Until recently, the starting point for an identification of the gene responsible for a particular inherited disease was a pattern of inheritance in particular families plus a knowledge of the tissues affected. It is very difficult to find the gene responsible for a disease when the identity of the normal protein is unknown. Very often, the first clue is the identification of other genes that are inherited along with the malfunctioning gene and that are therefore likely to lie close on the same chromosome (this is linkage, page 408). In the past, chromosome walking was then used to identify the disease gene. This is a slow and tedious process that involves isolating a series of overlapping clones from a genomic clone library. One starts by isolating a single clone and then uses this as a probe to find the next clone along the chromosome. The second clone is used to find the third, and this is used to find a fourth clone, and so on. Each successive clone is tested to find out if it might include all or part of the gene of interest (e.g., by using northern blotting, page 148, to see if the gene is expressed in a tissue known to be affected in the disease). Once a candidate gene is identified, one can find out if it was the gene of interest by sequencing it in unaffected individuals and in disease sufferers; if it is the disease gene, its sequence will be different in the affected people. With the publication of the entire human genome, chromosome walking to generate a series of overlapping clones for testing is unnecessary, but identifying the gene that is responsible for a particular inherited condition is even now a time-consuming task.
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