Sexual reproduction
Sexual reproduction is the creation of a new organism by combining the genetic
Genetics , a discipline of biology, is the science of genes, heredity, and variation in living organisms....

 material of two organisms. There are two main processes during sexual reproduction; they are: meiosis
Meiosis is a special type of cell division necessary for sexual reproduction. The cells produced by meiosis are gametes or spores. The animals' gametes are called sperm and egg cells....

, involving the halving of the number of chromosome
A chromosome is an organized structure of DNA and protein found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.Chromosomes...

s; and fertilization, involving the fusion of two gamete
A gamete is a cell that fuses with another cell during fertilization in organisms that reproduce sexually...

s and the restoration of the original number of chromosomes. During meiosis, the chromosomes of each pair usually cross over
Chromosomal crossover
Chromosomal crossover is an exchange of genetic material between homologous chromosomes. It is one of the final phases of genetic recombination, which occurs during prophase I of meiosis in a process called synapsis. Synapsis begins before the synaptonemal complex develops, and is not completed...

 to achieve homologous recombination.

The evolution of sexual reproduction is a major puzzle.