In Dogs black court colour is dominant over white.What coloured dog will you choose to breed with a given black dog in order to find its genotype .What is this type of cross known As?

Black coat is dominant. Let balck coat be represented by 'B'. Hence white coat is represented by 'b'. As black is dominant, and as we have black dog, we must breed it with homozygous recessive trait to know its genotype. So we choose white , homozygous dog (bb).

If our black dog is homozygous, BB,
we have BB * bb ,

BB	Bb
Bb	bb

We will have 3 black and 1 white puppies(3:1).

If our black dog is heterozygous dominant, bb,

we get
Bb*bb Bb Bb bb

bb

So, we have 2 white and 2 black offsprings(1:1).
Hence, from results , ie, ratio of white to black puppies, we can find genotype of our black dog.

Here, we breed 2 dogs, of same species, but different phenotype. Scientifically, this is called Phenotypic disassortive mating. You don't have to remember that name. According to NCERT, it is just called test cross. These types of crossing are called test cross.