Main Text
In July 2009 we published in this journal a report containing evidence for gene conversion between the X chromosome and the male-specific region of the Y chromosome1 at a translocation hotspot (hotspot A; HSA) between the PRKX (MIM 300083) and PRKY (MIM 400008) genes. In this issue, Cruciani and colleagues2 revisit our data and conclude that we overestimated the per-base-per-generation rate because of a failure to divide the number of conversion events within the sequence under study by the length of the sequence in base-pairs. We agree with Cruciani and colleagues2 that we made this error, thank them for pointing it out, and apologize to the readers of The Journal.
We observed two X-to-Y conversion events, but calculation of an average per-base-per-generation rate for the two events is complicated by the fact that the region sequenced was shorter for one of the events (in hgQ∗: 698 bp) than the other (in hgA2c: 1839 bp, excluding primers). In recalculating the conversion rate per base per generation we therefore consider only the twelve Y chromosomes that were sequenced for the entire 1839 bp region and which, measured according to the approach we used previously,1 encompass between 39,757 and 56,640 generations. Given the spacing of gametologous sequence variants, the single event observed here has a tract length between 4 and 125 bp. Using the equation of Cruciani et al.,2 we obtain a corrected rate range for X-to-Y gene conversion of between 3.8 × 10−8 and 1.7 × 10−6 per base per generation; this rate is comparable to that found by Cruciani et al.2 in their own data. The lower bound of the recalculated range is similar to the average base mutation rate (2.3 × 10−8 per base per generation3), and the upper bound of the range is two orders of magnitude slower than the Y-Y conversion rate within palindromes (2.2 × 10−4 per base per generation4).
Our report's other conclusions, including the similarity of the events-per-generation rates of crossover (translocation) (∼1 × 10−5; ref.1,5) and conversion (∼6.6 × 10−6 - ∼2.1 × 10−5; ref.1) at HSA, remain unchanged. We are gratified to observe that, in their independent resequencing of HSA, Cruciani and colleagues2 also discovered variants indicating conversion from the X chromosome.
References
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