When breeding mutations its very important to keep the standard genes in the lineage, as a mutation is a variance from the normal chinchilla gene, and the standard allows for healthier mutations. Generally speaking a mutation is a weaker gene, which often produces smaller animals, this is why its so important to introduce the standard gene to any line of mutations, as it is a stronger gene. This is also why when breeding mutations you often have to wait a little bit longer to put them into breeding as they do not develop as quickly as the standard.