Thread: Question about Chromosome Numbers

If we have 46 chromosomes and all the other primates have 48 chromosomes then there must have been a point during our evolutionary path that two chromosomes fused together to form one chromosome (from what i've read it is apparently chromosome 2). If this is true then the fusion of these 2 chromosomes must have been a random mutation according to darwinian evolution. However if this new mutant hominid had any offspring with any of the other hominids around it that still had the 'normal' chromosome number then it's children would be infertile (as in the cases of mules were the horse has 64 chromosomes and the donkey has 62 chromosomes giving an offspring of 63 chromosomes that does not pair properly and connot divide) So if the children where infertile then there is no way this mutation would suceed. The only way this could happen is if a large number of the hominidsall mutated within the same generation and were all in the same geological area and decided to mate with each other. The path of human evolution was very rapid in evolutionary terms and I think the odds of this random mutation happening lots of times within the space of about 20 years very slim indeed.

So my Questions are:

1. How do to chromosomes fuse together?
2. What is the evolutionary advantage of having less chromosomes
3. how did any offspring of the new mutant hominid born without being infertile.

I don't remeber the specifics of meiosis to answer number 1 completely, and I also don't have a good enough understanding of problems arising from genetic flukes to answer number 3...

2) Usually, large chunks of information are not lost, so all (or most) genes would still exist but would be moved around. This could jumble information quite severely...causing a sudden change. This could represent a significant jump in human evolution or it could just be a mostly-silent occurance...As for the advantageousness of any given number of chromosomes, there is no advantage to have any particular number of chromosomes. Look up chromosomes on wikipedia and you'll find that ferns have 1200, tobacco (cultivated) has 48, butterflys have 280, and fruit flies have 8. Each of these organisms is well suited for its evolutionary purpose based on its gene's functions, not the number of chromosomes.

-Ajain

interesting post and something I was not aware of (i've just had a quick scan of a few webpages on the topic).

My guess at why two chromosmes would fuse is that a recombination event may have occured during meisosis, but one involving a similar sequence in two non-homologous chromosomes.

An interesting point is made on the wikipedia chromosome 2 page that there is a short sequence at the join site which is from a different chromosome (ie not found in either of the two primate chromsomes). it says addition of the sequence probably just preceded the fusion event (in the same animal over several generations?). This probably precipitated the fusion event because telomeres, the normal ends of chromosomes, are specifically 'designed' not to fuse to one another easily (for obvious reasons).

are fewer chromosomes a selective advantage? who knows? but the 'new'sequence between the two fused sequences has several genes which could have been selected for (including one for vit b12 which may be involved in brain development, see wikipedia article).

differences in chromosome number do not necessarily lead to infertile offspring see: (http://www.gate.net/~rwms/hum_ape_chrom.html). this says wild (66 chr) and domestic horses (64 chr) can interbreed succesfully with fertile offspring.

I'm not sure whether a fused chromosome 2 would be able to undergo homologous recombination before meiosis with two unfused ones (2a and 2b), but the main problem I forsee would be during chromosome segregation. Two active centromeres on a chromosome might lead to it being 'pulled' in two directions. The solution is to silence or supress one centromere but the mechanism of this is unclear. this does appear to have happened with chromosome 2.

If a fused chromosome has a selective advantage over unfused ones (maybe the extra genes) and is not a barrier to fertile offspring it will soon spread.

Even if the offspring I don't see how it would be an advantage as it is more likely to bring problem to embryo development than to aid it.

And also this would create a new hominid species with 45 chromosomes not 46 as we have

I think this split of chromosones is a rather slow process. Its like a little part of one chromosone leaves to create its own. But since its so insignifican amount it doesnt matter. and ass mutation accumilate and that chromosone is reinforced it will grow in size and gene amount. This could explain why some are larger/smaller. Fussion is just the opposite. one chromosone would start to accumilate more DNA while another one loses and since its so insignifican amount in the begining ti doesnt really do much differens

Downs syndrome is an extra chromosome. The only figures I could find for prevalence suggested 1 per thousand,

If similar[ie Chromosome 2] happened in early man could they not have been 'expelled' from the pack? - met up mated, hence no missing link [yet found] due to scarcity/low population. Maybe those that were left developed religion.

Dump some research into "Robertsonian translocation". You might also like to look at the differences in centromeric and telomeric regions of DNA and then compare them to the sequences on our chromosomes which have fused fairly recently... Like #2.

1. How do to chromosomes fuse together?
2. What is the evolutionary advantage of having less chromosomes
3. how did any offspring of the new mutant hominid born without being infertile.

1. Chromosomes do not fuse together. But they can undergo recombination. It is proposed that recombination occurred between human chromosome 2 and one that would be equivalent to the "extra" chimp chromosome. The reason humans no longer hace this "extra" chromosome is because the recombination resulted in one large chromosome and one dwarf one. Since this dwarf one carried no functional genes it was lost.

2. A karyotype cannot directly offer any evolutionary advantage - only phenotypes can.

3. All of the functional genes were still present (and in the proper amounts) so that they were still fertile.