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VCF + Phylogenetic tree : how to calculate the distance between two set of samples
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15 months ago
France/Nantes/Institut du Thorax - INSE…

I'm trying to build a simple tool that would build a phylogenetic tree from a VCF.

My aim is to verify that the related + sequenced individuals are close in the phylogenetic tree.

My current code works with a small set of related individuals but it fails when some extra individuals are added. Current algorithm:

  • a genotype is an enumeration Gtype: HOM_REF, HOM_ALT, HET . 'N/A' is converted to HOM_REF.

  • I'm looking iteratively for the 'nodes' having the smallest distance. A node is a set of samples. The algorithm starts with all the possible pairs of samples.

  • For a pair of samples

    • AA vs AA: = distance=0
    • AA vs AB: = distance=5
    • AA vs BB: = distance=15
  • This is where I'm looking for the right algorithm : when merging two samples to create a 'merged node' how should I calculate the distance ?

my current algorithm for a position in the VCF is:

dist=0;
for(Gtype g1: allGenotypes(node1))
  for(Gtype g2: allGenotypes(node2))
      d += distance(g1,g2)
d /= ( countGenotypes(node1)+countGenotypes(node2))

what would be the best way to 'score' the distance between two sets of samples for a given position in the VCF ?

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4.8 years ago
David W 4.7k
New Zealand

It's not quite clear to me what makes a "merged" node, do you mean a group that is already "joined" as each other's closest relatives?

If so this seems like a classic heirechchal clustering problem. You could use UPGMA or neighbor-joining to create your tree from a distance matrix.

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16 months ago
United States

I would use pairwise identity by state for the distance matrix.

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