What is a genotype? What is the difference between genotype and phenotype?

Genotype is the genes, DNA nucleotide sequences have in the chromosomes of an individual that condition the phenotype. Phenotypes then are a biological manifestation of genotypes.

For example, the altered hemoglobin chain of sickle cell disease and the manifestation of the disease itself are the phenotype. The changed DNA nucleotide sequence in the gene that codifies the production of that abnormal hemoglobin chain is the genotype.