Transcript The Mechanism of X inactivation

Chromosomal Number
Variations
(CHAPTER 8- Brooker Text)
October 30, 2007
BIO 184
Dr. Tom Peavy
VARIATION IN CHROMOSOME
NUMBER
• Genome mutations
– changes in the number of
• Sets of chromosomes
– OR
• Numbers of individual chromosomes in a set
• Chromosome numbers can vary in two main ways
– Euploidy (Polyploidy)
• Variation in the number of complete sets of chromosome
– Aneuploidy
• Variation in the number of particular chromosomes within a set
Aneuploidy
• Aneuploidy commonly causes an abnormal
phenotype
– It leads to an imbalance in the amount of gene products
• Alterations in chromosome number occur frequently
during gamete formation
– About 5-10% of embryos have an abnormal chromosome
number
– Indeed, ~ 50% of spontaneous abortions are due to such
abnormalities
The Sex-linked phenotypic effects may be due to
1. The expression of X-linked genes prior to embryonic X-inactivation
2. An imbalance in the expression of pseudoautosomal genes
• Some human aneuploidies are influenced by the age
of the parents
– Older parents more likely to produce abnormal offspring
– Example: Down syndrome (Trisomy 21)
• Incidence rises with the age of either parent, especially mothers
Figure 8.19
Meiotic Nondisjunction
• Nondisjunction refers to the failure of chromosomes
to segregate properly during anaphase
• Meiotic nondisjunction can produce haploid cells
that have too many or too few chromosomes
– If such a gamete participates in fertilization
• The resulting individual will have an abnormal
chromosomal composition in all of its cells
During
fertilization,
these gametes
produce an
individual that
is trisomic
for the
missing
chromosome
During
fertilization,
these gametes
produce an
individual that
is monosomic
for the
missing
chromosome
All four gametes are abnormal
Figure 8.24
50 % Abnormal
gametes
Figure 8.24
50 % Normal
gametes
• In rare cases, all the chromosomes can undergo
nondisjunction and migrate to one daughter cell
• This is termed complete nondisjunction
– It results in a diploid cell and one without chromosomes
– The chromosome-less cell is nonviable
– The diploid cell can participate in fertilization with a
normal gamete
• This yields a triploid individual
The Mechanism of X inactivation
(formation of Barr bodies)
• Randomly, one of the two X chromosomes is
inactivated by the DNA becoming highly compacted
– Most genes on the inactivated X cannot be
expressed
• When this inactivated X is replicated during cell
division
– Both copies remain highly compacted and
inactive
• X inactivation of the same chromosome is passed
along to all future somatic cells
Examples of Barr body formations
– white and black variegated coat color is found in
certain strains of mice
– A female mouse has inherited two X chromosomes
• One from its mother that carries an allele conferring white coat
color (Xb)
• One from its father that carries an allele conferring black coat
color (XB)
The epithelial cells
derived from this
embryonic cell will
produce a patch of
white fur
While those from
this will produce a
patch of black fur
Figure 7.4
At an early stage of
embryonic development
Euploidy
• In contrast to animals, plants commonly exhibit
polyploidy
– 30-35% of ferns and flowering plants are polyploid
– Many of the fruits and grain we eat come from polyploid
plants
• In many instances, polyploid strains of plants display
outstanding agricultural characteristics
– They are often larger in size and more robust
• Sterility is generally a detrimental trait
• However, it can be agriculturally desirable because it
may result in
– 1. Seedless fruit
• Seedless watermelons and bananas
– Triploid varieties
• Asexually propagated by human via cuttings
– 2. Seedless flowers
• Marigold flowering plants
– Triploid varieties
• Developed by Burpee (Seed producers)
Interspecies Crosses
• Complete nondisjunction can produce an individual
with one or more sets of chromosomes
– This condition is termed autopolyploidy
How could this occur?
Interspecies Crosses
• A much more common mechanism for changes in
the number of sets of chromosomes is alloploidy
– It is the result of interspecies crosses
allodiploid
• Allopolyploidy can arise due to a combination of
nondisjuction, autopolyploidy and alloploidy
An allotetraploid:
Contains two
complete sets of
chromosomes
from two different
species