How did Humans end up with 46 Chromosomes?

I can understand how a single adventatious gene mutation can be passed along, but how does a species jump from one number of chromosme pairs to the next? Our closest relatives, the Apes, have 24 pairs, while we have 23 pairs, and all I could find on the internet is that 2 of our pairs probably fused into one somewhere back when. separating us from the apes . Since we can’t produce offspring with apes due to the chromosomal difference, how did the first proto-human with 46 chromosomes reproduce to create a new species?

This happens all the time, and it doesn’t mean that the two populations can’t interbreed. Horses and donkeys interbreed even though they a different chromosome count. And it doesn’t mean that the offspring has to be sterile, although that is sometimes the case.

The jury is still out on wether the different great apes (including us) can interbreed. No one has seriously tried it, but I don’t know of any other large mammal species that are as closely related as we are to chimps who have not been able to interbreed, if only under laboratory conditions. And there are many, like camels and llamas, that are more distantly related but which can interbreed.

Human/Great-Ape interbreeding is one of those things which I would never condone, yet which I secretly wish would happen someday because I am just so damn curious about the result.

I’m not a scientist but I have been studying up on this in my spare time. Here’s my attempt at an explanation , as much as I know.

The chromosome fusing happens all the time. I believe the actual chromosome has been isolated, IIRC, chromosomes 12 and 13 in the great apes fused together end to end to form human chromosome 2.

The thing about genome mutations is that they are not completely random. More active sections of the genome are wrapped more loosely and are therefore more prone to mutations. Long sequences of non-coding ( sometimes called junk, but the term is misleading ) DNA can sometimes get entangled with a nearby sequence on another chromosome. Other factors, such as long repeating sequences of base pairs, can also increase the chances of mutation.

The genotype evolves , just like the phenotype. If it is advantageous for a section of the genome to mutate on a regular basis, it will evolve a into structure that is vulnerable to mutation. OTOH, if a mutation to an area of the genome will be consistenly fatal, that area evolves into a stable structure…an example of one of these highly conserved genes would be the HOX genes that control segmentation ( the genes that give us a head sitting on a torso with limbs at both ends )…those genes are virtually the same in all mammal species.

The point here is that a mutation is not always a “one off” deal, if the genome evolves a vulnerablity at a certain point then that mutation can happen with some frequency…maybe not a lot but more than once. While most of these mutants would be non-fertile, if 2 people with opposing genitals and the same mutation were to hook up then they would be fertile with each other and a new species would begin to diverge.

There’s this idea going around lately that a change in chromosome number is essentially the same thing as speciation. That is, if a population gains or loses a chromosome, they are automatically a new species and instantly cannot interbreed with the old population, because they now have a different number of chromosomes. I don’t know where this idea comes from, but it’s completely false. What matters is the ability of chromosomes to pair up with their homologous counterpart during prophase of meiosis I. If they’re able to pair properly, then the DNA can segregate properly into the gametes, and all is well. If they cannot pair, the DNA won’t be distributed evenly, and bad things will result.

What does NOT matter is how many chunks (chromosomes) the DNA is broken into. Let’s take the human/ape change, where apes have two chromosomes to our one. Let’s just say for the sake of argument that all of our DNA is still similar enough to the ape DNA to allow homologous pairing and viable offspring. So what happens in meiosis I? Well, our one large chromosome lines up next to their two smaller chromosomes. There’s homology all the way down the pairing, with a few exceptions, like an extra centromere or telomere here or there, but generally they’re able to line up just fine. The kinetochores are thus able to correctly bind to opposite polar microtubules, meiosis happens, babies are born, and all is well.

Chromosome number is really largely irrelevant to pairing and hybridization. So this:

is incorrect. When closely related species are unable to interbreed, it’s because there’s been some change in some gene or genes that makes them incompatible, or because sequences have just drifted too far apart for homologous pairing, or that sort of thing. It’s generally not because of different chromosome numbers. Now, over evolutionary time, the two species can - and do - mix up their genomes with translocations and inversions and deletions and so forth so that it can be quite tricky to reconstruct with the original ancestral chromosomes looked like, but that’s a secondary event.

A guy at Slate has recently blogged a series about the human chromosomes, here’s the one concerning apes’ 12&13 fusing into humans’ 2. Pretty good series, I suggest reading them all.

The book The Violinist’s Thumb has a lot of really interesting takes on chromosomes and evolution.

One interesting factoid is that there is a family in China somewhere that has 44 instead of 46 - they dropped again sometime and it stuck.

Evil mad scientists must be stopped at all costs, and yet, somehow we don’t have enough of them. I agree with your sentiment.

I heard that Mussolini attempted this but the apes refused when he demanded that any offspring be brought up Catholic.

On a more serious side, each parent donates half of the chromosome pairs. If the mother’s egg has 24 chromosomes, and the sperm has only 23, how can fertilization occur? If it did, wouldn’t the child have 47 chromosomes? Isn’t Downs syndrome the result of an extra chromosome?

I thought that some mad scientist in the Soviet Union had attempted it … yup. And talk about needing a bit of new blood in a name, Ilya Ivanovich Ivanov. Elias John son of John … who named his son Ilya … [I like figuring out what russian names in english are].

Hm, he tried fertilizing chimps with human sperm but couldn’t manage the other way around.

I was gonna say, somebody somewhere has GOT to have tried this… Would it be a humimp?

Or a chiman?

What might be pertinent information to be drawn from DNA studies of Homo sapiens mating with hominids?

[cite from the Slate series referred to above]

Like I said: No one has seriously tried it. His attempts, if the documentation is even true, were laughably amateurish.

Which is why I’ve wanted to see a chimp/gorilla hybrid tried. If that can be done successfully, it almost certainly means humans and chimps could hybridize.

I think you overestimate the politeness and regimentation of the genetic process and fertilization. One theory says that a significant number of humans have too many sex chromosomes, and XYY are overrepresented in prisons. However, many of these men do not even realize they have a distinction. - about 1 in 1,000.

We only notice Downs because it has obvious results.

Ditto. The resulting court proceeding to determin whether the resulting hybrid is a person or not would certainly prove interesting.

It doesn’t cause problems so long as the “missing” chromosome has been completely fused to another chromosome in the other parent. If you think about it, if a chromosomal discrepancy between parents were detrimental, then the human chromosomes could never have fused in the first place. The first time a mutant appeared with two fused chromosomes, all its offspring would have to have been born crippled, since *all *potential partners would still retain the ancestral separate chromosomes. Therefore chromosomal fusion would be impossible.

Yeah, the offspring would have 47 discrete chromosomes, but that wouldn’t cause any problems because the offspring would receive exactly the* same amount of genetic material*… Fusing the chromosomes doesn’t add any or remove genetic material. It just prevents some combinations.

During cell division, the chromosome pairs line up side by side, so that the two copies of each chromosome literally lie next to each other. Like pairs of socks. During the production of sex cells, the chromosomes then get assorted, so that each daughter cell gets one copy of each pair. Even if two chromosomes do fuse, they can still line up in pairs. All that happens is that the “Left hand” side of the new chromosome aligns with chromosome Two, and the “right hand” side aligns with chromosome Three.

Imagine that you have a set of encyclopaedia, with volumes A-B, C-D and E-F etc. If someone re-printed the edition with volumes A-D and E-F, the encyclopaedia wouldn’t contain any more or less information. It wouldn’t confuse a reader at all. Fusion of volumes doesn’t change the information or the sense of it in any way.

Moreover you can throw those volumes into a warehouse with other volumes of the same encyclopaedia, shake the place up, and you can still arrange the volumes in perfect sequence as above. The only thing you need to do is make sure that when you get a A-D volume, you don’t pair it with a C-D or an A-B. But since you know that each encyclopaedia only gets one copy of each entry, that’s unavoidable.

And that’s exactly what happens during fertilisation with a fused chromosome. Each daughter cell only gets one copy of each chromosome from each parent, if it gets A-B from the A-D fusion, it simply doesn’t take a C-D from that parent either. The daughter cell doesn’t get any more or less genetic material. It gets exactly the same amount.

Yes it is. But that’s because the offspring have received a double helping of some genetic material. With a fused chromosome, the portion of genetic material remains identical. Having a fused chromosome doesn’t result in any genetic material being added or removed. Each offspring will only have two chromosomes of each number. The only difference is that chromosomes One and Two of one parent are fused. Just like when you join two volumes together, you still have exactly the same entriesl and exactly the same information. It’s exactly the same length. All that has changed is that it has become fused with another volume. All the information remains exactly the same.

When I took HS biology (1952) I was taught that humans had 48 chromosomes. So I decided that they were called A, B,…,X (the first 24 letters of the alphabet) and then, oh yeah, Y. Now they are simply numbered. But does anyone know if my folk etymology is actually the reason for the names X and Y? Otherwise what is the reason. I know that in species in which the female has the distinctive genes, the system is called W and Z. That is, males are WW and females are WZ. I think some or all species of birds use this system.

I believe it’s because that’s kind of how they look. Most chromosomes (when you get the right kind of cell at the right time that makes it easy to see chromosomes, anyway) look like two fat strands joined in the middle – an elongated X. The human male chromosome, though, has two very short strands joined almost at their ends. Probably more like a ‘V’ or “X with two arms almost chopped off” than a ‘Y’, but there was perhaps a little poetic license to get X and Y.

Damn, not only did you save me from posting this, which I came into the thread to do, but also saved all that google work, because I couldn’t remember exactly where I had read it.