h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Us humans have 23 pairs of chromosomes, and great apes have 24. What I want to know is, how do you lose a chromosome and still interbreed with others who still have an 'extra' one?

Presumably a few million years ago a chromosome got lost, and it wasn't damaging enough so that the individual it happened to still managed to reproduce. Fair enough. When s/he went about shagging though, his 23 chromosome sperm met a 24 chromosome egg, or vice versa. How's it possible for this to be viable?

The number of chromosomes in species is highly variable and not anything to do with the complexity (or lack of) of the animals.
There are fish and algae with far more chromosomes than us for example.

Here's a nice list.
http://en.wikipedia.org/wiki/List_of_number_of_chromosomes_of_various_organisms

A biologist may come along and correct me but I don't think we have any idea why this varies so much.

Also remember that about 95% of DNA in a cell is 'junk' and not used for any function that we know of.
So a species could junk most of its DNA theoretically with not loss of function.

What I mean is how does a homo erectus (or whatever) with 46 chromosomes get them to pair off with a sperm/egg with 48?

More generally, won't this make it really hard for the number of chromosomes to change in a species at any time? Yet as Mal says, there'a huge variation of chromosome number so it can't be a big problem on a long enough timescale anyway. So what gives? Is it actually quite easy to change then?

i'm no evolutionary biologist but i'm fairly certain evolution doesn't happen quite like that... more likely a population with 46 functioning chromosomes and one mis-functioning one became isolated and the essential roles of the mis-functioning one were taken over by genes in other chromosomes. then over time the population would become dominated by individuals with just 23 pairs...

hmm not sure anyway i am sure it was more complicated than a single mutation becoming a new species.

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That's what I'm getting at. How can individuals with a different number of chromosomes produce a functional embryo? There must be a period when that happens relatively frequently.

I´d imagine the chromosomes would not just disappear, but maybe shorten over time.

The simple answer to pedro's question is that if a genetic mutation produces an individual with a different number of chromosomes then he/she will NOT be able to reproduce. Unless, that is, he/she finds another genetic mutation with the same number of chromosomes.

Evolution in action, ladies and gentlemen.

B

Well, how do species have such a variable number of chromosomes? Presumably losing or gaining a chromosome is quite rare, so breeding with another mutant will be even rarer. Yet the number varies widely and seemingly randomly. What gives?

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i thought the cause of downs syndrome was an extra chromosone

regarding the quote

how does that work?????

Well, if 23 Chromosomes don't pair up with 23 other chromosomes, then the fertilised egg will simply fail to make copies of itself, and won't make it to the embryo stage.

B

That's what I thought, MB, but the fact that we have 23 pairs while the other great apes have 24 means that it does happen every so often. For it to spread through even a small population must (surely?) mean that 24 into 23 does go sometimes.

Mustn't it?

Yup. That's called evolution.

With all due respect to individuals suffering from Down's, they are unlikely to become a dominant species, which marks them out as a bit of an evolutionary dead end. Every now and then, though, a genetic anomaly will allow a species an advantage which will encourage further reproduction, etc etc (cf. Darwin)

B

And getting back to the original question, how does the number *change*?

A genetic accident. Creation of new cells is not a robot production line and cell DNA is not always perfectly copied. A whole chromosome could easily be 'mislaid' in meiosis.

I think the number is about 40% that actually a genetic defect of some kind. A lot of those don't survive and are re-metabolised by the body, but sometimes one or two do.

B

Down's is a slightly differnt question to the one regardign apes though, (If I remember correctly, which is, err doubtful)... its not that the egg or sperm has the extra copy of chrosome 23, but rahter a duplicated copy of that chromosome gets made at some point in the bubsequent divisions of the fertilised egg.. but fairly* early on in the diviosn so that most of the cells consequently have three copys (or I believe more than three).... A lot of embryos which have sizible chromosomal mutations are aborted...
I think with the ape issue, it may be one case where it occured as a gradule thing, shrinking Chromosome rather than perhaps it just utterly disappearing...
I also seem to recall that par tof the explination for the huge differnces in number of chromosomes between differnt species is through sideways interventions, such as viral or bacterophage DNA getting incorperated into existing Chromosomes, or existing Chromosomes due to their DNA sequence at a given point being inheriantly unstable; and effectively splitting into two Chromosomes...

I've a funny feeling this came up during the Dover trial.

I think it was used as evidence of a testable hypothesis that supported evolutionary theory

A chromosome has telomeres at either end, it was though one of these chromosomes in the evolution of human from the common ancestry shared with other primates, that one chromosome essentially joined with another, forming a longer chain with two telomeres at either end still. the hypothesis was that if this were true there would be a unique signature
in the middle formed when the telomeres at the opposite ends of the original separate chromosomes merged.

Until the advent of molecular genetics such a hypothesis wasn't testable but if true it would support the idea of micro evolution that distinguishes us from our closest lviing relative by common descent owing to evolution occurring.



The telomere marker it was claimed is there (I cannot recall on which chromosome alas) and that this was real scientific evidence of evolution which was used to dismiss the claims on intelligent design and Behe's idea of irreducible complexity since it could be shown through hypothesis prediction and experiment that the similarity and difference between chromosomes of two species had a natural explanation concomitant with the predictions of Evolution by natural selection.

See:
http://www.youtube.com/watch?v=zi8FfMBYCkk

Ken Miller on Human Evolution (with reference to the Dover trial)

http://www.talkorigins.org/faqs/dover/day1am2.html

Same guy's testimony from the Dover trial: The bit I'm referring to starts at Question 14 put by counsel to Dr Miller.

I'm still not sure how to answer Pedro's actual question though.

I think your previous post was the kind of answer I was looking for Clive. The how not the why. Cheers