Answer:
Incomplete dominance
Explanation:
According to Mendelian inheritance, when a gene is heterozygous, one of the alleles must be dominant while the other must be recessive. This is not the case with fur inheritance because heterozygous individuals showed a trait that is halfway between dominant and recessive (median fur thickness).
The allele for thick fur is not completely expressed over the allele for thin fur, a clear-cut case of incomplete dominance.