Difference between an allele and gene?

Alleles are versions of the genes present in the cell nucleus. In humans, there are always 2 versions of each gene (except for genes on the X chromosome in males because there is only 1 X chromosome). Each of the two alleles are obtained from one parent. Some organisms are not diploid but tetraploid, so they have 4 alleles of each gene (with minor exceptions). Each allele is a version of the gene, but we say that the organism which possesses Ss has 2 different alleles of the same gene. Typically there may be dozen or hundreds of different alleles for some genes in the human population whereas other genes don't have more than 1 allele. These genes are called "highly conserved" because any mutation of that gene is not compatible with life. Some genes don't have more than 2 or 3 different amino acids when comparing this protein between wildly divergent species, such as cows and peas.