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protection of individuals correcting the misalignment
of their brains and their anatomical sex, while supporting their transition
into society as hormonally reconstituted and surgically corrected citizens.
| Human Testes May Multiply Good & Bad Mutations |
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| SciMed - Biology | |||
| TS-Si News Service | |||
| Wednesday, 29 August 2007 20:00 | |||
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Can natural selection drive
 mutation frequency in human genes? 
London, United Kingdom. New findings argue for the persistence of sex-linked chromosomes, such as the male Y chromosome, refuting theories that the Y is doomed to extinction.
The results confirm that although these chromosom... DNA to offspring – but that can also backfire. The new theory is part of a study, appearing in PLoS Biology, that tries to explain the puzzlingly high frequency of deformity. In this case, the deformity is Apert syndrome, a genetic cranial deformity found in approximately one out of every 70,000 newborns. The study’s authors suggest that natural selection may favor “germline” cells – the precursors to sperm – carrying a mutation that causes the condition.
A competitive advantage for mutated sperm precursor cells could explain why Apert strikes 100 to 1,000 times more people than expected from a single mutation.
Useful mutations in sperm precursor cells also may be more likely to pass to the next generation. The authors suggest this is “because the effective mutation frequency is elevated beyond the level that can be achieved by the molecular mutation process alone.”
It is not yet understood why natural selection might favor sperm precursor cells carrying a disease mutation.
The authors based their conclusions on an analysis of four human testes and computer models of mutation frequency. They say their study is the first to check the location of mutant germline cells in the testes in any species. The result was surprising.
“But when we divided the testes up, we didn’t find that. What we found were some very big clusters of precursor cells that were mutant.”
The data did not support the theory that the site of the mutation in the Apert gene is unusually prone to DNA change. Another explanation – that the mutations arise very early in the life of a germline cell and multiply through subsequent divisions – also did not fit the data.
But Arnheim and Calabrese said the clusters of mutant cells could be explained if the mutant cells made copies of themselves more frequently than normal cells. If a mutant cell divided into two copies of itself every four to five years, the extra copies would be enough to explain the clusters, the researchers said.
They added that the model explains the increase in Apert risk with paternal age, while noting that other selection-based models also may be able to explain the same data.
Citing related studies along with their findings, the authors concluded that “it now seems very likely that (natural) selection can be a driving force acting to increase the mutation frequency at a number of genes in humans.”
The experimental work was initiated at USC by Jian Qin, now with Fluidigm Corp. Irene Tiemann-Boege, Deepali Narendra Shinde and Song-Ro Yoon of USC also carried out experiments. David Gelfand and Keith Bauer of Roche Molecular Systems provided critical advice and materials.
The Molecular Anatomy of Spontaneous Germline Mutations in Human Testes." Qin J, Calabrese P, Tiemann-Boege I, Shinde DN, Yoon SR, et al. PLoS Biology Vol. 5, No. 9, e224 doi:10.1371/journal.pbio.0050224.
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| Last Updated on Tuesday, 23 October 2007 17:58 |
“You would expect that when a new mutation arose, it could 
arise virtually anywhere in the organ,” said Norman Arnheim, Ester Dornsife Chair in Biological Sciences at the 
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and the TS-Si Research Service are collaborations of TS-Si officials, staff, contributors, and corresponding institutions. The contents do not necessarily convey official positions of TS-Si or its owners, participants, partners, or affiliates.