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Unformatted text preview: kes 45,X the most common human chromosomal anomaly. But
only about 3 percent of these fetuses survive to term. 45,X is also the
most common chromosomal anomaly found in spontaneously aborted
human fetuses—nearly 18 percent.4 In the end, about one in 2,700
live newborns has the 45,X karyotype.5 That is about fifteen hundred
newborns per year in the United States and perhaps as many as thirty
Just like the extra X chromosome, the absent X chromosome changes
things. Externally and internally, at birth these people are female, although
they are not identical to 46,XX females. The external genitalia are those
of a girl. The vagina and uterus usually develop normally as well, but
the ovaries do not. In place of ovaries, 45,X people have only streaks of
tissue called streak gonads, or sometimes streak ovaries. Streak gonads
do not produce normal levels of hormones. During fetal development
those hormones play a relatively small tune. But at puberty, people with
fully functional ovaries get a dose of estrogen as strong and focused as
the solo note of a French horn. In response to that call, breasts and pubic
hair appear, hips widen, and menstruation begins.
Streak gonads don’t know how to play that note. At puberty their
horns are silent. Because of that, in 45,X girls, secondary sexual changes
don’t occur, and only a very few of these people ever menstruate. Hormone supplementation can change most of that. And, using donor eggs,
these people can have children.
In 46,XX people, both X chromosomes play parts in oogenesis
(egg formation) and the development of the fetal ovaries. By the end
of fetal development the ovaries contain as many as seven million
oocytes. By puberty this number has dwindled to as few as four hundred thousand, and by menopause, fewer than ten thousand eggs Where Our Sexes Come From 65 survive. By contrast, in individuals with Turner syndrome, oogenesis
begins normally in early embryonic development, but without a second X chromosome, millions of oocytes begin to die. And they continue to die through the rest of fetal development and for the first
years of the child’s life. By age two, there are no ova left, and the
ovaries degenerate into fibrous streaks. In these women, menopause
happens before menarche.7
In addition, people with Turner syndrome are usually short (under
about four feet ten inches), have arched or “shield” chests, and webbed
necks.8 Sometimes the absence of that X chromosome affects people’s
brains as well, but rarely. Only about 5 percent of people with Turner
syndrome have any degree of mental retardation.
Mosaics: Putting the Pieces Together
Over four thousand years ago, people began to create art using little
pieces of broken stone. These mosaics usually contained stone quarried
or culled from very different sources so that the end product had at least
two dissimilar sets of fragments arranged together to create an image.
Some of the greatest of these were found when the ruins of Pompeii
Beyond the courtyards of Pompeii and the rotundas of our great
houses of state, some people are mosaics too, and not in some literary
sense. Through curious acts of living cells, some people have different numbers of chromosomes in different cells inside their bodies,
like different pieces gathered from different stones. When those cells
with differing numbers of chromosomes come from one zygote (one
fertilized egg), this is called “mosaicism,” and the affected people are
living mosaics in a very real biological sense. When the mosaicism
involves the sex chromosomes, we call these people sex-chromosome
Sometimes, as the zygote begins to grow, nondisjunctions occur
during the first few divisions. If that happens with the sex chromosomes,
and one cell gets two X chromosomes and the other none, then as the
individual develops, some of her cells are 45,X and others 46,XX. 66 Between XX and XY Even though it seems that this sort of nondisjunction should create
one cell line that contains three X chromosomes, this doesn’t seem to
happen. Other times, nondisjunction creates mosaics with three different sex-chromosome karyotypes, some even including normal male
and female karyotypes, like 46,XY/45,X/46,XX or 45,X/46,XX/47,XXX
cells—all inside one person.
All of these variants fall under the heading of Turner syndrome,
which includes people with mosaic mixes of cells including 45,X cells
with 46,XX cells or 46,XY cells. People who have cells with abnormal
X chromosomes (usually X chromosomes that have lost some of their
DNA) also fall under the heading of Turner syndrome.
During cell division, whole pieces of the long arm (called the q
arm) or chunks of the short arm (called the p arm) of the X chromosome may get left behind, creating X chromosomes that don’t do all
that they normally do. These, coupled with one or more complete X
chromosomes, can create karyotypes like 46,XXp- or 46,XXq- (missing pieces of the p arm or the q arm), 46,XXr (with a ring chromosome
formed from a piece of the X chromosome), 46,XXqi (another abnormal form of the q ar...
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This document was uploaded on 02/04/2014.
- Spring '14