In a cell undergoing meiosis, when a single crossover occurs between two genes of interest, the result is

The result is a recombination.

During meiosis, the homologous chromosomes that are paired side by side during phophase I, before being pulled apart by spindle fibres, can exchange genetic material in a process called crossing over

Sometimes when the homologous are pulled apart before the synapses process completes, one of the chromosomes may end up with a piece of the sister chromosome