you have to introduce the gene,you can't make it.Only brindle does can be reared all brindle bucks die at around 10 days,best to cull immediately so that they don't suffer.Any non brindle bucks and does in the litter won't be carrying the brindle gene so are also useless for creating more.Therefore you are always on a knife edge with them.You get litters with only brindle bucks or no brindles at all.It is quite exciting waiting to see what you get,good luck with them.