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Telomere

(b) Unbalanced Subtelomeric Rearrangements: Monosomies (□) and Trisomies (I

(b) Unbalanced Subtelomeric Rearrangements: Monosomies (□) and Trisomies (I

Fig. 2 Spectrum of telomeric involvement in reported MR studies (identical rearrangements within a family have been included only once). (View this art in color at www.dekker.com.)

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Telomere

Fig. 2 Spectrum of telomeric involvement in reported MR studies (identical rearrangements within a family have been included only once). (View this art in color at www.dekker.com.)

one-third of all reported 'de novo' monosomies and trisomies, over twice that for any other telomere. From these cases, a specific 1p36 deletion phenotype has been defined and is already aiding clinicians. Monosomies for 18q and 22q also occur frequently [Fig. 2(a)]. Cytogenet-ically visible deletions both of the long arm of 18q and 22q13.3 are known to be associated with MR, although the overlapping phenotypes of patients with smaller subtelomeric deletions are likely to help refine and further delineate the chromosomal regions contributing to the phenotypes.

Figure 2(b) shows a summary of the anomalies reported in MR individuals with unbalanced subtelomeric translocations. The vast majority are familial originating from a phenotypically neutral, balanced translocation in one of the parents. Although some telomeres are more frequently involved than others, to date, there is no indication that specific translocation events between any two subtelomeric regions are favored over others. This makes it difficult to use unbalanced translocations to dissect out exactly which parts of a patient's phenotype are associated with the monosomy or trisomy because there may always be uncertainty regarding the phenotypic consequences of having both. For example, patients with unbalanced rearrangements involving monosomy 4p may have phenotypes atypical of Wolf-Hirschhorn Syndrome because the associated trisomy masks the features.[3] Furthermore, the actual combination of subtelomeric regions involved in an unbalanced rearrangement may determine the overall severity or even lethality of a phenotype. Nonetheless, some unbalanced translocations may prove informative, and studies have been undertaken to compare the anomalies found with their clinical phenotype.[26,27]

Spectrum of Telomeres Involved in Unexplained Mental Retardation

Of all of the subtelomeric anomalies noted in unexplained MR, there are only seven telomeres (including four acrocentric p-arms) for which no deletions have been reported and four telomeres (including two acrocentric p-arms) for which no duplications have been reported. For the acrocentric telomeres, this is most likely because chromosome-specific probes do not exist and, in most cases, available probes have not been tested. For the other telomeres, it may be because such anomalies are comparatively rare in MR. Alternatively, the subtelomeric deletions or duplications might be lethal, although this is unlikely to be the case for 18p, 19p, and Yq monosomies or for Yq and 18q trisomies because microscopically visible cases do exist. Another possibility is that some anomalies are being missed because they occur more distal to the current probe sets. Finally, some subtelomeric anomalies simply might not occur or might be phenotyp-ically neutral or rare polymorphisms that will take more time to come to light. The latter do exist because a number of apparently normal individuals with subtelomeric anomalies have already been noted.[28]

Getting Started With Dumbbells

Getting Started With Dumbbells

The use of dumbbells gives you a much more comprehensive strengthening effect because the workout engages your stabilizer muscles, in addition to the muscle you may be pin-pointing. Without all of the belts and artificial stabilizers of a machine, you also engage your core muscles, which are your body's natural stabilizers.

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