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The molecular mechanism
of X chromosome inactivation,
presented
by Professor Neil Brockdorff
of the Department
of Biochemistry,
University of Oxford.
I'll begin my talk
with a short introduction
followed by a discussion
of critical steps
in the X inactivation process.
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X chromosome inactivation
is an important
form of epigenetic regulation
that evolved in mammals
to equalize the levels of expression
of genes on the X chromosome,
in XX females relative
to XY males.
Briefly,
during early development
of a female embryo,
each individual cell
triggers inactivation
of one of the two X chromosomes
that are present.
The process is normally random.
So in an individual cell,
there's an equal probability
of the X chromosome inherited
from the mother or the father
being selected
as the inactive X.
Once X inactivation
has occurred,
cells remember
which of the two X chromosomes
was inactivated through
all subsequent cell generations.
As a consequence,
female mammals or chimeras
comprised of a mosaic
or patchwork of cell populations
with either one
or the other X chromosome
being inactive.
A classical illustration
of X chromosome inactivation
is seen in the coat
of the calico cat,
which you can see in this image.
A gene that gives rise
to orange coat color
lies on the X chromosome.
And calico cats
are heterozygous
for this gene.
The wild type allele encodes
black coat color.
So when X inactivation occurs
in early development,
cells either
inactivate the chromosome
with the wild type gene,
giving rise
to orange coat color,
or the chromosome
with the orange gene,
giving rise to black coat color.
These individual cells then
expand into a clonal patch cells
during further development,
giving rise to these areas
with either orange
or black coat color.
