Answer: A male who is affected by a defective gene would have a symbol of shaded square that the horse breeder would use in the pedigree that is ■.
Pedigree can be described as a representation of a family and its lineage ( that is its ancestory and line of descent) in a form of flow chart ( using standard symbols), which depicts the pattern of inheritance of a particular disease.
For a male, a symbol of square is used and for a female, a symbol of square is used ( that are shaded if the individuals are affected with a disease)