Determining whether zippy carries this allele

Zippy is a puppy, you want to put him out to stud. However, his grandpa had a rare sickness that leads to brittle bones. The disorder is hereditary and results from homozygosity for a recessive allele. If your dog is heterozygous for the allele, it will not be possible to use him for stud, since the genetic defect may be passed on. How would you go about determining whether Zippy carries this allele?

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