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THE QUEST for an understanding of how genetic factors contribute to human disease is gathering speed. Most notably, the draft sequence of the human genome is now available. This rough sketch provides us with the first holistic view of our genetic heritage and is a significant landmark on the road to a complete human genome reference sequence. This will be a fundamental resource in future biomedical research.
   The 46 human chromosomes (22 pairs of autosomal chromosomes and 2 sex chromosomes) between them house almost 3 billion base pairs of DNA that contains about 30,000 - 40,000 protein-coding genes. This is a much smaller number than predicted - only twice as many as in the worm or fly model organisms. The coding regions make up less than 5% of the genome (the function of the remaining DNA is not clear) and some chromosomes have a higher density of genes than others.
   A great deal of effort over the past ten years has been put into creating a physical map of the human genome - ordering genes within the genome by placing landmarks to navigate by. This physical map has provided an excellent framework for completing the draft sequence, as well as assisting directly in the identification of about 100 disease genes.
   MOST OF THE genetic disorders featured on this web site are the direct result of a mutation in one gene. However, one of the most difficult problems ahead is to find out how genes contribute to diseases that have a complex pattern of inheritance, such as in the cases of diabetes, asthma, cancer and mental illness. In all these cases, no one gene has the yes/no power to say whether a person has a disease or not. It is likely that more than one mutation is required before the disease is manifest, and a number of genes may each make a subtle contribution to a person's susceptibility to a disease; genes may also affect how a person reacts to environmental factors. Unraveling these networks of events will undoubtedly be a challenge for some time to come, and will be amply assisted by the availability of the draft (and complete) sequence of the human genome.

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