Homologous genes are derived from common ancestors either due to orthology or paralogy.
The sequence in the gene that is homologous is generally termed conserved.
Homology in the gene is not similar to sequence similarity.
Similar sequences can be due to some ancestor of the chance event which is completely different from homologous genes.
Homologous genes can be due to:
Orthologous homologous genes: Orthologous genes in different species are the genes that originated from vertical descent. for example- regulatory protein sequences in Chlamydomonas and Arabidopsis share significant similarities in sequence and they share functional domains however Chlamydomonas has much more complex protein.
Paralogous homologous genes: The gene of an organism is replicated to give reserve two different sites in the same genome. they are usually present in some species but this is not compulsory. for example- myoglobin genes in chimpanzees are paralogous to the hemoglobin gene of humans.
