In the Quantitative Trait Locus (QTL) analysis,
composite interval mapping (CIM) method estimates the QTL positon with higher accuracy and statistical significance by combining interval mapping with multiple regression. This method also controls background noise resulting from genetic variations in other regions of the genome that affect the detection of the true QTL. In this post, my objective is to describe and walk you through all the basic steps one might need run the composite inteval mapping in
R software. For further descriptive information on R/qtl, please read the paper by Karl Broman at this link: http://www.rqtl.org/
To get started
Step 1. Download and install R software and R studio
- Download and install the latest version of the R software from this link
- Download and install R studio from this link
- Finally, install the library qtl in R
Step 1.1 - Install qtl library in R following the below steps:
Step 2. Prepare the input files
- Phenotype data
- Genetic map
genetic map infomation can be put together in a single
See below the screenshot to understand the formatting of the input file.
Step 2.1 - Setup working directory following the below steps:
Step 3. Running CIM in R
Please double check the formatting of the file is correct. Once ready, open
R studio and type the following commands to get started.
Step 3.1. Import qtl library
Step 3.2. Load the .csv file with the genotype and phenotype data
phenoGeno <-read.cross(format = "csv", file = "file.csv", na.strings= c('NA'), genotypes = c("A","B","H"))
In the above command, if your genetic data is formatted differently then specify the in
genotype section. To execute the above command, highlight the command and press
ENTER. You should see output in the
terminal window, and please check for any warnings or errors.
Step 3.3. Summary of the data file and plotting
You should see a plot similiar to shown below:
Step 3.4. Calculating genotype probabilities
In this step,
Hidden Markov model technology is used to calculate the probabilities of the true underlying genotypes.
phenoGeno <- calc.genoprob(phenoGeno, step=0, off.end=0.0, error.prob=1.0e-4,stepwidth = "fixed", map.function="kosambi") phenoGeno_cross <- sim.geno(phenoGeno, n.draws=32, step=0, off.end=0.0, error.prob=1.0e-4, stepwidth = "fixed", map.function="kosambi")
Important Make sure to rename
phenoGeno_cross in the second step of the above command.
Step 3.5. running QTL scan using CIM method with permutations
- Type the below command in the console to run the qtl scan using
scan.cim = cim(phenoGeno_cross, pheno.col=2, map.function="kosambi")
#valuetype the column number of the phenotype
permutationtest by typing the below command:
scan.cim.perm = cim(phenoGeno_cross, pheno.col=2, map.function="kosambi", n.perm=1000)
It is recommended to run at 1,000 permutation test.
- check the LOD threshold after permutations and significant QTL above the threshold.
summary(scan.cim.perm) summary(scan.cim, threshold = #value)
#valueAdd the LOD score at desired
alpha(0.05 0r 0.1)
- Plot the QTL scan
Step 4.0 QTL effect plot
Type the below command to plot the
effect plot of the QTL on the phenotype.
plotPXG(scan_cross, pheno.col = #value, marker = c("#value"))
Provide phenotype colum number in
pheno.col and marker name in
Step 5.0 Make QTL model with the significant marker
qtl <- makeqtl(scan_cross, chr=c(#value), pos=c(#value),what=c("prob")) fitqtl <- fitqtl(don, pheno.col=c(#value), formula = y~Q1, qtl= qtl, method = "hk", get.ests = T) summary(fitqtl)
#value Add the signifincat marker’s coordinates i.e.
Below is the screenshot of the output one might except to see in the R shell:
fitqtl summary Method: Haley-Knott regression Model: normal phenotype Number of observations : 93 Full model result ---------------------------------- Model formula: y ~ Q1 df SS MS LOD %var Pvalue(Chi2) Pvalue(F) Model 2 13.07 6.53 30.4 77.8 0 0 Error 90 3.724093 0.04137881 Total 92 16.795699 Estimated effects: ----------------- est SE t Intercept 3.01835 0.09467 31.882 firstname.lastname@example.org -0.22761 0.09566 -2.379 email@example.com -1.41852 0.18935 -7.492
Additive effect (a) = (BB - AA)/2 and
Dominance effect (d) = AB - (AA + BB)/2.
Step 6.0 Identify QTL intervals using LOD drop
lodint(results = scan.cim, chr = #value, drop = 1.8)
cM to drop in the
drop = and the
An example output is show below:
> lodint(scan.cim, chr=2, drop=1.8) chr pos lod 2_14850509 3 45.290 11.44917 2_15846197 3 48.827 14.43254 2_19297395 3 50.312 12.45010
--- End of Tutorial ---
Thank you for reading this tutorial. I really hope this helpful in giving you the concept and technology behind AmpSeq and the data analysis. If you have any questions or comments, please let comment below or send me an email.
Broman KW, Wu H, Sen Ś, Churchill GA (2003) R/qtl: QTL mapping in experimental crosses. Bioinformatics 19:889-890 [PubMed | pdf (236k) | Screen pdf (288k)]