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Unformatted text preview: mes that came from our mothers and our fathers gather, singly. Usually, as the cell pulls itself in two, one chromosome lands inside each
of the daughter cells with perfect parity, but not always. Sometimes
chromosomes cling to one another, and stick. Then, when the dividing
cell tries to pull them apart, the chromosomes don’t budge. A brief tugof-war ensues, then one side gives in, and both chromosomes fall into
only one of the daughter cells. The other cell gets none.
After that, when a sperm or an egg with two copies of chromosome 62 Between XX and XY 21 combines with a sperm or an egg containing the usual one copy of
chromosome 21, a baby begins, and each of his or her cells has three
copies of chromosome 21—trisomy 21.
We call this nondisjunction—chromosomes that fail to disjoin as
the mother cell divides. And we call these people abnormal, because
each of their cells holds one extra strand of DNA, because they have
forty-seven instead of forty-six chromosomes.
Nondisjunction can happen with any chromosome, including the
sex chromosomes X and Y. A single sperm or egg may end up with two,
three, or more X chromosomes, and a single sperm may hold more than
one Y chromosome. In truth, sperm and eggs come in variety packs.
If that alone isn’t enough to derail the simple XX/XY, female/male
idea, a mystery known as anaphase lag can also cause developing sperm
or ova to lose an X or a Y chromosome along the way. And even after
fertilization, sex chromosomes can be lost or gained. And even among
men with the normal 46,XY karyotype, the size of the Y chromosome
can vary.1 That means that my Y chromosome might be three times the
size of Arnold Schwarzenegger’s Y chromosome. Here certainly, quantity matters; perhaps size does as well.
The end product is a panoply of possible sexes by any definition, an
array of human beings as grand and as varietal as the fragrances of flowers: 45,X; 47,XXX; 48,XXXX; 49,XXXXX; 47,XYY; 47,XXY; 48,XXXY;
49,XXXXY; and 49,XXXYY.
People as Syndromes: Dr. Klinefelter’s Discovery
In 1937 Harry Fitch Klinefelter graduated from Johns Hopkins Medical
School and took a job at Harvard Medical School working with Fuller
Albright, a physician famous for his work in endocrinology. Among Dr.
Klinefelter’s first patients was an African American man with enlarged
breasts and small testes—something the doctor had not seen before.
But the more he searched, the more patients like this he found. In 1942
Dr. Klinefelter published a paper describing nine such men. Each was
sterile, each had gynecomastia (enlarged breasts), unusually long arms,
normal-sized penises, and small testes.2 Where Our Sexes Come From 63 It turned out that each of these men also had forty-seven chromosomes. Each was 47,XXY. This syndrome became known as Klinefelter
syndrome and grew to include all karyotypes with more than one X
chromosome plus one or two Y chromosomes. Because of the presence
of a Y chromosome (containing SRY), all of these people have penises.
Because of that we call them males, but they all have more than their
fair share of X chromosomes.
Only about 40 percent of fetuses with extra X chromosomes survive
to term. Nevertheless, somewhere between 1 in 500 and 1 in 1,000
boys born in any given day have one or more extra X chromosomes.3
That makes Klinefelter syndrome the most numerous among those syndromes with an unusual number of chromosomes.
In the end, this adds up to about three thousand live births every
year in the United States, and maybe as many as sixty thousand worldwide, of boys with extra X chromosomes. Most of these men are never
diagnosed. Nothing about these people would allow anyone except a
geneticist to even suspect that they differ from others. Of those diagnosed, most find out only when they marry, fail to have children, and
then undergo sterility testing.
The least common form of Klinefelter syndrome is the 48,XXYY
karyotype. These people very closely resemble those with the 47,XXY
karyotype except 48,XXYY men are taller, nearly five inches taller, on
average, than their fathers. Apparently, one of the things Y chromosomes do is provide the necessary genetic material for the tallness of
men in comparison to women. And in a double dose, it seems to push
people a little higher.
Dr. Turner’s Findings
The second largest group with unusual numbers of chromosomes
includes those with the 45,X karyotype, commonly called XO or Turner
When a sperm containing an X chromosome fertilizes an ovum with
no X chromosome, the zygote that forms has only forty-five chromosomes. Or if a chromosome gets lost because of anaphase lag or some 64 Between XX and XY other reason, the zygote that develops will have only forty-five chromosomes, 45,X.
Since Henry Turner first described some of these people in 1938,
we call it Turner syndrome. But of course it’s more than a syndrome,
much more. Just as with every other differently numbered collection of
chromosomes, for those who survive it, 45,X is a way of life.
The 45,X karyotype appears in about 0.8 percent of zygotes. That
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- Spring '14