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One way to analyze genome data is to consider where genes function. We can also categorize genes according to their roles at the molecular and cellular levels.

Each chromosome includes hundreds or thousands of genes. Since we have two copies of each chromosome, we also have two copies of each gene, each located at the same position on the homologous chromosome pairs. Sometimes the members of a gene pair are alike, their DNA sequences specifying the same amino acid sequence of the protein product. However, because a gene consists of hundreds of nucleotide building blocks, it may exist in variant forms, called alleles. An individual who has two identical alleles of a gene is said to be homozygous (ho"mo-zi'gus) for that gene. A person with two different alleles for a gene is said to be heterozygous (het'er o zi'guz) for it. A gene may have many alleles, but an individual person can have a maximum of two alleles for a particular gene.

The allele that causes most cases of cystic fibrosis was discovered in 1989, and researchers immediately began developing a test to detect it. However, other alleles were soon discovered. Today, hundreds of mutations (changes) in the cystic fibrosis gene are known. Different allele combinations produce different combinations and severities of symptoms.

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