How to Analyze X-linked Dominant Pedigree?

I will start my discussion based on the above pedigree.

In order to determine X-linked dominant trait, we need to determine the following things:

1. Make sure the pedigree is dominant
2. Make sure it follows DDD rule

In order to determine the dominant trait, it should have the following characters:
a. There should not be skipping of generation
b. Either of the parent should be affected

Now what is DDD rule?

Once we assure that the pedigree is a dominant one, we can apply DDD rule.
The first D represents dominance
The second D represents affected Dad
The third D represents that if dad is affected, all his daughters should also be affected.

Now, take a look at the picture:

There is no skipping of generation, and in every generation, one of the parents is affected. So, the above pedigree shows dominant trait.

Now, we can use DDD rule. Look at the affected dad in the second generation. So, the rule says that if the dad is affected, all his daughters will also be affected, so in the third generation, we can see that all daughters are also affected. So, its an X-linked dominant trait.