Before the split, however, a serious error can occur. Palindromes on one Y chromosome can occasionally reach over and form a fatal attraction with the counterpart palindrome on its neighbor. The two Y’s fuse at the point of joining, and everything from the juncture to the end of the chromosome is lost

The double-Y’s so generated come in a range of lengths, depending on which of the palindromes makes the unintended liaison. Like other chromosomes, the Y has a left arm and a right arm with the centromere in between. The male-determining gene lies close to the end of the left arm. If the palindromes at the very end of the right arm make the join, a very long double-Y results in which the two centromeres are widely separated. But if the joining palindromes are just to the right of the centromere, a short double-Y is formed in which the two centromeres lie close together.

Dr. Page detected among his patients both short and long double-Y’s and those of all lengths in between. He and his colleagues then noticed a surprising difference in the patients’ sexual appearance that depended on the length between the centromeres of their double-Y’s.

The patients in whom the distance between the Y’s two centromeres is short are males. But the greater the distance between the centromeres, the more likely the patients are to be anatomically feminized. A few of the patients were so feminized that they had the symptoms of Turner’s syndrome, a condition in which women are born with a single X chromosome.

The explanation for this spectrum of results, in Dr. Page’s view, lies in how the double-Y’s are treated in dividing cells and in the consequences for determining the sex of the fetus.

When the centromeres are close together, they are seen as one and dragged to one side of the dividing cell. As long as the Y’s male-determining gene is active in the cells of the fetal sex tissue, or gonad, the gonads will turn into testes whose hormones will masculinize the rest of the body.

But when the centromeres lie far apart, chromosomal chaos results. During cell division, both centromeres are recognized by the cell division machinery, and in the tug of war the double-Y chromosome may sometimes survive and sometimes be broken and lost to the cell.