Recombination
The process by which progeny derive a combination of genes different from that of either parent. In higher organisms, this cur by crossing over.
2. A process of DNA strand exchange between homologous molecules that leads to new combinations of alleles on chromosomes.

3. A event, occurring by the crossing-over of chromosomes during meiosis, in which DNA is exchanged between a pair of chromosomes of a pair.

Recombination often follows DNA transfer in bacteria and, in higher organisms, is a regular feature of sexual reproduction. The basic feature of sexual reproduction is the cyclical alternation between the diploid state, with a double set of chromosomes (two of each kind), and the haploid state, with a single set. Animals and most flowering plants are diploid, and their germ cells (eggs and sperm, or eggs and pollen grains) are haploid. In most fungi, the organism is normally haploid, but sexual fertilization creates a transient diploid cell that immediately divides to form a tetrad of four haploid spores.
In all sexually reproducing organisms, the transition from the diploid to the haploid state is brought about by the process of meiosistwo rounds of cell division accompanied by only one division of the chromosomes. The two chromosomes of each kind present into different haploid meiotic products.

The fact that recombinants types Ac and aC occur at all is due to reciprocal exchanges between chromosomes (crossing over) that take place in the first meiotic division. The reciprocal nature of these events is shown most clearly in fungi such as yeast and the filamentous fungus Neurospora crassa, where to four products of a single meiosis can be recombinant type (Ac) is generally accompanied in the same tetrad by the reciprocal product (aC).