Homolog: It describes a divergent evolutionary relationship between genes and proteins based on their sequence similarity/identity.
Orthologs: They have the same gene/protein in different species. These are derived from a common ancestor gene through vertical descent.
Paralogs: They have the same genes/proteins within the genome of a species. These are derived from a common ancestor gene through duplication events.
Differenciate Between Homolog , Ortholog, Paralog
A gene related to a second gene by descent from a common ancestral DNA sequence. The term, homolog, may apply to the relationship between genes separated by the event of speciation ortholog or to the relationship betwen genes separated by the event of genetic duplication paralog.
Orthologs are genes in different species that evolved from a common ancestral gene by speciation, and, in general, orthologs retain the same function during the course of evolution. Identification of orthologs is a critical process for reliable prediction of gene function in newly sequenced genomes.
Paralogous genes (or paralogs) are a particular class of homologous genes. They are the result of gene duplication and the gene copies resulting from the duplication are called paralogous of each other. Paralogous genes can be retained in the genome after their duplication, but some copies can also be lost.