Changes in quantity of chromosomes?

[Cycloneman]

Chromosomes are the organized form of DNA in cells, and there are known differences in the number of chromosomes between species. There is specific evidence to support the idea that a change in the chromosomes is possible, such as the fact that humans have 23 pairs of chromosomes, as opposed to great apes’ 24, combined with the fact that one of the humanity's chromosomes exhibits obvious signs of being two chromosomes stuck together.

But how does this shift work in a population? Two mates having different numbers of chromosomes would obviously make them pretty much incapable of reproducing. So how does that work out?

[Surlethe]

Well, we know it's more than possible for the number of chromosomes to change -- e.g., Down's Syndrome. Most chromosomal mutation in humans is fatal. But there's no reason that every once in a while, a mutation to the number of chromosomes pops up that doesn't harm anything, and permits reproduction.

In fact, talkorigins has something to say:

Chromosome counts are poor indications of similarity; they can vary widely within a single genus or even a single species. The plant genus Clarkia, for example, has species with chromosome counts of n = 5, 6, 7, 8, 9, 12, 14, 17, 18, and 26 (Lewis 1993). Chromosome counts in the house mouse species (Mus domesticus) range from 2n = 22 to 40 (Nachman et al. 1994).

Chromosomes can split or join with little effect on the genes themselves. One human chromosome, for example, is very similar to two chimpanzee chromosomes laid end to end; it likely formed from the joining of two chromosomes (Yunis and Prakash 1982). Because the genes can still align, a change in chromosome number does not prevent reproduction. Chromosome counts can also change through polyploidy, where the entire genome is duplicated. Polyploidy, in fact, is a common mechanism of speciation in plants.

[Alyrium Denryle]

Chromosomes are the organized form of DNA in cells, and there are known differences in the number of chromosomes between species. There is specific evidence to support the idea that a change in the chromosomes is possible, such as the fact that humans have 23 pairs of chromosomes, as opposed to great apes’ 24, combined with the fact that one of the humanity's chromosomes exhibits obvious signs of being two chromosomes stuck together.

But how does this shift work in a population? Two mates having different numbers of chromosomes would obviously make them pretty much incapable of reproducing. So how does that work out?

The problem is not with the number of chromosomes, but with the number of alleles which cause problems in the cell. In cases where a chromosome fusion occurs the ratio of allele copies does not change, so the animal is capable of reproducing.

[Alyrium Denryle]

Well, we know it's more than possible for the number of chromosomes to change -- e.g., Down's Syndrome. Most chromosomal mutation in humans is fatal. But there's no reason that every once in a while, a mutation to the number of chromosomes pops up that doesn't harm anything, and permits reproduction.

In fact, talkorigins has something to say:

Chromosome counts are poor indications of similarity; they can vary widely within a single genus or even a single species. The plant genus Clarkia, for example, has species with chromosome counts of n = 5, 6, 7, 8, 9, 12, 14, 17, 18, and 26 (Lewis 1993). Chromosome counts in the house mouse species (Mus domesticus) range from 2n = 22 to 40 (Nachman et al. 1994).

Chromosomes can split or join with little effect on the genes themselves. One human chromosome, for example, is very similar to two chimpanzee chromosomes laid end to end; it likely formed from the joining of two chromosomes (Yunis and Prakash 1982). Because the genes can still align, a change in chromosome number does not prevent reproduction. Chromosome counts can also change through polyploidy, where the entire genome is duplicated. Polyploidy, in fact, is a common mechanism of speciation in plants.

Yep. having an odd n count (haploid, 3 n, 87 n)will cause sterility because the chromosomes cannot align. But chromosome fusions are a bit different last I checked...

[Cycloneman]

The problem is not with the number of chromosomes, but with the number of alleles which cause problems in the cell. In cases where a chromosome fusion occurs the ratio of allele copies does not change, so the animal is capable of reproducing.

Okay, so a proto-human with 23 pairs of chromosomes that mates with a proto-great ape with 24 pairs of chromosomes can produce fertile offspring?

I thought mating with a creature that has a different number of chromosome pairs (almost always) results in infertile offspring, though. Does that not always hold true? If so, what situations can two creatures with a different number of chromosome pairs mate?

[Alyrium Denryle]

It is not a hard and fast rule. And has, again,more to do with the genes than the chromosomes themselves.

[Superman]

There are a whole host of chromosomal abnormalities that can be seen in humans... and they never work out for the better.

An example would be Trisomy 21; what most of us know as Down's Syndrome. This occurs when the 21st pair of chromosomes triplicates instead of expressing itself as the usual pair. There are some pretty wild ones too, check out conditions like Klinefelter's Syndrome or Turner's Syndrome.

[Superman]

I thought mating with a creature that has a different number of chromosome pairs (almost always) results in infertile offspring, though. Does that not always hold true? If so, what situations can two creatures with a different number of chromosome pairs mate?

Within humans, a person with a normal chromosomal structure can successfully reproduce with someone who possesses a chromosomal abnormality. The chances of the offspring having a chromosomal disorder significantly increases though.

I'm not quite sure what you're asking, but the very definition of 'species' has to do with creatures that can mate and produce fertile offspring. This is not a simple function of the number of chromosomes each animal possesses. A dove and a guinea pig both have 16 pairs, but we've yet to see a "Guinea Dove" or "Dove Pig."

[Zablorg]

If you had a chimp with 26 chromosomes, and it mates with one with the regular 28, wouldnt that produce an offspring with 47 chromosomes?

[Zablorg]

EDIT: *46 and 48 chromosomes...

[Superman]

If you had a chimp with 26 chromosomes, and it mates with one with the regular 28, wouldnt that produce an offspring with 47 chromosomes?

No, those are pairs of chromosomes. Chimps, I think, have 24 pairs (48 in total).

The most common I know of are donkeys and horses. Donkeys have 62 chromosomes, while horses have 64. Cross them and you get a mule, which has 63 chromosomes. 63 doesn't evenly divide, so you get sterile offspring.

[Zablorg]

Yeah, I stuffed up with the chromosome numbers.

The reason I ask is I was thinking about the whole thing with chromosome pair two actually being a fusion of two different chromosome pairs found in many primates.

But I really don't see if that could relly hit off. I mean, if you have this one mutant ape with 23 pairs, it's not going to be able to breed with any other apes that have 24.

Unless it managed to find another chimp with the exact same condition and of a different gender, I'm pretty baffled how this mutation would be able to spread among the community.

[Superman]

But I really don't see if that could relly hit off. I mean, if you have this one mutant ape with 23 pairs, it's not going to be able to breed with any other apes that have 24.

It might be able to. Chances aren't great, and it will increase the chances of producing offspring with a chromosomal abnormality, but this can still happen. We're talking about members of the same species. Nature has a way of selecting these types out though.

Unless it managed to find another chimp with the exact same condition and of a different gender, I'm pretty baffled how this mutation would be able to spread among the community.

One way is through recessive genes and recessive alleles. Members of a species can carry potentially dangerous recessive genes, be unaffected, but produce offspring that are. There are a number of ways that this happens too; incest being one of them.

[Alyrium Denryle]

Yeah, I stuffed up with the chromosome numbers.

The reason I ask is I was thinking about the whole thing with chromosome pair two actually being a fusion of two different chromosome pairs found in many primates.

But I really don't see if that could relly hit off. I mean, if you have this one mutant ape with 23 pairs, it's not going to be able to breed with any other apes that have 24.

Unless it managed to find another chimp with the exact same condition and of a different gender, I'm pretty baffled how this mutation would be able to spread among the community.

It is not, I repeat,not that the chromosomes will not pair up. It is an issue of allele ratios. In the case of an individual with a fused chromosome pair and an individual with no fusion, the alleles are still just fine. The offspring still has two copies of each relevant allele and thus will not suffer from problems as a result of only having one copy of a potentially deleterious allele. IE they still have heterozygosity.

If the chromosomes themselves are different IE they have different genes, then there may be gene interactions or nuclear cytoplasmic interactions, or nuclear-mitochondrial interactions that are deleterious And if they end up with an odd number of sister chromosomes (IE are three N instead of the 2 N) there could be problems, but it is not the number of chromosomes per se, but the homozygosity having unpaired chromosomes causes.

[Alyrium Denryle]

I did forget to mention that there CAN be odd position effects in cases where the location of genes changes....

[Zablorg]

Ah, I see. Thank you both for your info, Supes and Alyrium.

[LadyTevar]

So.... (Rar) ... can a chimp have fertile offspring with a human, since the alleles can match up?

[Alyrium Denryle]

So.... (Rar) ... can a chimp have fertile offspring with a human, since the alleles can match up?

Probably not. There are genetic differences and nuclear-cytoplasmic interactions that prevent that. But they are not necessarily due to the chromosome counts IIRC