Paralogous genes have the same function and have sequence identity
while residing in the same
organism (opposite of orthologous
genes). Humans have other globin
s as well as the famous haemoglobin, most expressed during their time in the womb. So delta globin and haemaglobin are paralogs.
Often occurs due to gene duplication, which can lead to pseudogenes. Indeed, there are globin pseudogenes which are not expressed, and are subject to neutral drift.