The pewter boa is a combination of the blood and the anerythristic boa.

The genetic combination was present in the wild-caught animals. Apparently the original wc blood boas also carried a gene for the anerythristic trait. When bred together, a few pewter boas appear in a litter of blood boas. In such a litter some of those blood boas are also het anery, but the hets are not identifiable. Both blood and anerythristic traits are simple recessive traits.