GBS snp calling

One of the main ways we call SNPs in GBS data is using a maximum likelihood script derived from the original Hohenlohe stickleback RAD paper. It uses a likelihood function based on the number of reads for each base at a site to determine if its homozygous or heterozygous.

It’s been known for a while that it seemed to be biased against high coverage heterozygous sites. I took the script apart and realized that there was actually a serious problem with it. The likelihood function used factorials, and when there were over 175 reads at a site, perl broke down and started describing things as infinite. Then the script would call it as a homozygote by default.

Letting perl work with the huge numbers necessary makes it incredibly slow, so I put in a logic gate for high coverage sites. If there is over 150 reads at a site, and the second most numerous base makes up atleast 10% of the reads (i.e. at minimum 15), then it is heterozygous. Otherwise it is homozygous.

The rest of the script is the same. One could debate how best to call the high coverage scripts, but I suggest you use this script over the old one.