The process that produces sex-cells  (called meiosis) differs from normal body-cell production (called mitosis), in that it produces cells "without" paired chromosomes.

Chromosomes in the Mother cell (Fig. 1) DO NOT duplicate themselves as in normal body cell production. Therefore, the end results are "half cells" with unpaired chromosomes (Fig. 2).  Understanding how meiosis and mitosis differ is essential for the understanding how genes are transferred from parent to offspring; and the continuous mixing of genes in each generation.

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Figure 1.

Mother cell showing nucleus with paired chromosomes prior to the onset of the duplicating process.

 
Figure 2.

Mother cell shown dividing without the duplicated chromosomes.  Only one chromosome of each pair migrate to the eventual half-cell (or, gamete).  Unlike mitosis, which produces copies of the Mother cell, meiosis produces cells with half the original number of chromosomes.