A paired gene whose difference from a normal gene may be responsible for one of the 3,500 chromosomally linked human diseases. Normally, the alleles have the same function (e.g., two alleles control eye color, but one is for blue eyes and the other is for brown eyes). In terms of addiction, one allele may be responsible for normal alcohol metabolism while the other does the same job but does it poorly, so the alcohol has a greater effect.
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