How can I add introns to a translated feature?
View the Translated Feature

In this example, a Saccharomyces cerevisiae tubulin gene has been annotated as a translated feature. The intron has not yet been specified, so the translation contains stop codons.
Select the Intron Region

According to the Saccharomyces Genome Database, the TUB1 intron spans positions 26 - 141.
To specify such a numerical range click Edit → Select Range... , type the range, then click Select.
Delete the Selected Feature Segment

To annotate the selected region as an intron, click Features → Delete Feature Segment....

The deletion will be applied to the largest overlapping feature.
The Edit Feature dialog will appear, showing the deletion as applied to the overlapping feature. Two feature segments will flank the deleted region.
Name and edit the color of the new segments if desired.
Click OK to accept the changes.
View the Intron

A dashed line will now mark the intron. The translation will adjust to span the intron boundaries. In this example, the codon for Valine-9 (GTC) is split between the two exons.
Convert Introns to Lowercase

To convert introns in a translated feature to lowercase, click the feature to select it, then click Features → Introns→ Convert Introns to Lowercase... .

The number of introns converted to lowercase is shown. Click OK.
Sequence text in the intron region will be shown as lower case.
Annotate Introns Marked with Lowercase Characters
If the introns in a feature are marked with lowercase characters, you can use this property to annotate the introns. First, click the feature to select it.

Then click Features → Introns→ Annotate Lowercase Regions as Introns... .

Dashed lines will now mark the introns.
Save to keep the changes.