So I have 2 files and something peculiar occurred.

My origin file is a list of SELECT genes like this:

A1BG
A2M
A2MP1

And I have a second file with gene synonyms which is like this:

A1BG   A1B;ABG;GAB;HYST2477
A2M    A2MD;CPAMD5

So, if I read the second file in R and do summary() then the output for the synonyms column is each gene individually. like so:

symbol     synonyms
A1BG: 1    A1B: 7
A2M: 1     TRNAL_CAA: 2

This basically means that in the second file, R can tell that the ';' is a separator in the 2nd column.

But when I append to the 1st file the info from the synonyms file and do summary() for the produced file I get this:

symbol     synonyms
A1BG: 1    A1B;ABG;GAB;HYST2477 :1
A2M: 1     A2MD;CPAMD5: 1

I read both files like this:

synonyms file:

df <- read.csv('homo_sapiens_synonyms.csv', header=TRUE, sep='\t')

joined file:

df <- read.csv('synonyms.csv', header=TRUE, sep='\t')

Why R doesn't separate the values in the synonyms column on the joined file?