The portrait is by Elliott & Fry; a bromide print taken on 29 March 1951, one of three from the National Portrait Gallery collection

Solution

This one is deliberately misleading: it’s not Morse code. You can tell because several of the symbols exist.

In fact the dots and dashes are placeholders for characters in the sentence The portrait is by Elliott & Fry; a bromide print taken on 29 March 1951, one of three from the National Portrait Gallery collection.

The dashes are the characters you need to extract:

The portrait is by Elliott & Fry; a bromide print taken on 29 March 1951, one of three from the National Portrait Gallery collection

The answer is FATHERING.