If a grandfather and grandchild are the only two with a genetic disorder, the disorder must be

The disorder where the grandfather and the grandchild are affected is related with the X chromosome and is called Sex linked or X linked disorder.

All the sex linked disorders are recessive in character i.e the normal allele is dominant over the mutated allele. In females, there are 2 X chromosomes, so the mutated allele is only expressed when there are both the mutated alleles, else its masked by the dominant normal allele. But in males, there's only one X chromosome, so if a mutated allele is present, it's readily expressed.

If the Grandfather is diseased, then he must have that mutated allele in X chromosome. Through reproduction, its received by the mother, but she is normal because the other allele received from grandmother was normal. But mother has one of the X chromosomes with mutated allele, which is received by the grandson who again becomes diseased. So the disorder must be X linked disorder

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