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The following example illustrates how to compute the “scaled population recombination rate” \(\rho_{ij}\) between SNPs \(i\) and \(j\) using HapMap genetic map.
We take six SNPs in chromosome 22 from HapMap CEU Phase 2, with the
genetic map shown below. We also get the effective diploid population
size \(N_e=11418\) from IMPUTE
software document.
position COMBINED_rate(cM/Mb) Genetic_Map(cM)
1: 14431347 8.096992 0.00000000
2: 14432618 8.131520 0.01029128
3: 14433624 8.131967 0.01847159
4: 14433659 8.132625 0.01875620
5: 14433758 8.129606 0.01956133
6: 14434713 8.024772 0.02732511
To compute recombination rates from two SNPs, we use the following formula from Li and Stephens (2003):
\[ \rho = 4 \times \text{effective diploid population size} \times \text{genetic distance}. \]
For example,
As a validation, we compare our calculations above with the results
from BLIMP
(Wen and
Stephens, 2010).
The file rmb.ceu.ch22
lists recombination rate between
all adjacent markers in the panel.
library(data.table)
genetic.map.chr22 <- data.table::fread("genetic_map_chr22.txt")
legend.ceu.chr22 <- data.table::fread("legend.ceu.ch22", header = T)
rmb.ceu.chr22 <- data.table::fread("rmb.ceu.ch22")
# locate all the snps specified by the legend file
selected.pos <- intersect(genetic.map.chr22$position, legend.ceu.chr22$position)
map.index <- which(genetic.map.chr22$position %in% selected.pos)
leg.index <- which(legend.ceu.chr22$position %in% selected.pos)
# check the location is correct
location.check <- prod(genetic.map.chr22$position[map.index] == legend.ceu.chr22$position[leg.index])
if (location.check != 1) stop('locate the snps wrongly')
# calculate the shrinking coefficient: exp(-rho_{ij}/(2*m)) for adjacent (i,j)
# general formula: rho = 4 * effective population size * genetic distance
# hapmap phase 2 ceu: effective population size (Ne)=11418, haplotype size (m)=120
num.snp <- length(map.index);
ro.coef <- rep(0, num.snp);
ro.coef[1] <- 1;
map.selected <- genetic.map.chr22[map.index, ]
rcb.selected <- map.selected[[3]]
for (i in 2:num.snp){
ro.coef[i] <- exp(-4*11418*(rcb.selected[i]/100-rcb.selected[i-1]/100)/120)
}
# compare the result with the one provided in BLIMP (Wen and Stephens, 2010)
# make sure rss compute rho in the same way as BLIMP
results <- matrix(0, nrow=num.snp, ncol=2)
results[, 1] <- unlist(rmb.ceu.chr22)
results[, 2] <- ro.coef
> head(results)
[,1] [,2]
[1,] 1.0000000 1.0000000
[2,] 0.9615886 0.9615886
[3,] 0.9693454 0.9693454
[4,] 0.9989173 0.9989173
[5,] 0.9969404 0.9969404
[6,] 0.9708834 0.9708834
> tail(results)
[,1] [,2]
[34021,] 0.9963701 0.9963701
[34022,] 0.9909937 0.9909937
[34023,] 0.9986032 0.9986032
[34024,] 0.9879030 0.9879030
[34025,] 0.9962124 0.9962124
[34026,] 0.9847357 0.9847357
> max(abs(results[,1]-results[, 2]))
[1] 4.999806e-08
─ Session info ───────────────────────────────────────────────────────────────
setting value
version R version 4.4.1 (2024-06-14)
os macOS Sonoma 14.6.1
system aarch64, darwin20
ui X11
language (EN)
collate en_US.UTF-8
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tz America/Los_Angeles
date 2024-09-16
pandoc 3.1.11 @ /Applications/RStudio.app/Contents/Resources/app/quarto/bin/tools/aarch64/ (via rmarkdown)
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