Gametes are formed by the process of meiosis. Germinal cells undergo meiosis. Meiosis is known as reductional division i.e. each daughter cell receives only half set of each gamete contains only one gene set
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The gametes contains only one Gene set because they are formed by the meiotic division.This division reduces the chromosome number to half.This is necessary as the two types of gametes fuse during sexual reproduction to form diploid zygote and thus maintaining chromosome number in the off-spring.