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Extra credit problem for
Lecture #6
For extra credit question,
please use the index cards
provided.
• Print your name and
section # at top of card.
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What are the Barr
bodies that occur
in the cells of
female
mammals?
Announcements
Second Homework Assignment
– only your first grade will count!
Second set of Recommended Problems
Polyploidy
Autopolyploidy
Allopolyploidy
Triploidy
Triploid
Tetraploid
P:
2n
2n
P:
2n
2n
G:
n
2n
G:
2n
2n
F1:
3n
F1:
4n
Many plants are polyploids
• As many as 30-70% of flowering plants are of
polyploid origin
• In many genera, different species will have
different ploidy levels (multiples of a base
number) representing a series of polyploids.
In the genus Chrysanthemum, different
species have chromosome numbers = 18, 36,
54, 72, 90, and 198 - all multiples of a base
chromosome number of n=9.
Why less common in animals?
Many plants have meristematic tissue
throughout their lives and are self-fertile: In
plants, about 5 out of every 1000 gametes
produced is diploid!
Many animals have chromosomally determined
sex, and polyploidy interferes with this.
Many animals have isolating mechanisms
(geographic, temporal, behavioral etc.) that
tend to prevent natural interbreeding between
species. So hybridization is less common.
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
Testes
Male hormones
Normal male
mating behavior
Sterile
Incidence in
humans: 1/20,000
male births
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 random.
As development proceeds, all
cells arising by cell division
after than time have the same
X inactivated as the parent
cell.
In 64-cell embryos
Adult female
mammals are
mosaics for
genes on the X
chromosome.
Genetic Mosaics
calico cats are almost always
female
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 randomly silenced
daughter cells inherit active or
inactive X chromosomes,
creating a cat with patches of
coat color
XOXo
Tortoiseshell (calico) coat color
Tortoiseshell (calico) coat color
Tortoiseshell (calico) coat color
X inactivation is different in
marsupials
kangaroos, wombats,
bandicoots, opossums
absolute paternal X
chromosome
inactivation
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.