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Fill in the paragraph below by choosing the most suitable word for each gap. A [BLANK] is made up of one long DNA molecule that is divided into sections called genes. Genes may be copied and passed to the next generation. Each gene may have two or more alternative forms called [BLANK]. One of these forms may be [BLANK] and the other recessive. A change in the structure of a gene or in the number of chromosomes is called a [BLANK]. Chemicals and [BLANK] may increase the rate at which these changes occur.