It is an exchange of the corresponding parts between the Chromatids of Homologous Chromosomes. In the case above the recessive hemophilic gene merely changed places with its normal Allemorph. A diagram will make it clearer. Under such circumstances, the Chromosomes would be the units of heredity rather than the genes. We can see that it is because of Crossing Over that no two pups of a litter can be exactly the same, in fact no two pups, or humans for that matter, are ever identical in genetic make up.
Variation is most important to the breeder and the organism.
Many have fallen by the wayside because they produced no variations which could adapt to these changes. The dog was, in all probability, the first animal domesticated. Domestication did not eliminate natural selection although it reduced its importance and substituted human selection in its place. It is primarily human selection wich has produced the great variety of pure bred dogs we have today. There are other forces which contribute to variation and thus make selection more effective but none are more constant than crossing over.