 The information within a particular gene is not always exactly the same between one organism and another, so different copies of a gene do not always give exactly the same instructions.Each unique form of a single gene is called an allele.As an example, one allele for the gene for hair color could instruct the body to produce much pigment, producing black hair, while a different allele of the same gene might give garbled instructions that fail to produce any pigment, giving white hair.