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Announcements
Homework due 9:00 Thursday
– only your first grade will count!
Review Session Sunday 3:00 to 4:00 PM
124 Burrill Hall
Exam Tuesday 9:00 AM, in Lecture room
Sex Chromosomes and Sex
Determination
X and Y are clearly not homologous chromsomes.
How do they segregate normally at Meiosis?
During what part of Meiosis do they pair and
then segregate?
Human Y chromosome
• Yellow: pseuoautosomal regions
• Red: genes with homologs on the X
chromosome
• Blue: genes without homologus on
the X
Genetically, what determines
gender in mammals?
Presence of a single gene (SRY) that
usually, but not always, occurs on the Y
chromosome.
If the Y chromosome is missing (this gene
deleted) or has a non-functional
mutation in the gene, an XY individual
can be a perfectly normal female.
If the SRY gene becomes translocated to
another chromosome, an XX individual
can be a phenotypically normal (but
sterile) male.
XX (+SRY) mice
Dosage Compensation
Do males have half as much of the products of
genes on the X as do females?
X Inactivation
Barr Body: Inactive X
Interphase: Chromomes can’t be stained, but a
dark-staining body is visible in the nuclei of
cells of female mammals
Which X gets inactivated?
Mary Lyon & Lianne Russell
(1961) proposed that one or
other of X becomes
inactivated at a particular time
in early development. Within
each cell,which X becomes
inactivated is ____________.
As development proceeds, all
cells arising by cell division
after than time have _______
_________________.
In 64-cell embryos
Adult female
mammals are
___________
for genes on
the X
chromosome.
Genetic Mosaics
Calico cats are almost always
____________.
One X chromosome carries the
allele for black coat color
The other X chromosome carries
the allele for orange coat color
In 64-cell embryos, one of each
pair of X chromosomes and its
genes are _____________
________________.
Daughter cells inherit active or
inactive X chromosomes,
creating a cat with patches of
coat color
XOXo
Tortoiseshell (calico) coat color
allele
allele
Tortoiseshell (calico) coat color
Tortoiseshell (calico) coat color
Carbon Copy
X inactivation is different in
marsupials
kangaroos, wombats,
bandicoots,
opossums
Sex chromosome aberrations
47,XXY Klinefelter syndrome
male
tall stature
testes do not ma ture
steril e
lowered IQ is comm on
1/700 male bir ths
45,X Turner Syndrom e (XO)
female
sho rt stature
rudimentary ova ries
steril e
IQ typicall y nor mal
1/3000 female births
47,XYY Doubl e-Y syndrome
male
above ave rage height, otherwise pheno typicall y
normal.
At one time , it was claim ed that XYY males are prone
to violent or antisocial behav ior, based on eleva ted
incidenc e of 47,XYY a mong inca rcerated men. Now
though t to be due to higher incidence of moderate
mental retardation than for XY males.
47,XXX
Trisomy-X syndrome
many pheno typicall y no rmal
the frequency of lowered IQ is higher than among
XX females.