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dkysh t1_j506qis wrote

Reply to comment by fruticosa in Given that reproduction is difficult or impossible when both animals have different numbers of chromosomes, how did so many species evolve to have so many different numbers of them? by MercurioLeCher

Another (close) example: Human's chromosome 2 is the result of the fusion of other 2 smaller chromosomes present in all other great apes, chromosomes 2a and 2b.

The content of chr2a+2b is almost identical to human's chr2, even with genes following the same order. This makes them much more compatible and probable to recombine and produce viable offspring.

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VictoriousEgret t1_j5076sh wrote

How do we know this/find this out? Is it that someone noticed similarities between two chromosomes common in great apes and chromosome 2 in humans?

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ikefalcon t1_j508nej wrote

As the previous commenter said, human chromosome 2 has nearly identical sequences to chromosomes 2a and 2b in the other great apes. It’s believed that human ancestors like Neanderthals had 23 chromosome pairs like humans.

Also, chromosomes usually have 1 centromere (center link between pairs of chromatids) and 2 telomeres (basically end caps of the chromosome). Human chromosome 2 has a vestigial (unused due to no longer being needed) extra centromere and 2 vestigial telomeres found inside the chromosome sequence.

This is pretty good evidence that there used to be 2 chromosomes before they fused together.

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wheatgrass_feetgrass t1_j50lv6a wrote

>Human chromosome 2 has a vestigial (unused due to no longer being needed) extra centromere and 2 vestigial telomeres found inside the chromosome sequence.

Goddammit that's cool.

I can't be sure of it quite yet, but I have a feeling sequencing the entire genome is the single greatest human breakthrough in my lifetime. I was in middle school when my science teacher told us it had been done for the first time. When I was in college I toured a sequencing facility where it was being done for outrageous cost per sequence on machines bigger than my house. Last year I got my own DNA fully sequenced, every single base pair, for about two day's wage.

Reading your own ancient programming code letter by letter is a weird, almost disassociating experience.

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dkysh t1_j50geul wrote

Building a "reference genome" for a species from scratch is a whole field of science.

When people do "normal sequencing", the genome is broken into an infinity of small pieces. The genome of chimpanzees, gorillas, and orangutans fit so well the human genome, that scientists usually use the human reference genome to study great apes.

Some scientists compared both the human and the chimpanzee reference genomes built independently from scratch, and they found just minimal differences. All this shows that the human chr2 and the great ape chr2a and 2b are almost identical in they just happened to fuse in proto-humans sometime in the last 6 million years.

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JFSOCC t1_j50r6a8 wrote

So are people with down syndrome a form of speciation?

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dkysh t1_j50ssc6 wrote

If we were bacteria, yeah, sure. But we are extremely complicated multicellular organisms. A whole genome trisomy screws up the balance of gene expression to such an extreme that most trisomies are simply lethal and never observed (they end up in miscarriage).

A gene fusion is a less drastic event, where 2 chromosomes happen to be connected, but the genetic load is identical.

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