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The gene product might be detected only chemically or might lead to a visible trait, such as eye pigment. An alteration (mutation) of the gene might compromise the gene function and result in a disease, such as cystic fibrosis. The position on the chromosome where a particular gene resides is its locus.

Alternative forms of a gene, such as those producing normal and sickle-cell hemoglobin, are called alleles. If the same allele is present in both chromosomes of a pair, the person is homozygous; if the two alleles are different, the person is heterozygous. (The corresponding nouns are homozygote and heterozygote.) A person's genetic makeup is the genotype. Genotype can refer to a single gene locus with two alleles, A and a, in which case the three possible genotypes are AA, Aa, and aa; or it can be extended to several loci or even to the entire set of genes. In forensic analysis, the genotype for the group of analyzed loci is called the DNA profile. (The word fingerprint is sometimes used, but to avoid confusion with dermal fingerprints we shall use the word profile.)

The number of human genes is thought to be between 50,000 and 100,000; the number is quite uncertain. It is known, however, that genes make up only a small fraction of all the DNA in the genome. Even functional genes, especially larger ones, contain noncoding regions (introns). In fact, the great bulk of DNA has no known function. The chromosomal segments used most often in forensic analysis are usually in nonfunctional regions.

The sequence of nucleotides in the genome determines the genetic difference between one person and another. But the DNA of different persons is actually very similar. Corresponding sequences from the same genes in two people differ by an average of less than one nucleotide in 1,000 (Li and Sadler 1991). Yet the total number of nucleotides in a haploid genome is so large, about 3 billion, that any two people (unless they are identical twins) differ on the average in several million nucleotides. Most of the differences are outside the coding regions (genes), so the average number of nucleotide differences in the functional regions between two unrelated persons is much less. Nevertheless, the number of differences in the functional regions is large enough to account for the genetic diversity in the human population that is so apparent in such things as body shape, hair color, and facial appearance.

Before a cell divides, each chromosome is copied. In this process, the two strands of DNA in a short stretch separate, and each single strand copies its opposite, according to the A-T, G-C rule. The process proceeds, zipper-like, along the chromosome until there are two double strands where there was one before. (The entire chromosome is not actually copied sequentially from end to end—this would require more time than the interval between cell divisions; rather, there are multiple starting points along the chromosome.) When the cell divides, the two identical chromosomes, each half-old and half-new, go into separate daughter cells and ensure the genetic identity of the two cells.

Genes that are on the same chromosome are linked; that is, they tend to be inherited together. However, during the formation of a sperm or egg, the two