Gamete cells the sex cells reproduce using Meiosis or sexual reproduction. 1 cell becomes 4 new cells. Genes are the smallest chemical parts that form segments of chromosomes. Genes determine what we look like. There are not two organisms alike in the world unless you are an identical twin or a clone.
Alleles are an alternate form of a gene for one trait.
E.g. Brown and blue eyes are two different alleles for eye color. We have 2 alleles for any, one trait, one from our mother and one from our father. They may be the same form of the gene or two different forms. The gene may or may not be expressed when you observe a living organism. Phenotype is the expressed appearance of a living organism.