Estimated QTL effects

Overview

Teaching: 10 min
Exercises: 20 min
Questions
  • How do I find the estimated effects of a QTL on a phenotype?

Objectives
  • Obtain estimated QTL effects.

  • Plot estimated QTL effects.

The scan1() function returns only LOD scores. To obtain estimated QTL effects, use the function scan1coef(). This function takes a single phenotype and the genotype probabilities for a single chromosome and returns a matrix with the estimated coefficients at each putative QTL location along the chromosome.

For example, to get the estimated effects on chromosome 2 for the liver phenotype, we’d do the following:

c2eff <- scan1coef(pr[,"2"], iron$pheno[,"liver"])

The result is a matrix of 39 positions × 2 genotypes. An additional column contains the y-axis intercept values.

dim(c2eff)
[1] 39  3
head(c2eff)
               SS       SB       BB
D2Mit379 86.47730 93.86458 104.9718
c2.loc39 86.34738 93.74505 105.3583
c2.loc40 86.26104 93.63473 105.6668
c2.loc41 86.21877 93.54212 105.8779
c2.loc42 86.21960 93.47498 105.9766
c2.loc43 86.26170 93.43885 105.9547

To plot the effects, use the function plot_coef(). Use the argument columns to indicate which coefficient columns to plot.

par(mar=c(4.1, 4.1, 1.1, 2.6), las=1)
col <- c("slateblue", "violetred", "green3")
plot(c2eff, map["2"], columns=1:3, col=col)
last_coef <- unclass(c2eff)[nrow(c2eff),] # pull out last coefficients
for(i in seq(along=last_coef))
    axis(side=4, at=last_coef[i], names(last_coef)[i], tick=FALSE, col.axis=col[i])

plot of chunk plot_effects_liver_c2

The default is to provide phenotype averages for each genotype group. If instead you want additive and dominance effects, you can provide a square matrix of contrasts, as follows:

c2effB <- scan1coef(pr[,"2"], iron$pheno[,"liver"],
                    contrasts=cbind(mu=c(1,1,1), a=c(-1, 0, 1), d=c(-0.5, 1, -0.5)))

The result will then contain the estimates of mu, a, and d.

dim(c2effB)
[1] 39  3
head(c2effB)
               mu        a         d
D2Mit379 95.10455 9.247233 -1.239968
c2.loc39 95.15023 9.505449 -1.405183
c2.loc40 95.18752 9.702880 -1.552789
c2.loc41 95.21294 9.829582 -1.670822
c2.loc42 95.22372 9.878490 -1.748738
c2.loc43 95.21841 9.846482 -1.779555

Here’s a plot of the additive and dominance effects, which are in the second and third columns.

par(mar=c(4.1, 4.1, 1.1, 2.6), las=1)
plot(c2effB, map["2"], columns=2:3, col=col)
last_coef <- unclass(c2effB)[nrow(c2effB),2:3] # last two coefficients
for(i in seq(along=last_coef))
    axis(side=4, at=last_coef[i], names(last_coef)[i], tick=FALSE, col.axis=col[i])

plot of chunk plot_effects_liver_c2_contr

If you provide a kinship matrix to scan1coef(), it fits a linear mixed model (LMM) to account for a residual polygenic effect. Here let’s use the kinship matrix from the LOCO method.

c2eff_pg <- scan1coef(pr[,"2"], iron$pheno[,"liver"], kinship_loco[["2"]])
dim(c2eff_pg)
[1] 39  3
head(c2eff_pg)
               SS       SB       BB
D2Mit379 84.99427 93.95099 104.6940
c2.loc39 84.71083 93.87206 105.1537
c2.loc40 84.47392 93.80197 105.5385
c2.loc41 84.28851 93.74761 105.8280
c2.loc42 84.15801 93.71501 106.0058
c2.loc43 84.08450 93.70797 106.0625

Here’s a plot of the estimates.

par(mar=c(4.1, 4.1, 1.1, 2.6), las=1)
col <- c("slateblue", "violetred", "green3")
plot(c2eff_pg, map["2"], columns=1:3, col=col, ylab="Phenotype average")
last_coef <- unclass(c2eff_pg)[nrow(c2eff_pg),]
for(i in seq(along=last_coef))
    axis(side=4, at=last_coef[i], names(last_coef)[i], tick=FALSE, col.axis=col[i])

plot of chunk plot_effects_pg_liver_c2

You can also get estimated additive and dominance effects, using a matrix of contrasts.

c2effB_pg <- scan1coef(pr[,"2"], iron$pheno[,"liver"], kinship_loco[["2"]],
                       contrasts=cbind(mu=c(1,1,1), a=c(-1, 0, 1), d=c(-0.5, 1, -0.5)))

Here’s a plot of the results.

par(mar=c(4.1, 4.1, 1.1, 2.6), las=1)
plot(c2effB_pg, map["2"], columns=2:3, col=col)
last_coef <- unclass(c2effB_pg)[nrow(c2effB_pg),2:3]
for(i in seq(along=last_coef))
    axis(side=4, at=last_coef[i], names(last_coef)[i], tick=FALSE, col.axis=col[i])

plot of chunk plot_effects_pg_liver_c2_contr

Another option for estimating the QTL effects is to treat them as random effects and calculate Best Linear Unbiased Predictors (BLUPs). This is particularly valuable for multi-parent populations such as the Collaborative Cross and Diversity Outbred mice, where the large number of possible genotypes at a QTL lead to considerable variability in the effect estimates. To calculate BLUPs, use scan1blup(); it takes the same arguments as scan1coef(), including the option of a kinship matrix to account for a residual polygenic effect.

c2blup <- scan1blup(pr[,"2"], iron$pheno[,"liver"], kinship_loco[["2"]])

Here is a plot of the BLUPs (as dashed curves) alongside the standard estimates. Note that the BLUPs are centered at 0, while the coefficient estimates are centered at the phenotype average.

par(mar=c(4.1, 4.1, 1.1, 2.6), las=1)
col <- c("slateblue", "violetred", "green3")
ylim <- range(c(c2blup, c2eff))+c(-1,1)
plot(c2eff, map["2"], columns=1:3, col=col, ylab="Phenotype average", ylim=ylim,
     xlab="Chr 2 position")
plot(c2blup, map["2"], columns=1:3, col=col, add=TRUE, lty=2)
last_coef <- unclass(c2eff)[nrow(c2eff),]
for(i in seq(along=last_coef))
    axis(side=4, at=last_coef[i], names(last_coef)[i], tick=FALSE, col.axis=col[i])

plot of chunk plot_scan1blup

The scan1coef function can also provide estimated QTL effects for binary traits, with model="binary". (However, scan1blup has not yet been implemented for binary traits.)

c2eff_bin <- scan1coef(pr[,"2"], bin_pheno[,"liver"], model="binary")

Here’s a plot of the effects. They’re a bit tricky to interpret, as they are basically log odds ratios.

par(mar=c(4.1, 4.1, 1.1, 2.6), las=1)
col <- c("slateblue", "violetred", "green3")
plot(c2eff_bin, map["2"], columns=1:3, col=col)
last_coef <- unclass(c2eff_bin)[nrow(c2eff_bin),] # pull out last coefficients
for(i in seq(along=last_coef))
    axis(side=4, at=last_coef[i], names(last_coef)[i], tick=FALSE, col.axis=col[i])

plot of chunk plot_effects_binary

Finally, to plot the raw phenotypes against the genotypes at a single putative QTL position, you can use the function plot_pxg(). This takes a vector of genotypes as produced by the maxmarg() function, which picks the most likely genotype from a set of genotype probabilities, provided it is greater than some specified value (the argument minprob). Note that the “marg” in “maxmarg” stands for “marginal”, as this function is selecting the genotype at each position that has maximum marginal probability.

For example, we could get inferred genotypes at the chr 2 QTL for the liver phenotype (at 28.6 cM) as follows:

g <- maxmarg(pr, map, chr=2, pos=28.6, return_char=TRUE)

We use return_char=TRUE to have maxmarg() return a vector of character strings with the genotype labels.

We then plot the liver phenotype against these genotypes as follows:

par(mar=c(4.1, 4.1, 0.6, 0.6))
plot_pxg(g, iron$pheno[,"liver"], ylab="Liver phenotype")

plot of chunk plot_pheno_geno

par(mar=c(4.1, 4.1, 0.6, 0.6))
plot_pxg(g, iron$pheno[,"liver"], SEmult=2, swap_axes=TRUE, xlab="Liver phenotype")

plot of chunk plot_pheno_geno_se

Challenge 1

In the code block above, we use swap_axes=TRUE to place the phenotype on the x-axis. We can use SEmult=2 to include the mean ± 2 SE intervals. 1) How would you decide which chromosome to plot? Discuss this with your neighbor, then write your responses in the collaborative document. 2) Calculate and plot the best linear unbiased predictors (blups) for spleen on the chromosome of your choice.

Solution to Challenge 3

Key Points

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