Biology Reference
In-Depth Information
Inactive
X
Active
X
Proto X
Proto Y
X
Y
PAR
PAR
PAR
PAR
PAR
PAR
PAR
Testis-determining
gene is combined
with genes that
increase male
fitness
Y
Addition of
autosomal
material to
the PAR
F
M
Müller's
ratchet
Dosage
compensation
Pair of
ordinary
autosomes
XIST
on
XIST
off
XIST
on
XIST
off
PAR
PAR
a
b
c
d
e
Allelic system
of sex
determination
Recombination
suppression
War between the sex
chromosomes; increased
X-gene dosage and
Y deterioration
Regional
X-inactivation
X-autosome and Y-autosome
translocations that are
fertile and viable
Mutation
Double-dose
Single-dose
Inactivated
Figure 2.10.
Model for the evolution of the mammalian sex chromosomes (redrawn from Ellis, 1998). (a) Mammalian sex chromosomes evolved
from a pair of ordinary autosomes. Initially, sex was determined by a simple diallelic system, F and M, with the male being the heterogametic sex.
(b) Sex chromosome differentiation began when the proto-Y chromosome acquired at least one additional gene that together with the M allele
conferred a selective advantage upon males. Combinations of syntenic genes were then selected for a meiotic system in which recombination was
suppressed. (c) The consequence of recombination suppression was the acquisition by the Y chromosome of recessive mutations in the many genes
unrelated to sexual development by the process known as Müller's ratchet. Recessive mutations in the recombination-suppressed segment are not
subject to selection. The loss of fitness of males resulting from hemizygosity of an increasing number of X chromosomal genes was counterbalanced
by increased expression of the X chromosomal genes, thereby raising their level of expression in females. Crossing over is confined to the
pseudoautosomal regions (PAR) and is required for the segregation of the sex chromosomes at meiosis. (d) The inactivation of one X chromosome
evolved in females. Initiation of the inactivation process was controlled by the
XIST
locus. (e) PARs can be sites to which autosomal material can be
translocated. The newly translocated material becomes subject to Müller's ratchet.
