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Question on Human Chromosome 2

  1. May 23, 2012 #1
    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?
     
  2. jcsd
  3. May 23, 2012 #2
    A recent thread that addresses your question -

    https://www.physicsforums.com/showthread.php?t=579244&highlight=chromosome+2
     
    Last edited: May 23, 2012
  4. May 23, 2012 #3
    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.
     
  5. May 24, 2012 #4
    Have you checked the links posted by the contributors, those are credible sources.
     
  6. Jun 5, 2012 #5
    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.
     
  7. Jun 5, 2012 #6
    "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.
     
    Last edited: Jun 5, 2012
  8. Jun 7, 2012 #7
    I think you mean translocation.
     
  9. Jun 11, 2012 #8
    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).”
     
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