We made an interesting accidental discovery a little while ago: d1 (nd1) is extremely common in Arabian horses!
A batch of Arabian horses were accidentally tested for dun/d1 (nd1) instead of CA, and when I looked at the results every single horse had either one or two copies of d1 (nd1). None were dun. It was only a small batch, but still an interesting result. If the gene frequency we saw was a good random sample of the Arabian population I’d say less than 10% of Arabians are ’n n’, and any Arabian cross is odds on for having at least one copy of d1 (nd1).