I am reading a paper that describes the identification of "single copy genes" in plant species.
I'm trying to understand why the below described process is useful:
"To establish a useful criterion for declaring a gene as single copy, each of the five data sets was blasted against itself using BLASTN"
If the gene has been duplicated, there will be 2 of it naturally so how does blasting it against itself tell you that there's two or more of it?