ldest {ldsep}

Pairwise LD estimation in polyploids.

Description

Estimates either haplotypic or composite measures of LD using either genotypes are genotype likelihoods via maximum likelihood. The usual measures of LD are estimated (D, D', and r) along with the Fisher-z transformation of r (called "z"). All estimates are returned with standard errors. See Gerard (2021) for details.

Usage

ldest(
  ga,
  gb,
  K,
  se = TRUE,
  type = c("hap", "comp"),
  model = c("norm", "flex"),
  pen = ifelse(type == "hap", 2, 1)
)

Arguments

ga

One of two possible inputs:

1. A vector of counts, containing the genotypes for each individual at the first locus. When type = "comp", the vector of genotypes may be continuous (e.g. the posterior mean genotype).

2. A matrix of genotype log-likelihoods at the first locus. The rows index the individuals and the columns index the genotypes. That is ga[i, j] is the genotype likelihood of individual i for genotype j-1.

gb

One of two possible inputs:

1. A vector of counts, containing the genotypes for each individual at the second locus. When type = "comp", the vector of genotypes may be continuous (e.g. the posterior mean genotype).

2. A matrix of genotype log-likelihoods at the second locus. The rows index the individuals and the columns index the genotypes. That is gb[i, j] is the genotype likelihood of individual i for genotype j-1.

K

The ploidy of the species. Assumed to be the same for all individuals.

se

A logical. Should we calculate standard errors (TRUE) or not (FALSE). Calculating standard errors can be really slow when type = "comp", model = "flex", and when using genotype likelihoods. Otherwise, standard error calculations should be pretty fast.

type

The type of LD to calculate. The available types are haplotypic LD (type = "hap") or composite LD (type = "comp"). Haplotypic LD is only appropriate for autopolyploids when the individuals are in Hardy-Weinberg equilibrium (HWE). The composite measures of LD are always applicable, and consistently estimate the usual measures of LD when HWE is fulfilled in autopolyploids. However, when HWE is not fulfilled, interpreting the composite measures of LD could be a little tricky.

model

When type = "comp" and using genotype likelihoods, should we use the proportional bivariate normal model to estimate the genotype distribution (model = "norm"), or the general categorical distribution (model = "flex")? Defaults to "norm".

pen

The penalty to be applied to the likelihood. You can think about this as the prior sample size. Should be greater than 1. Does not apply if model = "norm", type = "comp", and using genotype likelihoods. Also does not apply when type = "comp" and using genotypes.

Value

A vector with some or all of the following elements:

D

The estimate of the LD coefficient.

D_se

The standard error of the estimate of the LD coefficient.

r2

The estimate of the squared Pearson correlation.

r2_se

The standard error of the estimate of the squared Pearson correlation.

r

The estimate of the Pearson correlation.

r_se

The standard error of the estimate of the Pearson correlation.

Dprime

The estimate of the standardized LD coefficient. When type = "comp", this corresponds to the standardization where we fix allele frequencies.

Dprime_se

The standard error of Dprime.

Dprimeg

The estimate of the standardized LD coefficient. This corresponds to the standardization where we fix genotype frequencies.

Dprimeg_se

The standard error of Dprimeg.

z

The Fisher-z transformation of r.

z_se

The standard error of the Fisher-z transformation of r.

p_ab

The estimated haplotype frequency of ab. Only returned if estimating the haplotypic LD.

p_Ab

The estimated haplotype frequency of Ab. Only returned if estimating the haplotypic LD.

p_aB

The estimated haplotype frequency of aB. Only returned if estimating the haplotypic LD.

p_AB

The estimated haplotype frequency of AB. Only returned if estimating the haplotypic LD.

q_ij

The estimated frequency of genotype i at locus 1 and genotype j at locus 2. Only returned if estimating the composite LD.

n

The number of individuals used to estimate pairwise LD.

Haplotypic LD

This section describes the methods used when type = "hap" is selected.

Haplotypic LD measures the association between two loci on the same haplotype. When haplotypes are known, estimating haplotypic LD is simple using just the haplotypic frequencies.

When haplotypes are not known, we can still estimate haplotypic frequencies using the genotypes or genotype likelihoods in autopolyploids as long as Hardy-Weinberg equilibrium (HWE) is satisfied. We do this via maximum likelihood using gradient ascent. Gradient ascent is performed over the unconstrained parameterization of the 3-simplex from Betancourt (2012). The estimated haplotype frequencies are then used to estimate haplotypic LD.

Standard errors are provided using standard maximum likelihood theory. In brief, the Hessian matrix of the log-likelihood is calculated at the MLE's of the haplotype frequencies. The negative inverse of this Hessian matrix is approximately the covariance matrix of the MLE's of the haplotype frequencies. Since all haplotypic LD measures are functions of the haplotype frequencies, we use the delta-method to obtain the standard errors for each LD estimate.

A Dirichlet(2,2,2,2) prior is placed over the frequencies of haplotypes 00, 01, 10, and 11. This corresponds to the "add two" rule of Agresti (1998). You can change this prior via the pen argument.

When you either do not have autopolyploids or when HWE is not satisfied, then the estimates using type = "hap" are nonsensical. However, the composite measures of LD are still applicable (see below).

Composite LD

This section describes the methods used when type = "comp" is selected.

When HWE is not satisfied, haplotype frequencies are not estimable. However, measures of association between two loci are still estimable. These associations may be caused by LD either on the same haplotype or between different haplotypes. Cockerham and Weir (1977) thus called such measures "composite" measures of LD.

When the genotypes are known, these composite measures have simple correspondences to well-known statistical measures of association. D is the covariance of genotypes between loci divided by the ploidy. r is the Pearson correlation of genotypes. D' is D divided by a term that involves only mean genotypes.

When genotypes are not known, we estimate the joint genotype frequencies and use these to estimate the composite measures of LD using genotype likelihoods. The distribution of genotypes is assumed to either follow a proportional bivariate normal model (by default) or a general categorical model.

These estimates of composite measures of LD estimate the haplotypic measures of LD when HWE is fulfilled, but are still applicable when HWE is not fulfilled.

When genotypes are known, standard errors are calculated using standard moment-based approaches. When genotypes are not known, standard errors are calculated using standard maximum likelihood theory, same as for the haplotypic LD estimates (see above), or using a bootstrap.

Author(s)

David Gerard

References

• Agresti, Alan, and Brent A. Coull. "Approximate is better than "exact" for interval estimation of binomial proportions." The American Statistician 52, no. 2 (1998): 119-126. doi:10.1080/00031305.1998.10480550

• Betancourt, Michael. "Cruising the simplex: Hamiltonian Monte Carlo and the Dirichlet distribution." In AIP Conference Proceedings 31st, vol. 1443, no. 1, pp. 157-164. American Institute of Physics, 2012. doi:10.1063/1.3703631

• Cockerham, C. Clark, and B. S. Weir. "Digenic descent measures for finite populations." Genetics Research 30, no. 2 (1977): 121-147. doi:10.1017/S0016672300017547

• Gerard, David. "Pairwise Linkage Disequilibrium Estimation for Polyploids." Molecular Ecology Resources 21, no. 4 (2021): 1230-1242. doi:10.1111/1755-0998.13349

See Also

ldfast()

Fast, moment-based approach to LD estimation that also accounts for genotype uncertainty.

mldest()

For calculating pairwise LD among all pairs of a collection of SNPs.

sldest()

For calculating pairwise LD along a sliding window of SNPs.

ldest_hap()

For the function that directly estimates haplotypic LD when HWE is fulfilled.

ldest_comp()

For the function that directly estimates composite LD.

Examples

set.seed(1)
n <- 100 # sample size
K <- 6 # ploidy

## generate some fake genotypes when LD = 0.
ga <- stats::rbinom(n = n, size = K, prob = 0.5)
gb <- stats::rbinom(n = n, size = K, prob = 0.5)
head(ga)
head(gb)

## generate some fake genotype likelihoods when LD = 0.
gamat <- t(sapply(ga, stats::dnorm, x = 0:K, sd = 1, log = TRUE))
gbmat <- t(sapply(gb, stats::dnorm, x = 0:K, sd = 1, log = TRUE))
head(gamat)
head(gbmat)

## Haplotypic LD with genotypes
ldout1 <- ldest(ga = ga,
                gb = gb,
                K = K,
                type = "hap")
head(ldout1)

## Haplotypic LD with genotype likelihoods
ldout2 <- ldest(ga = gamat,
                gb = gbmat,
                K = K,
                type = "hap")
head(ldout2)

## Composite LD with genotypes
ldout3 <- ldest(ga = ga,
                gb = gb,
                K = K,
                type = "comp")
head(ldout3)

## Composite LD with genotype likelihoods and normal model
ldout4 <- ldest(ga = gamat,
                gb = gbmat,
                K = K,
                type = "comp",
                model = "norm")
head(ldout4)

## Composite LD with genotype likelihoods and general categorical model
ldout5 <- ldest(ga = gamat,
                gb = gbmat,
                K = K,
                type = "comp",
                model = "flex",
                se = FALSE)
head(ldout5)

ldout1[["D"]]
ldout2[["D"]]
ldout3[["D"]]
ldout4[["D"]]
ldout5[["D"]]