Human Chromosome 2 Question

[Gofre]

Hi all,

I was recently having a discussion with a friend regarding evolution/creation, when I brought up human chromosome 2 and the evidence for a fusion event, indicating common descent between humans and the other great apes. He responded by saying that the fusion may simply between two human chromosomes some time in our past and does not necesarrily indicative of common ancestry. How would you respond to this claim?

Many thanks.

 

[quantumfireball2099]

http://rationalwiki.org/wiki/Common_descent#RNA_.2F_DNA_codeAnother piece of evidence that makes clear the factual reality of common descent is chromosome fusion that occurs across a wide variety of species in different ways. [28] Fusion of chromosome generates decreases the chromosome numbers in a descendant species. (Alternatively, a split in a chromosome increases the chromosome number.) The pattern of these fusion events generates characteristic phylogenetic trees offering proof of common descent.

One famous example is a fusion event that indicates the evolution of Homo sapiens from a common ancestor of humans and chimpanzees. [29] While all other great apes have 24 pairs of chromosomes, humans have 23 pairs of chromosomes. Human chromosome 2 looks almost identical to two of the chimpanzee's chromosomes stacked on one on top of the other, indicating chromosome fusion. [30]

Chromosomes form light and dark bands on a karyotype that can be compared to see how similar they are. The light and dark banding patterns of the two chimp chromosomes match that of the single human one. Also, on the end of each chromosome are a series of repetitive DNA sequences called telomeres. These sequences are found typically only on the tips of the chromosome. For chromosome 2, scientists find them in the middle, as if the tips of two chromosomes merged together.

In addition, chromosomes have a centromere which link the two sister chromatids together. While there are usually only one of these distinct regions on a chromosome, there are two centromeres on chromosome 2. One is "non-functional", but its genetic code matches the other centromere from the second chimpanzee chromosome.

Clearly from this evidence human chromosome 2 is a fusion event between two chromosomes found in chimpanzees in distinct form. This means that humans inherited these chromosomes from a common ancestor with chimpanzees.

Biologists have discovered other gene fusions that are consistent with common descent.[31][32]

Also



I am no geneticist, so I can't tell you first hand that I know this, but it makes sense to me.

 

[MRaverz]

Chromosome 2 shows the same sequence of genes as two of the common ancestor's chromosomes.

There's also evidence of telomeres in the centre of chromosome 2, telomeres being found on the ends of chromosomes.


I'm only an undergrad, but that's my understanding of it.

 

[RichardMNixon]

Chromosome 2 shows the same sequence of genes as two of the common ancestor's chromosomes.

If I may play the creationist, ahem.

Common design, common designer!!1! :ugeek:

It comes down to Occam's Razor. We know chromosomes can fuse. We know human chromosome 2 looks exactly as it would if two ape chromosomes fused to form it. You can believe that happened, or you can believe there is a great creator that created humans and chimps in such a specific manner as to deliberately mislead us. Is god uninvolved in creation, or is god deceitful? Your friend's call.

 

[MRaverz]

Chromosome 2 shows the same sequence of genes as two of the common ancestor's chromosomes.

If I may play the creationist, ahem.

Common design, common designer!!1! :ugeek:

That's such a silly argument, if someone made everything they designed look the same - they'd lose business.

 

[Pulsar]

He responded by saying that the fusion may simply between two human chromosomes some time in our past

That would mean that early humans and other great apes had indeed the same amount of chromosomes. Which is evidence of common descent. So, what is his argument? I don't get what he's aiming at.

 

[Squawk]

Look at it this way.

Humans have 23 chromosome pairs. One of those chromosomes shows evidence of a fusion event. Actually that chromosome is pretty much indisputably a fusion event. The only conclusion is that at some point in the past an ancestor of humans experienced a chromosome fusion.

Whats the impact of this? Well, on it's own not much, but you must consider it in the wider context. If humans and apes do share a common ancestor, yet have different numbers of chromosomes, then it must be true that either we lost an entire chromosome, or a fusion event took place. Losing a chromosome is unlikely (if not impossible, but I'd have to research).

Finding the fused chromosome was essential to evolutionary theory. That it was found is simply yet more corrobarative evidence.

 

[borrofburi]

While we're at it, someone should preemptively answer the inevitable response "well that's impossible because even if the mutation happened to someone, this someone wouldn't be able to breed with the rest of the apes and the weird fused chromosome would die off".

 

[MRaverz]

While we're at it, someone should preemptively answer the inevitable response "well that's impossible because even if the mutation happened to someone, this someone wouldn't be able to breed with the rest of the apes and the weird fused chromosome would die off".

I guess all those blue-eyed females really hate having that mutation from brown.

 

[Squawk]

While we're at it, someone should preemptively answer the inevitable response "well that's impossible because even if the mutation happened to someone, this someone wouldn't be able to breed with the rest of the apes and the weird fused chromosome would die off".

You need to read my debate more carefully

http://forums.leagueofreason.org.uk/viewtopic.php?p=88855#p88855

 

[Deleted member 619]

It should be noted also that the fused chromosome in humans expresses identically to the unfused chromosome in chimpanzees, which is to say that it accounts for none of the difference between them. That's pretty hard to explain without common ancestry.

 

[DWisker]

While we're at it, someone should preemptively answer the inevitable response "well that's impossible because even if the mutation happened to someone, this someone wouldn't be able to breed with the rest of the apes and the weird fused chromosome would die off".

You need to read my debate more carefully

http://forums.leagueofreason.org.uk/viewtopic.php?p=88855#p88855

That's a very good start, but needs some beefing up, given our ID creationist friends' penchant for throwing up misleading and irrelevant objections.

One of the most popular IDC arguments is to quibble over definitions, because, as we all know, they much prefer dictionaries over data. "Telomere-Telomere fusions are not Robertsonian translocations", they will say, with an air of sophistication, "therefore, your fertility examples do not hold". This tactic is surprisingly effective with laypeople. It is easily dismantled, but needs to be done slowly and methodically. First, for the fusion to occur, breaks have to occur in the telomeric regions of both chromosomes. When this happens, each will lose a small portion of their telomeres, and will fuse together, head-to-head. Second, the fusion that formed human chromosome 2 (HC2) was between two <i>acrocentric</i> chromosomes. Acrocentrics have one long arm and one much shorter arm. Robertsonian translocations by definition involve two acrocentric chromosomes that break near the centromeres, and can occur two ways. The first (and most common) is where one break occurs in the long arm and the other occurs in the short arm. This results in a metacentric chromosome (one with two arms of roughly equal length) with one centromere, and a small fragment with the other centromere (which is usually lost). The other way involves both breaks in the short arms, which results in a metacentric chromosome, but with two centromeres very close together, and a very small acentric (no centromere) fragment which is lost. This second type is a special form of Robertsonian translocation called a centric fusion. It should be clear that HC2 is the result of a centric fusion: it produced a metacentric chromosome with two centromeres in close proximity), and a fragment(s) of telomeric material.

Confronted with the above, the IDc's will try and say "Aha! But a chromosome with 2 centromeres will be torn apart at meiosis!". Again, this may impress laypeople, but all you have to do is point out that, in humans, 90% of the dicentric chromosomes formed by Robertsonian translocations are mitotically and meiotically stable (Sullivan et al, 1994). The reason for this is not known, but the Sullivan et al reference is very good at suggesting mechanisms.


Thwarted yet again, our IDC friends will then say that Robertsonian translocations result in severe losses in fertility as heterozygotes. Laypeople are very vulnerable here, because the IDC will trot out complicated tables of aneuploid vs euploid gametes, which sounds all sciency and stuff. The problem is, as you began to explore, Robertsonian translocation (and especially centric fusion) heterozygotes often have normal fitness, especially in mammals. In fact, one cannot say with certainty that a Robertsonian translocation heterozygote will have reduced fitness. To quote Coyne and Orr (1998):

A further problem with chromosomal speciation is that it depends critically on the semisterility of hybrids who are heterozygous for chromosome rearrangements. It is not widely appreciated, however, that heterozygous rearrangements theoretically expected to be deleterious (e.g., fusions and pericentric inversions) in reality often enjoy normal fitness, probably because segregation is regular or recombination is prevented"¦ Any putative case of chromosomal speciation requires proof that different rearrangements actually cause semisterility in heterozygotes, and almost no studies have met this standard. [p. 295].

Human population cytogenetic data show that Robertsonian translocations' detrimental effect on fitness is minimal, ~ 10% (Bengtsson & Bodmer, 1976; Jacobs, 1977). Not only that, the fitness disadvantage is offset by significant meiotic drive in human females, which preferentially pass fusions to their eggs during oogenesis (Pardo-Manuel de Villena & Sapienza, 2001).

Hope this gives you some ammunition in the wars.

References:

Bengtsson BO & WF Bodmer (1976). The fitness of human translocation carriers. Ann. Hum. Genet. Lond. 40: 253-257.

Coyne JA & HA Orr (1998). The evolutionary genetics of speciation. Phil.Trans. R. Soc. Lond. B 353: 287-305.

Jacobs PA (1977). Structural rearrangements of the chromosomes in man. In Population Cytogenetics: Studies in Humans, EB Hook and IH Porter, eds. Academic Press, New York, San Francisco & London, pp. 81-97.

Paredo-Manuel de Villena F & C Sapienza (2001). Transmission ration distortion in offspring of heterozygous female carriers of Robertsonian translocations. Hum. Genet. 108: 31-36
Sullivan BA, DJ Wolff, & S Schwartz (1994). Analysis of centromeric activity in Robertsonian translocations: implications for a functional acrocentric hierarchy. Chromosoma 103: 459-467.

 

[Squawk]

If only all first posts on the boards were like that one. Excellent stuff, and thanks for the education.

 

[DWisker]

If only all first posts on the boards were like that one. Excellent stuff, and thanks for the education.

Thanks. for the kind words. There are more ID creationsit arguments regarding this that we can also talk about, as well as some misconceptions about it in the reality-based community.

 

[Deleted member 619]

If only all first posts on the boards were like that one. Excellent stuff, and thanks for the education.

Indeed. Stellar work.

 

[CosmicJoghurt]

Just a question. Chromosome fusion would happen when? When the first cell of the organism is created? What's the order of events, I don't understand. How does it happen?

 

[DWisker]

Just a question. Chromosome fusion would happen when? When the first cell of the organism is created? What's the order of events, I don't understand. How does it happen?

The fusion happened during meiosis, when an individual was producing sperm or eggs.

 

[Gunboat Diplomat]

He responded by saying that the fusion may simply between two human chromosomes some time in our past

That would mean that early humans and other great apes had indeed the same amount of chromosomes. Which is evidence of common descent. So, what is his argument? I don't get what he's aiming at.

He's trying to say that it's possible that it's just a coincidence and is therefore not evidence of evolution.

It's much like a debate I once had with a poster on a thread on another forum. He tried to claim that "The lack of credible evidence supporting the existence of ghosts says nothing about whether they exist." As if this weren't disingenuous enough, when someone challenged him on this claim by saying "It does speak volumes about the probability of them existing though..." he retorted by saying "It only speaks volumes about the probability that there exists credible evidence of the existence of ghosts..."

It's a stupid response, compelling only to people with a vested interest in denying the truth. Ken Miller said it best in the video of him posted by quantumfireball2099. Not only is the fused chromosome consistent with the number and sizes of chromosomes of humans and other primates but it was predicted by the theory of evolution. That's yet another example of how powerful the theory of evolution is, as well as being evidence of its veracity.

 

[DWisker]

HC2 is not evidence for common ancestry in the sense that it could not conceivably be explained any other way. It's certainly consistent with the idea, however. It's also consistent with the Intelligent Design hyppthesis as well. The only difference is, the evolutionary explanation has an actual mechanism underlying it, whereas ID can only say that it is conceivable that a designer could have done it.

 

[Pulsar]

Not only is the fused chromosome consistent with the number and sizes of chromosomes of humans and other primates but it was predicted by the theory of evolution. That's yet another example of how powerful the theory of evolution is, as well as being evidence of its veracity.

Precisely.

It's also consistent with the Intelligent Design hyppthesis as well.

Everything is "consistent" with ID, because ID invokes magic.