Imagine that you want to breed a species of rose that has a wonderful smell. You have many different types of roses, but none of the individual roses has the wonderful smell that you want. What is the best way to go about breeding a pure species rose with the trait that you want?

Answer: if i want to breed a rose and i don't have a certain kind of rose, i have to cross between them until i have a kind of rose that express the phenotype i was looking for. Once i've got it, i'll try to cross it with another rose and generate more species like that to cross with the rose that express the phenotype, this favors to generate a specie that have a pure phenotype.

Explanation: The answer is explain by the Mendel's laws.

Mendel's second law:

if you have two roses that are heterozygous and you cross them, their offspring have a 25% probability of generating a homozygous phenotype for a certain recessive characteristic (such as the rich smell of roses that is not a dominant character), 50% of generate heterozygotes with a dominant phenotype, and 25% generate other homozygotes with a dominant phenotype.

Once the offspring with the recessive homozygous character are obtained, it is possible to start making crosses between those of a recessive nature and there Mendel's first law applies

All the homozygous recessive individuals that are crossed will have a 100% homozygous recessive offspring.

Also, if you cross heterozygotes with another heterozygotes the offspring will be 100% heterozygotes