SMN1 has a high rate of de novo deletion mutations. There is a high frequency of recombination and crossover events at 5q13, presumably secondary to the presence of the large inverted repeat as well as smaller repeats around the SMN1 and SMN2 loci. The high rate of de novo mutations in SMN1 likely underlies the high carrier frequency for SMA in the general population despite the high genetic lethality of the disease. De novo mutations of paternal origin are much more frequent (with a mutation rate of ~ 1.6 x 10"4) than those of maternal origin.
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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.