Aresearcher is studying mice that carry a mutant allele of a gene called pax3. animals heterozygous for this mutation demonstrate a malformed nervous system, a cleft palate, and abnormal fur pigmentation. as this mutant pax3 allele affects several mouse traits, this allele is:

This allele is a wild type.

Usually, the most prevalent allele, which means is the allele more frequent of that gene, is the wild type. It seems that this is the case, since it's very frequent to find a Pax3 allele in mice.

In this case, the "Mutant" alleles became the wild type. It may be a result from a genetic shift within the population.