How can a person receive two diffrent alleles?

If a person has a single sickle cell allele they will have some sickles red Since diploid organisms have two copies of each chromosome, they have two of each gene. Since genes come in more than one version, an organism can have two of the same alleles of a gene, or two different alleles. This is important because alleles can be dominant, recessive, or co-dominant to each other. blood cells, and some normal red blood cells.