Why do gametes have only one allel while the other cells have a pair of allels?

Gametes have only half the number of chromosomes as compared to normal somatic cells. Gametes i.e. sperm and eggs are special because while every other cell in our bodies has two copies of our genes (one from mom and one from dad), sperm and eggs have only one copy of each gene, i.e. they are haploid. Sperm and eggs are created through a process called meiosis. So for each character, the gametes carry only one gene. When these gametes unite, then the resulting zygote will be diploid and will carry a pair of alleles for each character.