Download Mammalian X Chromosome Inactivation

Types of Chromosome Mutations
Refer to Figure 17-2, Griffiths et al., 2015.
Polyploidy in Animals
Parthenogenesis - development of unfertilized egg into
embryo
polyploidy in leeches, flatworms, brine shrimp
polyploidy in salamanders, lizards
Polyploid frogs and toads undergo sexual reproduction.
Polyploid fish (such as salmon, trout) are not unusual.
Triploid oysters are of economic value.
In general, polyploid mammals are not viable.
Aneuploidy (2n +/- m)
Aneuploids differ
from wild-type
individuals by part of
a chromosome set.
Nondisjunction is a
common mechanism
for aneuploidy.
Aneuploidy can be
the cause of
disorders such as
Down Syndrome.
Examples of Aneuploidy
Nullisomy (2n -2)
Usually lethal in diploids
Tolerated in some polyploids
Monosomy (2n - 1)
Gene imbalance results (missing chromosome)
Expression of genes on lone chromosome (hemizygosity)
Trisomy (2n + 1)
Gene imbalance results (extra chromosome)
Viability depends on chromosome and species
Disomy (n + 1)
Phenomenon in haploid organisms
Meiotic
Nondisjunction
Generates
Aneuploid Products
Refer to Figure 17-12, Griffiths et al., 2015.
Turner Syndrome
XO
Klinefelter Syndrome
XXY
Meiotic Products of a Trisomic
Refer to Figure 17-14, Griffiths et al., 2015.
Characteritstics of Down Syndrome (Trisomy 21)
Refer to Figure 17-16, Griffiths et al., 2015.
The Maternal-Age Effect in Down Syndrome
Refer to Figure 17-17, Griffiths et al., 2015.
Trisomy 13: Patau Syndrome
Rarely do fetuses go to term. Rarely do babies survive.
Symptoms include:
* cleft palate
* atrial septal defect
* inguinal hernia
Polydactyly is common.
Trisomy 13: Patau Syndrome
Severity of symptoms is variable.
7-year old survivor (deaf and
legally blind).
Others are not
as fortunate.
CYCLOPIA
Trisomy 18: Edward’s Syndrome
Rarely do fetuses go to term. Rarely do babies survive.
Symptoms include:
* cleft palate
* small face
* short sternum
* low-set ears
* overlapping fingers
* clenched fist
* clubfeet
Trisomy 18: Edward’s Syndrome
These two girls are rare survivors.
Heterochromatin
Features of Constitutive Heterochromatin
1. Present at homologous sites on pairs of chromosomes
2. Always genetically inert
3. DNA sequences that are not organized into genes
4. Telomeric and centromeric regions
Features of Facultative Heterochromatin
1. Referred to as silent chromatin
2. Potential to become heterochromatic (Barr body)
3. Genetic information that is not expressed once
chromatin becomes condensed
Mammalian X Chromosome Inactivation
Calico cats are usually females heterozygous for
orange allele and black allele at an X-linked locus.
Male calico cats are very rare. Why is this the case?
X Chromosome Inactivation
Most genes on the inactivated
X chromosome are silenced
(turned off, not expressed).
Genes on the inactivated X
chromosome remain silenced
in descendant cells.
Heritable alteration of this
type is an example of
epigenetic inheritance.
Chromosomal DNA sequence
remains unchanged.
Mammalian X Chromosome Inactivation
Inactivated X chromosome becomes highly condensed,
darkly staining structure called Barr body. State of
inactivation is transmitted down the cell lineage.