The ψη-globin gene as another example.
"There are very many examples of redundant pseudogenes shared between primates and humans. One is the ψη-globin gene, a hemoglobin pseudogene. It is shared among the primates only, in the exact chromosomal location, with the same mutations that destroy its function as a protein-coding gene (
Goodman et al. 1989). Another example is the steroid 21-hydroxylase gene. Humans have two copies of the steroid 21-hydroxylase gene, a functional one and a untranslated pseudogene. Inactivation of the functional gene leads to congenital adrenal hyperplasia (CAH, a rare and serious genetic disease), giving positive evidence that the 21-hydroxylase pseudogene lacks its proper function. Both chimpanzees and humans share the same eight base-pair deletion in this pseudogene that renders it incapable of its normal function (
Kawaguchi et al. 1992)."
I already made that argument. Due to the randomness of mutation, if the gene was knocked out independently in each lineage then we should see mutations at different positions. If the gene was knocked out once in a common ancestor, then we should see the same mutation at the same position. That is the test. Guess what we see? The same mutation.