Chromosome Fusion in Humans - or Not?

[Witchdoc]

IIRC, tandem repeats are very inaccurately replicated by DNA polymerase due to slippage and stalling leading to length and sequence variation, so its absolutely unsurprising that the fusion site is “degenerated” rather than a well preserved.

[CrisprCAS9]

@Witchdoc

so its absolutely unsurprising that the fusion site is “degenerated” rather than a well preserved.

More likely still is that the repeats were already degenerate. The other DDX11Ls are 10-15kb from the ends, with several kb of subtelomeric junk between it and the actual repeats, and those regions commonly include a degenerate ‘spacer’.

[scd]

the problem with the fusion argument isnt about the fusion itself, but about the fact that it doesnt support common descent, as you can see in this figure:

so we can only use it to prove that humans share a common descent with…humans.

[CrisprCAS9]

@scd

but about the fact that it doesnt support common descent, as you can see in this figure:

It is required for common descent. As with every other potential falsification, we test the world and find it within the narrow range of possibilities consistent with common descent.

@scd

so we can only use it to prove that humans share a common descent with…humans.

We can’t even use it to ‘prove’ that. Proof isn’t a thing in science.

[evograd (Blogging Graduate Student)]

@CrisprCAS9

She mentions that there are ‘some’ telomeres there, and fails to mention that the number there is many orders of magnitude more than what you’d otherwise expect outside of a subtelomeric region.

I assume she also didn’t mention that they’re found in a head-to-head orientation?

@CrisprCAS9

Alphoid sequences aren’t unique to centromeres in the same way that telomeric sequences aren’t unique to telomeres. Hopefully the reason that’s an unimpressive argument is obvious. Can you guess how many non centromeric alphoid sequences are found in the human karyotype? Hint: It is a very small number.

Indeed, as I note in this post:

Native Creationists and Chromosomal Fusion

I recently did two analyses into chromosome 2 fusion, which I’ll eventually include in a comprehensive blog post dedicated to the evidence for the fusion when I get around to it. For now, here’s the summary for those interested. It’s commonly argued, by Tomkins and other creationists, that two of the primary pieces of evidence for chromosome 2 fusion - the presence of telomeric repeats at the alleged fusion site and the presence of alphoid repeats in the alleged cryptic centromere - are actually very poor evidence because these repeats are in fact found all over the genome, not just in those locations alleged to be related to the fusion event. They argue that since telomeric repeats and alphoid repeats can be found all over the genome, there’s really nothing special about the sites claimed to represent the fusion site and cryptic centromere. In effect, they’re claiming that “evolutionists” are cherry-picking these sites because they fit the fusion story while ignoring all the other …

[evograd (Blogging Graduate Student)]

@scd

so we can only use it to prove that humans share a common descent with…humans.

It’s an old blog post now, and likely poorly written compared to my current standards, but it’s still valid:

Chromosome 2 Fusion and Bayes Theorem: Support for Common Ancestry After All

[CrisprCAS9]

@evograd

I assume she also didn’t mention that they’re found in a head-to-head orientation?

Which again, is a completely unnecessary finding.

Not finding telomers wouldn’t rule out fusion.
Not finding them literally touching head-to-head wouldn’t rule out fusion.
Not finding it next to a pseudogene exclusively found by telomeres wouldn’t rule out fusion.

What we have is a hilariously gratuitous surplus of evidence.

Creationists would be better off arguing that only a divine entity could engineer a fusion so perfectly, obviously, and exclusively concordant with fusion than arguing it isn’t one at all!

But instead they just start waving their hands again…

[Mercer (John Mercer)]

@thoughtful

I’ve set it at the time stamp. Listen until about 28 minutes. So 8 minutes long.

@thoughtful

My question is - is there an explanation of when this happened? Would it happen it one hominid first? And then be passed down until selected for? Obviously it has to happen in both sexes? I guess my questions are more pragmatic that way If it’s happened in other animals like zebra crosses and it’s been observed - I suppose that would answer those questions. So if anyone has a link to that explanation on my kind of level, it’d be appreciated. Thanks, I’m curious.

So many questions. Awesome curiosity!

@CrisprCAS9

Okay, but remember: You asked for it…

That was a heroic effort.

@thoughtful

Thanks, that was very detailed. It will help when I’m ready to understand all the biology related to this better.

I’m puzzled by this response. Why did you ask so many questions 21 hours ago and ask @CrisprCAS9 to address a very specific part of that video, if you’re not currently ready to understand any of those exhaustive answers (with evidence)?

[Rumraket (Mikkel R.)]

@scd

the problem with the fusion argument isnt about the fusion itself, but about the fact that it doesnt support common descent, as you can see in this figure:

That’s just so obviously wrong. Please consider the context that, if humans are to share common ancestry with our closest primate relatives, given that we do not have equal numbers of chromosomes, the hypothesis of common ancestry demands that either we or our closest relatives must have evolved a different number of chromosomes.

Given that humans stand out from the rest by having fewer chromosomes, the most probable explanation is that humans must have somehow evolved fewer chromosomes after we split off from the most recent common ancestor shared with chimpanzees. One way this can happen is by chromosomal fusion. A chromosomal fusion is thus a prediction of common ancestry, and that is what we find.

Do you understand this argument? Not believe, or accept, but do you at least understand it?

There is a hypothesis that demands a particular thing to have happened in order to make sense of the data. The creationism hypothesis does not demand that ancestral humans started out with 48 chromosomes like other apes, we could have been created with 46.

[thoughtful (Valerie (non-scientist))]

@Mercer

if you’re not currently ready to understand any of those exhaustive answers (with evidence)?

I have nausea going on six weeks from morning sickness it’s not going away and it and hormones make my mood rather volatile. Sometimes I am very interested and curious, other times I want to throw my phone in the lake because this forum drives me crazy. I was just trying to be honest that I won’t respond much. I just also know I need to learn a lot before understanding the biology in order to assess this and don’t have the energy. Though my brain still always has questions I guess!

[Michael_Okoko]

@CrisprCAS9

But instead they just start waving their hands again…

They are not doing science but apologetics, otherwise no one would see the clear signs of a fusion event in human chromosome 2 and deny it.

[scd]

@CrisprCAS9

It is required for common descent.

@evograd

It’s an old blog post now,

@Rumraket

A chromosomal fusion is thus a prediction of common ancestry, and that is what we find.

since evolution has no problem with gene loss, that fusion isnt a prediction of the evolution theory. evolution would be just fine with missing chromosomes.

[CrisprCAS9]

@scd

since evolution has no problem with gene loss, that fusion isnt a prediction of the evolution theory. evolution would be just fine with missing chromosomes.

No, evolutionary theory wouldn’t be ‘just fine’ with the simultaneous loss of the entire gene complement of a full chromosome.

[Faizal_Ali]

@scd

since evolution has no problem with gene loss, that fusion isnt a prediction of the evolution theory. evolution would be just fine with missing chromosomes.

As has been explained, but which you somehow managed to miss, the fusion is a prediction that follows from the observation that humans have one fewer chromosome pair than other great apes.

If an entire chromosome was missing, that would be practically impossible to reconcile with common ancestry with chimps only 6 million years ago. Hence the hypothesis that the chromosome was not missing, but fused. A hypothesis what was eventually confirmed.

What hypotheses did Creation Scientists propose to explain this observation? How well did they fare in terms of confirmation?

[Faizal_Ali]

I will just add this comment that Ken Miller made regarding this subject on Sandwalk:

Creationists discover that human and chimp genomes are only about 70% similar!!!

[Rumraket (Mikkel R.)]

@scd

since evolution has no problem with gene loss

… of genes that don’t positively contribute to reproductive fitness. Do the genes on the two fused chromosomes not contribute to reproductive fitness?

[Faizal_Ali]

@Rumraket

Do the genes on the two fused chromosomes not contribute to reproductive fitness?

Well, if they’re still there, they’re not lost. Are they?

@scd isn’t talking about the genes that are still there and, therefore, aren’t lost.

He’s talking about, well, the genes that were lost, but somehow are still in our genome and not lost at all.

Just what difficulty are you having following that?

[evograd (Blogging Graduate Student)]

@scd

since evolution has no problem with gene loss, that fusion isnt a prediction of the evolution theory. evolution would be just fine with missing chromosomes.

Gene loss and chromosome loss aren’t quite the same thing now are they? it’s losing one gene versus losing 1000 genes. We have plenty of evidence of gene loss happening today, but I don’t think there are any examples of a mammalian species losing an entire chromosome without severe fitness consequences.

Besides, if you prefer we can move one step later in the chronology of the prediction. Instead of saying “the observation that humans and chimps have different number of chromosomes led to the prediction of a fusion event”, we can say “the observation that the banding patterns of two chimpanzee chromosomes seem to line up with human chromosome 2 lead to the prediction of a fusion”. The evidence of the fusion would then still be the remnants of telomeric sequences at the fusion site and the degenerate second centromere.

[swamidass (S. Joshua Swamidass)]

That needs to be elevated into a post in and of itself.

[scd]

@CrisprCAS9

No, evolutionary theory wouldn’t be ‘just fine’ with the simultaneous loss of the entire gene complement of a full chromosome.

@Faizal_Ali

the fusion is a prediction that follows from the observation that humans have one fewer chromosome pair than other great apes.

@Rumraket

of genes that don’t positively contribute to reproductive fitness. Do the genes on the two fused chromosomes not contribute to reproductive fitness?

@evograd

Gene loss and chromosome loss aren’t quite the same thing now are they? it’s losing one gene versus losing 1000 genes.

i know that horse has 64 chromosomes, mule has 63 and donkey 62 (although I have not checked yet if this is the result of chromosomal fusion, chromosomal loss etc). fruit fly and soil nematode have lost more than 1000 genes. elephant lost more than 1000 OR genes etc:

https://science.sciencemag.org/content/317/5834/86

so yes- evolution has no problem to explain gene loss, even at large scale.