Dunbar's gold is a true chimera. He has both female DNA and male DNA cell lines. Interestingly enough, his reproductive organs are male... They did have some difficulty DNA testing his foals back to him in fact. The skin vs blood DNA was different if I remember correctly.
If brindle becomes replicable like Batty Atty, it is because of a genetic mutation that is being passed on. It is possible for a complication, but highly unlikely for a foal to still be viable with such a big mutation. It is an error in DNA replication which is fusing twins. A big error.