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. 2012 May 29;7(5):e38154. doi: 10.1371/journal.pone.0038154

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Liao et al.

This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are properly credited.

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Assuming the fractional fetal DNA concentration in chrRef is f, the F-S ratio would be f/(2-f) on chrRef irrespective of the aneuploidy status of the fetus. On the other hand, the F-S ratio on chr21 would be f/(2-f) if the mother is carrying a euploid fetus, 2f/(2-f) if the mother is carrying a paternally-derived T21 fetus, and f/2 if the mother is carrying a maternally-derived T21 fetus. Therefore, the would be 1 if the mother is carrying a euploid fetus, would become 2 if the mother is carrying a paternally-derived T21 fetus, and would become (1-f/2) if the mother is carrying a maternally-derived T21 fetus.

Inline graphic — Assuming the fractional fetal DNA concentration in chrRef is f, the F-S ratio would be f/(2-f) on chrRef irrespective of the aneuploidy status of the fetus. On the other hand, the F-S ratio on chr21 would be f/(2-f) if the mother is carrying a euploid fetus, 2f/(2-f) if the mother is carrying a paternally-derived T21 fetus, and f/2 if the mother is carrying a maternally-derived T21 fetus. Therefore, the would be 1 if the mother is carrying a euploid fetus, would become 2 if the mother is carrying a paternally-derived T21 fetus, and would become (1-f/2) if the mother is carrying a maternally-derived T21 fetus.