What is a gene, genome and allele?

A gene is a segment of DNA that contains the instructions for building one or more molecules that help the body work. It is the basic physical and functional unit of heredity. Genes are made up of DNA and each chromosome contains many genes.

A genome, on the other hand, is the complete set of genes or genetic material present in a cell or organism. The human genome, for example, includes not only the genes, but also the noncoding DNA as well as the genetic blueprint for life.

An allele is a variant form of a gene. Some genes have a variety of different forms, which are located at the same position, or genetic locus, on a chromosome. Humans are called diploid organisms because they have two alleles at each genetic locus, with one allele inherited from each parent. Each pair of alleles represents the genotype of a specific gene. Alleles contribute to the organism's phenotype, which is the observable physical properties of the organism, including the organism's appearance, development, and behavior.