1 that means that my y chromosome might be three

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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 syndrome. 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 ma...
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