MCB 104; Handout #8
, will mutations that prevent the functioning of the dosage
compensation machinery be lethal in males, females, or both?
dosage compensate by increasing the expression of X-linked genes in
So without dosage compensation, females would have a normal (2x) dose of X-
linked genes, but males would only have a one-half dose (1x) of X-linked genes, and not
will mutations that prevent the functioning of the dosage
compensation machinery be lethal in males, hermaphrodites, or both?
dosage compensate by decreasing the expression of genes
on each of the X chromomosome in hermaphrodites.
Without dosage compensation,
males would have a normal dose of X-linked genes (1x), but hermaphrodites would have
a double dose (2x), and would not survive.
In mammals, will deletion of the X inactivation center be lethal to males, females, or
Lethal to females—provided the X inactivation center is mutant on both X’s.
If at least
one X has an X inactivation center, it can be silenced and form a Barr body, and therefore
allow the female to dosage compensate (and only have a single dose of X-linked genes).
However, if a female cannot silence either X chromosome, she will have a double dose of
X-linked genes, which will most likely be lethal.
Males would not be adversely affected
by a deletion of the X inactivation center.
What is the Lyon hypothesis?
If a human female is heterozygous for the deletion of
an X-linked gene that is essential for the survival of toenail cells, what will be her profile
of X chromosome inactivation in toenail cells?
In each cell, one X chromosome is randomly silenced to form a Barr
The X that is silenced will be silenced in all progeny of the original cell (i.e. the
whole cell lineage will have the same silenced X chromosome).
All of the toenail cells will have silenced the X that carries the deletion of the toenail-
This way, the toenail cells will be expressing genes from the wild-type
X, and thus will express the gene that is essential for toenail development.
The precursor cells might have been heterogeneous for X inactivation, but the cells that