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change in `ploidy`
change in `ploidy`

... VI. Mutation A. Overview B. Changes in Ploidy - These are the most dramatic changes, adding a whole SET of chromosomes 1. Mechanism #1: Complete failure of Meiosis - if meiosis fails, reduction does not occur and a diploid gamete is produced. This can occur because of failure of homologs OR sister ...
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The importance of having two X chromosomes - Neuroscience
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CHAPTER 13 MEIOSIS AND SEXUAL LIFE CYCLES
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... Male-specific pressures on the Y chromosome caused a region that was spatially linked to Sry to diverge from the X chromosome, leading to a loss of homology and recombination of those portions of the two chromosomes. The loss of recombination was important because it led to progressive degeneration ...
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... 1. Generally refers to spontaneous (rather than induced) mutations. There are two types relative to population genetics: a) Recurrent -- mutations that occur repeatedly, generally at some characteristic frequency b) Non-recurrent -- mutations that arise only once in the history of a lineage 2. Chang ...
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Skewed X-inactivation

Skewed X chromosome inactivation occurs when the inactivation of one X chromosome is favored over the other, leading to an uneven number of cells with each chromosome inactivated. It is usually defined as one allele being found on the active X chromosome in over 75% of cells, and extreme skewing is when over 90% of cells have inactivated the same X chromosome. It can be caused by primary nonrandom inactivation, either by chance due to a small cell pool or directed by genes, or caused by secondary nonrandom inactivation, which occurs by selection. Most females will have some levels of skewing. It is relatively common in adult females; around 35% of women have skewed ratio over 70:30, and 7% of women have an extreme skewed ratio of over 90:10. This is of medical significance due to the potential for the expression of disease genes present on the X chromosome that are normally not expressed due to random X inactivation. X chromosome inactivation occurs in females to provide dosage compensation between the sexes. If females kept both X chromosomes active they would have twice the number of active X genes than males, who only have one copy of the X chromosome. At approximately the time of implantation (see Implantation (human embryo), one of the two X chromosomes is randomly selected for inactivation. The cell undergoes transcriptional and epigenetic changes to ensure this inactivation is permanent. All progeny from these initial cells will maintain the inactivation of the same chromosome, resulting in a mosaic pattern of cells in females.
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