break for this exchange to take place, break apart and come together again, so it isn't exactly right to say that a whole GENE is being moved, is it? Is it right to say that whatever alleles are getting paired for particular genes are getting placed on the opposite chromosome or something like that?Edited by Faith, : No reason given.

They're numbered in order of length. 1 is the longest, etc, except that the sex-determining chromosomes will be labeled X and Y. (Or sometimes Z and W, to indicate the different sexing scheme that some birds, lizards, and insects have.) Since every cell has at least one of each chromosome, you can lay them out in order of size and just count. That's called a "karyotype." Also, when you dye ("stain") a chromosome, they produce a distinct pattern of banding that can also be used to identify them. For the most part that's species-specific.To address Percy's comment, the reason that I mentioned that some species have intraspecific variations in chromosome count is 1) it's true, and 2) you should get used to the idea that the only absolute rule in biology is that there are no absolute rules in biology. Life exists in infinite and continuous diversity. That's not just some spiritual-sounding mumbo-jumbo. Life really is a lot more varied than we usually imagine.

Hello Adversary Anglagard. Perhaps you'll be able to find time for a zinger or two between your courses.Thanks for the recommendation but I've got such a long list of books I'd like to read and never will, that one doesn''t stand a chance. The point of this thread is to raise some particular questions I've had about DNA for some time now, about DNA in general and not particularly about human DNA if at all.I think the thread really should be titled something like "Questions About DNA" rather than "Introduction to Genetics" which would better describe what Dr. A has in mind for a future project.

Aha, "numbered in order of length." Thanks, Crash, a nice simple direct practical answer.I actually have some questions about this occasional occurrence of a different number of chromosomes in some individuals, but the questions are pretty blurry at the moment, will probably get back to them later.Edited by Faith, : No reason given.

For the most part, genes do not change chromosomes. For example, for almost every human being on the planet gene OCA2 lies on chromosome 15. But the copying and combining of DNA that occurs for sexual reproduction is not perfect, and as Crash says, anything can happen. There no doubt are some human beings for whom the OCA2 gene does not lie chromosome 15, but genetic accidents like this shouldn't be of interest in an introductory course. Gene swapping between chromosomes is rare. Extremely rare. The "jumping genes" that have been mentioned a couple times change location on a chromosome, not between chromosomes. Jumping between chromosomes is not impossible, but it doesn't happen very often. We can give the label Crashfrog's Law to the fact that things that are not impossible will happen at least occasionally. This is a good question that involves the mechanics of protein production and the copying of reproduction. I'd be interested in the answer myself.--Percy