Question on Human Chromosome 2

Kaldanis

I was having a discussion about the evidence for and against evolution. I brought up the fact that Humans have one less pair of chromosomes as a result of the fusion of two ape chromosomes and all the evidence that supports this. Someone then told me that this is 'rubbish', because:

1) If a member of a species suddenly had 2 of it's chromosomes fused, it would be rendered incapable of mating with the other members of that species. The mutation would then die with this creature as it could not have children with such a drastic change. If by some unlikely chance that a twin male and female were born from the same mother with the same mutation, it would not be possible for those two to mate with each other and continue to have their descendents mate with each other to populate the earth with Humans that had this mutation

2) It's impossible for chromosomes to even fuse in the first place as they aren't designed to do that. He then went on to compare it to pressing two ends of a magnet together and having them repell - chromosomes just can't fuse

Are either of his two points valid? If not, can someone explain why not so I can educate myself and then him in the process?

thorium1010

A recent thread that addresses your question -

http://www.physicsforums.com/showthr...t=chromosome+2

Kaldanis

Based on that thread it seems that it's not only possible, but it happens frequently. Is there absolutely no doubt about this then? I don't know too much about biology so I feel like I'm relying on the word of someone else, so I want to be sure.

thorium1010

Have you checked the links posted by the contributors, those are credible sources.

Darwin123

A chromosome fusion in balanced form does not cause either health difficulties nor does it cause a significant hybridization barrier. A balanced chromosome fusion between two chromosomes results in one chromosome with two centromeres. The balanced chromosome fusion does not significantly affect either the health or fertility of the mutation that caused it. Balanced chromosome fusions have very little effect on
the phenotype of the individual.
The chromosome fusion that led to chromosome 2 was apparently balanced. Chromosome 2 has two centromeres. This is unusual among chromosomes. One of the centromeres in chromosome 2 is often referred to as vestigial. This “vestigial centromere” has no known function in the present population of humans.

If anyone asks whether balanced chromosome fusions actually occur, then tell them about the Robertson fusion. There is a rather common mutation in humans referred to as a Robertson fusion. It is often unbalanced, which does result in health problems. However, many people with a Robertson fusion are unaffected by the fusion. The mutation that caused this fusion was balanced in the ancestors of these people.
The Robertson fusion is not the same as the fusion that caused chromosome 2. However, the Robertson fusion is probably very similar to the mutation that caused chromosome 2, with different chromosomes.

Darwin123

"Based on that thread it seems that it's not only possible, but it happens frequently. Is there absolutely no doubt about this then? I don't know too much about biology so I feel like I'm relying on the word of someone else, so I want to be sure."
I can not post links here. There is a rule about not being able to post links
until one has answered enough threads. However, I was readily able to find quite
a number of links. Some of the best were on Wikipedia.
A chromosome fusion is a type of chromosome transposition. Chromosome
transpositions occur when a segment of a chromosome is separated and then
reattached to a chromosome. A transposition can have the segment reattach
to the chromosome that the segment came from. Transpositions that don't
change the arrangement or number of genes have very little effect.
A chromosome fusion is an extreme form of transposition where an entire
chromosome is joined, practically unchanged, on another chromosome. If
the joining is perfectly balanced, there is no effect on the animal. Mutations
that cause chromosome fusions are extremely rare, since the end of the
chromosome is so small. However, chromosome transpositions of different
types are common. There have been observed transpositions that are
balanced in such a way as to not effect the offspring.
There is a common type of variation in human beings called a Robertson
transposition. This is very similar to the mutation that supposedly caused
chromosome 2. You can look up "Robertson transposition".
There are other balanced chromosome transpositions that have been
documented. However, one could try to argue on first principles here.
If during a fusion the centromeres are joined in the new chromosome, then
there is no reason that the fused chromosome can coordinate with unfused
chromosomes during meiosis. The genes will largely keep the same arrangement
after a balanced fusion. Thus, the effect of a balanced fusion on the offspring
would have to be small. The resulting offspring would have to be almost as fertile
as the parents.
An individual with a fused chromosome would be indistinguishable from an
individual where the corresponding chromosomes were unpaired. There would
only be a small hybridization barrier.
The individual with the fused chromosome, crossing with an individual with unfused chromosomes, would have almost the same number of healthy offspring as a
homozygous individual. There could be a small increase in defective grandchildren
caused by an occasional "tangle" between fused an unfused chromosomes. This
partial hybridization barrier would be very minor.
Again, look up transpositions. Mutations that merely move segments of
chromosomes around don't have significant effects on the offspring. The mutation
that caused chromosome 2 was one of these transpositions.
Yes, chromosome fusions are very rare. However, there is no general principle
that stops chromosome fusions from occurring. A chromosome fusion is just a type
of transposition that occurs near the very tip of the chromosome. The reason that
it is rare has nothing to do with chromosomes repelling. Chromosomes are not magnets.
Mutations with transpositions occur often enough so that scientists understand
transpositions. Balanced transposition mutations do not significantly affect the
offspring of the mutant. There is no fundamental difference between transpositions
of the more common variety and chromosome fusions. Chromosome fusions have to
occur occasionally, just like other transpositions.
Ask your Creationist friend why the Designer chose to put two centromeres
onto chromosome 2. Also ask why one of the centromeres of chromosome 2
has no known function.

mishrashubham

I think you mean translocation.

Darwin123

You are right. Thank you!
The fusion that created chromosome 2 was a balanced chromosome translocation, not a balanced chromosome transposition. In any case, it was balanced. It had very little effect on the mutated offspring or it immediate descendents.
Here is a link describing chromosome translocations..
http://en.wikipedia.org/wiki/Chromosomal_translocation
“In genetics, a chromosome translocation is a chromosome abnormality caused by rearrangement of parts between nonhomologous chromosomes. A gene fusion may be created when the translocation joins two otherwise separated genes, the occurrence of which is common in cancer. It is detected on cytogenetics or a karyotype of affected cells. There are two main types, reciprocal (also known as non-Robertsonian) and Robertsonian. Also, translocations can be balanced (in an even exchange of material with no genetic information extra or missing, and ideally full functionality) or unbalanced (where the exchange of chromosome material is unequal resulting in extra or missing genes).”