Exercise 2: Measuring differential expression

Select the reference sequence again, and you should now see Expression level annotation tracks for each sample condition loaded on it. If you can't see these tracks, check that they are enabled in the Annotation and Tracks tab to the right of the sequence viewer. To find genes that are differentially expressed between the two sample conditions go to Annotate and Predict → Compare Expression Levels.

Check that the tracks you created in the previous exercise are chosen for comparison: they should be Track 1 - Expression: Sample_condition_1 and Track 2 - Expression: Sample_condition_2. Choose to compare Transcripts normalized by Median of Gene Expression Ratios - this is the recommended method (see expression level measures for more information on these methods). Click OK.

You'll now see a third annotation track added to your reference sequence called "Diff Expression: Sample_condition_1 vs Sample_condition_2", and as with the individual expression level tracks, annotations are colored according to the results. Mouse over one of the annotations and you'll see a popup window listing the raw read and transcript counts for each sample condition, plus a list of differential expression scores.

The Differential Expression p value tells you whether the differential expression observed is statistically significant (see here for information on how this is calculated). The Differential Expression Confidence is the negative base 10 log of the p value, adjusted to be negative for genes that are under-expressed in sample 2 compared to sample 1, or positive for over-expressed genes. By default, Geneious uses this value to color the annotations: from blue for under-expressed genes, through to white for genes that are not differentially expressed, through to red for genes that are over-expressed.

Try filtering based on this field to find all the genes that show significantly higher expression in sample condition 2 than sample condition 1: Display the Annotations and Tracks tab () to the right of the sequence viewer and type "Differential Expression Confidence">2 (including the quotation marks) in the Search box, as in the screenshot below. A Differential Expression Confidence level above 2 equates to a p value of less than 0.01. You should now see only 29 annotations displayed, all of which are pink or red in color, indicating significantly higher expression of that gene in sample condition 2 than sample condition 1 (at p<0.01). To scroll through these annotations, click the arrows to the right of the annotation track name in the Annotations and Tracks tab.


You can also display the results in tabular form by clicking the Annotations tab above the sequence viewer and displaying the "Diff Expression" Annotation track only. You may need to add the columns for Differential Expression Confidence and Ratio by clicking the Columns button. To sort the results in order of most over-expressed in sample 2 → most under-expressed in sample 2 click the "Differential Expression Confidence" header twice so that the highest values are at the top.


You can export this table in .csv format if you wish by clicking Export table (this may be under the double arrows >>).

Congratulations, you have now completed the Expression Analysis tutorial