Linkmapper Tutorial: Linkage Mapping with the F2 Demo Dataset

Introduction

This tutorial walks through a complete linkage mapping analysis using the demo dataset bundled with Linkmapper. The dataset is an F2 intercross population comprising 188 individuals, 62 markers, and 2 phenotype traits. It represents the validation dataset used in the Linkmapper paper and reproduces the results reported there.

By the end of this tutorial you will have:

  • Uploaded the demo file and completed prior analysis
  • Grouped 62 markers into 5 linkage groups
  • Ordered markers within each group using the RECORD algorithm
  • Generated a linkage map totalling approximately 675.6 cM
  • Run a QTL scan on one of the two phenotype traits

Each step below corresponds to one tab in the Linkmapper interface.

Step 1: Uploading data and prior analysis

Uploading the demo file

From the Upload & Analyse tab, click Download demo dataset to save the bundled F2 demo file to your computer, then click Browse and select it. The file is in MAPMAKER format with the header:

data type f2 intercross
188 62 2

Linkmapper reads the file and displays a confirmation message showing the number of individuals (188), markers (62), and phenotype columns (2) detected.

Expected prior analysis results

Once the file is loaded, click Run prior analysis. The output should show:

  • Missing data: The demo dataset is fully genotyped - no missing data bars appear in the missing data plot (0% missingness across all markers and individuals).
  • Segregation distortion: The chi-squared test is run for all 62 markers under the expected 1:2:1 (AA:AB:BB) ratio for an F2 intercross. In the demo dataset, 0 markers show significant distortion at the default threshold (p < 0.05 after Bonferroni correction), so all 62 markers proceed to grouping.
  • Traits detected: 2 phenotype columns are listed, available for QTL analysis in Step 5.
  • Suggested LOD threshold: A LOD of approximately 3.84 is computed based on dataset size. This value is pre-filled in the grouping panel.

Step 2: Marker grouping

Navigate to the Group Markers tab. The suggested LOD (≈ 3.84) and a default maximum recombination frequency of 0.50 are pre-filled.

Click Run grouping. Expected output:

Linkage group Markers assigned
LG 1 13
LG 2 12
LG 3 13
LG 4 12
LG 5 12
Total linked 62
Unlinked 0

All 62 markers form 5 groups with no unlinked markers, consistent with the known 5-chromosome structure of the mapping population. If you obtain a different number of groups, try adjusting the LOD threshold slightly (±0.5) and re-running.

Step 3: Marker ordering

Navigate to the Order Groups tab. Select RECORD as the ordering algorithm (recommended for F2 intercross data; RECORD minimises the total number of recombinations across the ordered sequence).

Click Order all groups. Ordering runs sequentially for all 5 linkage groups. Progress messages appear as each group completes. For the demo dataset:

  • LG 1 - 13 markers ordered; log-likelihood ≈ −1722.934
  • LG 2-5 - ordered with comparable log-likelihoods

If you prefer to compare algorithms, you can re-run with RCD or UG and compare the resulting log-likelihoods. For the demo dataset, RECORD and RCD produce similar orderings; UG may produce slightly longer maps on some groups.

Step 4: Linkage map generation

Navigate to the Linkage Map tab. Choose a mapping function (Kosambi is the default and is appropriate for most plant mapping populations), enter a map title, marker label prefix, and colour preference, then click Generate map.

Expected output

The linkage map is displayed as an interactive plotly figure. Hover over any marker to see its name and cM position. A summary table below the map shows statistics per linkage group:

LG Markers Length (cM)
1 13 ~145
2 12 ~130
3 13 ~140
4 12 ~125
5 12 ~136
Total 62 ~675.6

Exact values may vary slightly depending on the mapping function selected.

Click Download map (PNG) to save a publication-quality static version, and Download statistics (CSV) to save the per-group summary table.

Step 5: QTL analysis

Navigate to the QTL Analysis tab. This step requires that Steps 1–4 are complete and that the data file contains at least one phenotype column (the demo dataset has 2).

Running a scan

  1. Select a phenotype from the dropdown (e.g. Trait 1).
  2. Choose a scanning method: Interval Mapping (IM) uses the standard EM algorithm; CIM (Composite Interval Mapping) accounts for background markers and typically gives sharper QTL peaks.
  3. Set a LOD significance threshold. A threshold of 3.0 is commonly used as a genome-wide suggestive threshold for F2 populations with ~60 markers; 3.5–4.0 is more stringent.
  4. Click Run QTL scan.

Interpreting the output

The LOD profile plot shows the LOD score at every position across all five linkage groups. Peaks exceeding the threshold are highlighted and listed in the QTL summary table with their linkage group, peak position (cM), and peak LOD score.

Note: QTL results depend on the phenotype selected, the scanning method, and the significance threshold. Run the scan and interpret the LOD profile in the context of your study organism and research question. The demo dataset results are provided for workflow validation only and should not be used as biological conclusions.

Interpreting linkage group statistics

The summary table produced in Step 4 gives three key statistics per linkage group:

  • Number of markers: More markers per group generally gives better map resolution. Very few markers per group may indicate poor coverage of that chromosome.
  • Total length (cM): Expected total genome length for the organism should be known approximately. Groups that are unusually long may contain markers from multiple chromosomes that were incorrectly grouped (consider tightening the LOD threshold).
  • Average inter-marker interval (cM): Computed as length / (markers − 1). For QTL mapping, an average interval < 20 cM is generally adequate; < 10 cM gives good resolution.

In a breeding context, the linkage map is used to:

  • Position QTL relative to flanking markers for marker-assisted selection
  • Calculate recombination frequencies between loci of interest
  • Serve as the foundation for comparative genomics or genomic selection studies

Downloading outputs

The following files are available for download after completing the workflow:

Step Download Format
Prior analysis Missing data plot PNG
Prior analysis Segregation test table CSV
Marker grouping Group assignment table CSV
Marker ordering Ordered sequence summary CSV
Linkage map Linkage map figure PNG
Linkage map Per-group statistics table CSV
QTL analysis LOD profile plot PNG
QTL analysis QTL summary table CSV