Why is A right

I was very close to choosing A as it definitely is on the right track for the answer. However I thought that the statement of having "one less gene" was too strong. The passage in my view only told us that one gene had been changed or removed it didn't however tell us that overall it had one less gene in general. I considered the possibility that they simply swapped it out in which case that answer would be wrong. Am I over thinking this or is there something I missed that confirms that was actually 1 less gene in total?