gt_extract_f2: Compute and store blocked f2 statistics for ADMIXTOOLS 2

In tidypopgen: Tidy Population Genetics

Compute and store blocked f2 statistics for ADMIXTOOLS 2

Description

This function prepares data for various ADMIXTOOLS 2 functions from the package ADMIXTOOLS 2. It takes a gen_tibble, computes allele frequencies and blocked f2-statistics, and writes the results to outdir. It is equivalent to admixtools::extract_f2().

Usage

gt_extract_f2(
  .x,
  outdir = NULL,
  blgsize = 0.05,
  maxmem = 8000,
  maxmiss = 0,
  minmaf = 0,
  maxmaf = 0.5,
  minac2 = FALSE,
  outpop = NULL,
  outpop_scale = TRUE,
  transitions = TRUE,
  transversions = TRUE,
  overwrite = FALSE,
  adjust_pseudohaploid = NULL,
  fst = TRUE,
  afprod = TRUE,
  poly_only = c("f2"),
  apply_corr = TRUE,
  n_cores = 1,
  quiet = FALSE
)

Arguments

.x	a gen_tibble
outdir	Directory where data will be stored.
blgsize	SNP block size in Morgan. Default is 0.05 (5 cM). If blgsize is 100 or greater, if will be interpreted as base pair distance rather than centimorgan distance.
maxmem	Maximum amount of memory to be used. If the required amount of memory exceeds maxmem, allele frequency data will be split into blocks, and the computation will be performed separately on each block pair. This doesn't put a precise cap on the amount of memory used (it used to at some point). Set this parameter to lower values if you run out of memory while running this function. Set it to higher values if this function is too slow and you have lots of memory.
maxmiss	Discard SNPs which are missing in a fraction of populations higher than maxmiss
minmaf	Discard SNPs with minor allele frequency less than minmaf
maxmaf	Discard SNPs with minor allele frequency greater than than maxmaf
minac2	Discard SNPs with allele count lower than 2 in any population (default FALSE). This option should be set to TRUE when computing f3-statistics where one population consists mostly of pseudohaploid samples. Otherwise heterozygosity estimates and thus f3-estimates can be biased. minac2 == 2 will discard SNPs with allele count lower than 2 in any non-singleton population (this option is experimental and is based on the hypothesis that using SNPs with allele count lower than 2 only leads to biases in non-singleton populations). Note that, While the minac2 option discards SNPs with allele count lower than 2 in any population, the qp3pop function will only discard SNPs with allele count lower than 2 in the first (target) population (when the first argument is the prefix of a genotype file; i.e. it is applied directly to a genotype file, not via precomputing f2 from a gen_tibble).
outpop	Keep only SNPs which are heterozygous in this population
outpop_scale	Scale f2-statistics by the inverse outpop heterozygosity (1/(p*(1-p))). Providing outpop and setting outpop_scale to TRUE will give the same results as the original qpGraph when the outpop parameter has been set, but it has the disadvantage of treating one population different from the others. This may limit the use of these f2-statistics for other models.
transitions	Set this to FALSE to exclude transition SNPs
transversions	Set this to FALSE to exclude transversion SNPs
overwrite	Overwrite existing files in outdir
adjust_pseudohaploid	Genotypes of pseudohaploid samples are usually coded as 0 or 2, even though only one allele is observed. adjust_pseudohaploid ensures that the observed allele count increases only by 1 for each pseudohaploid sample. If TRUE (default), samples that don't have any genotypes coded as 1 among the first 1000 SNPs are automatically identified as pseudohaploid. This leads to slightly more accurate estimates of f-statistics. Setting this parameter to FALSE treats all samples as diploid and is equivalent to the ADMIXTOOLS inbreed: NO option. Setting adjust_pseudohaploid to an integer n will check the first n SNPs instead of the first 1000 SNPs. NOW DEPRECATED, set the ploidy of the gen_tibble with gt_pseudohaploid().
fst	Write files with pairwise FST for every population pair. Setting this to FALSE can make extract_f2 faster and will require less memory.
afprod	Write files with allele frequency products for every population pair. Setting this to FALSE can make extract_f2 faster and will require less memory.
poly_only	Specify whether SNPs with identical allele frequencies in every population should be discarded (poly_only = TRUE), or whether they should be used (poly_only = FALSE). By default (poly_only = c("f2")), these SNPs will be used to compute FST and allele frequency products, but not to compute f2 (this is the default option in the original ADMIXTOOLS).
apply_corr	Apply small-sample-size correction when computing f2-statistics (default TRUE)
n_cores	Parallelize computation across n_cores cores.
quiet	Suppress printing of progress updates

Value

SNP metadata (invisibly)

Examples

bed_file <-
  system.file("extdata", "lobster", "lobster.bed", package = "tidypopgen")
lobsters <- gen_tibble(bed_file,
  backingfile = tempfile("lobsters"),
  quiet = TRUE
)
lobsters <- lobsters %>% group_by(population)
f2_path <- tempfile()
gt_extract_f2(lobsters, outdir = f2_path, quiet = TRUE)
admixtools::f2_from_precomp(f2_path, verbose = FALSE)

tidypopgen documentation built on Aug. 28, 2025, 1:08 a.m.