Difference between Disjoint and Excess genes in NEAT?

Question

I was reading Stanley's paper but I couldn't figure out what exactly are Disjoint and Excess genes in NEAT. I understand they appear to be related in some particular way with the fact that all of them contain innovation numbers not pertaining to both parents. But what distinguishes them?

Could anyone shed some light into the issue?

Answer 1

When aligning two genomes by gene ID (innovation number), the mismatches at the ends are referred to as excess genes, and all other mismatches are referred to as disjoint genes. As far as I am aware no NEAT implementation has ever treated disjoint and excess genes differently. The distinction was made in early NEAT papers, most probably because treating the types of mismatches differently was being suggested as a possible future research topic.

(FYI - speaking as the author of SharpNEAT).