Yet, careful students of genetics knew that the Y chromosome was an inhospitable place for genes. Unlike any other chromosome, the Y is “unpaired”—i.e., it has no sister chromosome and no duplicate copy, leaving every gene on the chromosome to fend for itself. A mutation in any other chromosome can be repaired by copying the intact gene from the other chromosome. But a Y chromosome gene cannot be fixed, repaired, or recopied from another chromosome; it has no backup or guide (there is, however, a unique internal system to repair genes in the Y chromosome). When the Y chromosome is assailed by
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