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Unformatted text preview: f tightly clustered grapes. Then, following some direction
laid down a hundred million years ago, a few cells condense into a mass
on one side of the cluster, and a fluid-filled cavity opens at the opposite
side. Surrounding all of this is a thin crystal-clear membrane, a shell.
At the end of the fifth day, the embryo begins to rhythmically
expand and contract. As those contractions intensify, the shell bursts,
and the embryo hatches. All of this happens as the embryo slowly wanders through the labyrinth of the Fallopian tube. But by about day six
this odyssey comes to an end, and the embryo implants itself in the
frothy wall of the uterus.
The embryo now has two inner cavities—one will become the
amniotic sac that will surround the developing fetus, and the other the
secondary yolk sac. In between those two cavities is a thin strip of cells. Where Our Sexes Come From 51 Within that narrow thread live the cells that will become the child—the
living bridge between placenta and yolk, a bridge between amoeba and
ape, between an exploding universe and the darknesses still to come.
Out of that cyclone of dividing cells, one of the first groups of cells
to commit itself to a final future is the group that will form the gonads.
These cells may become ovaries inside a new baby girl or testes in a
new baby boy—bits of life destined by the finger of a force beyond our
imagining to generate eggs or sperm. And all of that happens before we
are even four weeks old.
By about week six, the embryo has assumed a vaguely recognizable form, and at a point just below the umbilical attachment the germ
cells have settled into a structure called the gonadal or urogenital ridge.
If some mutation prevents the formation of the urogenital ridge, no
gonads and no sexual characteristics will ever develop, and usually no
kidneys or adrenal glands will develop either. In this case, the embryo
turns out its small lamps and slips back beneath its mother’s blankets;
it stops developing, and the uterus reabsorbs it.
But mutations that prevent the formation of the urogenital ridge
are rare, which seems nearly a miracle, because, along the way to the
gonadal ridge, dozens, perhaps hundreds of things need to occur with
Swiss precision—enzymes must appear just as they are needed, then
just as quickly fade. Pieces of DNA must be awakened at a precise hour,
then lulled again to sleep. RNA must ratchet through the embryo’s
world, grinding out proteins like cars off an assembly line. Groups of
cells must migrate from south to north, east to west, and back again.
Like an orchestra, each section plays its part—the reeds, the brass,
the percussion, and the strings all make precisely timed and critical contributions. Occasionally in the brass section, French horns are swapped
for flutes, and among the violins, snare drums appear; new musicians
arrive and depart, the floor raises and lowers periodically, as does the
curtain. And without any one piece of this, things change, the music
shifts, often in unpredictable ways.
But if everything goes more or less as usual, at about seven weeks
a pair of gonads appears near the urogenital ridge—gonads already
filled with cells that will one day become eggs or sperm. At this point 52 Between XX and XY the gonads are somewhere between male and female; the final sex of
the fetus depends on what happens next. If nothing (or nearly nothing) happens, the primitive gonads become ovaries, and the embryo
develops the external and internal machinery of a girl—ovaries, uterus,
Fallopian tubes, vagina, clitoris, labia. Female, in other words, is development’s default mode; without orders to the contrary, women’s bodies
make more women. Perhaps that isn’t as surprising as it might seem,
given that sponges, starfish, whiptail lizards, and others do the same
thing—all without males. Since we evolved from one of those all-female
species, maybe it isn’t startling that reproduction’s first choice is female.
Only the gradual evolution of the Y chromosome (from an X chromosome) changed that.
Compared to making girls, making boys is harder work. And
though many things can go wrong on the way to making girl babies,
there are even more potential detours, pitfalls, and wrong turns on the
road to making baby boys.
The human X chromosome is long, wasp-waisted, and full of information. The Y chromosome, on the other hand, is an odd-looking little
blob of DNA. But there are genes on the Y chromosome that can make
all the difference when it comes to determining sex. One part in particular of the Y chromosome plays a critical role. We call it the sexdetermining region of the Y chromosome, or SRY, and it is essential for
making baby boys.
Genes in this part of the Y chromosome operate like switches to
turn on other genes that are critical to male development. Without SRY,
the primitive gonads do not develop into testes, and sometimes don’t
develop at all.1 With SRY, the gonads, until now amorphous little tissues, transform into testes and by about four months begin to do some
of the things that testes do.
The SRY part of the Y chromosome is essential for making baby boys,
but it is not sufficient. After th...
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- Spring '14