Transcript Abnormal Chromosome Number

Abnormal Chromosome
Number
A Change in Chromosome
Number
• If the spindle fails at meiosis, this causes
an incorrect number of chromosomes in
the gametes.
• This is called Non-disjunction.
Aneuploidy
• After non-disjunction, one cell may have 2
copies of a homologous chromosome and
the other cell may have none.
• If the 2 cells are fertilised by a normal
gamete, you can get
– A zygote with 3 copies of the homologue,
which is called Trisomy.
– A zygote with only 1 homologue – a condition
called Monosomy.
Tisomy
Monosomy
Anueploidy
• This can occur with the autosomes or the
sex chromosomes.
• Most autosome non-disjunction leads to
death of the foetus.
Autosome Aneuploids Formed
By Non-disjunction
Trisomy 21- Down’s Syndrome
• (47 Chromosomes)
• 3 copies of chromosome 21.
• Produces a child with a sunny nature,
characteristic facial features which include
heavy skin folds above the eye, an
enlarged tongue, short stature, and
various internal disorders such as heart
defects and susceptibility to respiratory
infections.
Trisomy 21- Down’s Syndrome
• These children are usually mentally
retarded, (severity varies).
• Most are sterile.
• They are prone to develop leukaemia and
Alzheimer’s disease (these are known to
be on chromosome 21)
• The incidence of the syndrome can be
related to the mother’s age at conception.
Trisomy 21- Down’s Syndrome
• It is suggested when the mother is over 35
years she should have an amniocentesis
test for Down’s Syndrome.
• The extra chromosome forms by nondisjunction during gamete production in
one of the parents.
Trisomy 18- Edward’s Syndrome
• These babies have 3 copies of
chromosome 18.
• It occurs once in every 10 000 live births,
and affects almost every body organ.
• Babies rarely survive for more than a year,
most die within a few weeks of birth.
• The mother’s age affects that occurrence
as with Down’s Syndrome.
Trisomy 13- Patau Syndrome
• Babies have 3 of chromosome 13.
• Occurs 1 in every 5000 live births.
• Babies have serious eye, brain and
circulatory defects as well as a cleft palate.
• Babies rarely survive for more than a year,
most die within a few weeks of birth.
• The mother’s age affects that occurrence
as with Down’s Syndrome.
Trisomys
• The trisomys of most other autosome
chromosome aneuploids result in
spontaneous abortion or miscarriage. Only
Down’s Syndrome and the sex
chromosome aneuploids are exceptions.
Sex Chromosome Aneuploids
formed by Non-disjunction
• This results in live babies and abnormal
adults.
Turner’s Syndrome XO
• called monosomy X; 45 chromosomes
• Produces sterile females who are
abnormally short, have webbed neck and
do not develop mature sexual organs.
They usually have normal mental function.
Trisomy X
• XXX, 47 chromosomes
• Produces tall females who are frequently
sterile and may be mentally retarded.
Klinefelter’s Syndrome
• XXY; 47 chromosomes
• Produces males who are tall and thin,
sterile and underdeveloped sexually.
• Most have breast development resembling
that of a mature woman.
• Often suffer from thyroid dysfunction,
chronic pulmonary distress and diabetes.
• They often have a high-pitched voice and
may be mentally retarded.
XYY Syndrome
• 47 chromosomes
• Produces males who are normal except
for their extreme height and a tendency to
severe acne.
• They are genetically sterile and have
reduced IQ.
Extreme Cases
• XXYY; XXXY; XXXYY; XXXX; XXXXX;
even XXXXYY
• Each syndrome has its own distinguishing
characteristics, however there are 4
generalisations we can make about sex
chromosome abnormalities.
Generalisations
– A human must have at least 1 X
chromosome to live.
– The presence of a Y causes the
individual to develop as a male, if Y is
absent the individual is female.
– The reason why these syndromes are
not fatal is probably that all but 1 of the
Xs will form a barr body
– The more sex chromosomes a person
has the taller they will be.
Polyploidy
• Sometimes all the chromosome pairs
undergo non-disjunction simultaneously.
• This results in gametes that have a double
set of chromosomes, ie they are 2n.
• If these are fertilised by an n gamete, the
zygote will be Triploid or 3n.
• If 2 diploid zygotes fuse, a tetraploid or
4n.
Polyploidy
• In animals, a polyploid cross between 2
species is usually infertile (if they survive
at all).
• In plants, polyploidy is relatively common,
however these plants often fail to set seed
as their homologous chromosomes do not
match, although they can grow
vegetatively.
Polyploidy in Gamete Formation
Autopolyploidy
• Remember Auto means self.
• This is an individual with more than 2
chromosome sets, all delivered from a
single species.
• Infertile offspring when the result is a 3n
zygote. The triploid offspring can form if
the non-disjunction occurs in only 1
gamete.
Autopolyploidy
• They are infertile because there are no
matching pairs of homologous
chromosomes to line up at meiosis, so
they can’t make gametes.
• Fertile offspring form when diploid
gametes formed by individuals of the
same species fuse to form tetraploid
offspring.
Autopolyploidy
• They are fertile because they have
matching pairs of homologous
chromosomes to line up at meiosis, so can
form gametes.
• Ref pg 146 txtbk
Allopolyploidy
• This occurs when individuals of different
but related species are accidentally
hybridised.
Advantages of Polyploids
• They usually have larger cells which result
in larger and more resistant plants.
– E.g. modern cotton and wheat
• Polyploids with uneven numbers are
sterile, so result in fruit with no seeds.
– E.g. seedless watermelons
– The banana is an infertile triploid- although it
forms fruits, the seeds inside are
underdeveloped, and the plant must be
propagated by asexual means.
Seedless Watermelon and
Bananas
Advantages of Polyploids
• The greatest advantage is the fact that it
extends the range of adaptations of the
plant, as a result of the combination of
characteristics from 2 species.
• Ref pg 147 txtbk