Here's why:
* Genetics: Fawn coloring in pugs is caused by a recessive gene. This means that for a puppy to be fawn, it needs to inherit two copies of the fawn gene, one from each parent.
* Black pugs: Black pugs have two copies of the dominant black gene.
* Inheritance: When two black pugs breed, their puppies will inherit one black gene from each parent.
Therefore, it's almost impossible for two black pugs to produce a fawn puppy. The only way this could happen is if one or both parents have a hidden fawn gene. This could occur if one or both parents had a fawn ancestor in their lineage, even if they themselves appear black.
However, it's important to note: Even if both parents carry the recessive fawn gene, it's still not guaranteed that they will have fawn puppies. It's a matter of chance which genes are passed down to each puppy.