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genes and alleles

Genetic code, DNA (deoxyribonucleic acid), is packaged in cells in the form of chromosomes, or a string of genes. A chromosome contains many genes and elements, such as proteins, that regulate genes. An allele is a viable DNA coding occupying a specific position on a chromosome. Alleles define a genotype. For example, a single gene controls the colour of petals on most flowers. But the gene may comprise several alleles, including one that produces red petals and another that produces white petals. The flower's colour will reflect the interaction of the alleles; white, red or pink (a combination of the two). Also see Nature Genetics.