"brown hair in dogs is controlled by a recessive allele b. black hair is controlled by a dominant allele B at the same locus. what is the probability of two heterozygotes producing a black puppy?

The Punnett square demonstrate the genetic results when two individuals cross. In order to make the square, we have to draw a big square with small squares inside, each representing all possible combinations of alleles. Since we have 2 alleles we have 4 possible combinations, that means 4 small squares.

In the up part of the square we introduce the alleles of the parent 1 and in the left part the alleles of parent 2. The results will be written in the small squares.

The attached image represents the crossing of two heterozygotes Bb x Bb

Represented by red there's 25% of probability of the puppy be homozigot BB, that means he will be black.

Represented by green there's 50% of probability of the puppy be heterozigot Bb, that means he also will be black, since B is dominant.

Represented by blue there's 25% of probability of the puppy be homozigot bb. That means he will be brown, since there's no dominant B there.

Total: 25% (BB) + 50%(Bb) = 75% of being black