10.2 Nondisjunction
&
Karyotypes
Disjunction
• The normal separation of chromosomes in
meiosis I or sister chromatids in meiosis II
is termed disjunction.
• When the separation is not normal, it is
called nondisjunction.
nondisjunction
• Nondisjunction ("not coming apart") is the
failure of chromosome pairs to separate
properly during meiosis stage 1 or stage 2.
• This could arise from a failure of
homologous chromosomes to separate in
meiosis I, or the failure of sister
chromatids to separate during meiosis II or
mitosis.
Nondisjunction
nondisjunction
• The result of this error is a cell with an
imbalance of chromosomes. Such a cell is
said to be aneuploid.
Aneuploidy
• Aneuploidy is an abnormal number of chromosomes,
and is a type of chromosome abnormality. An extra or
missing chromosome is a common cause of genetic
disorders (birth defects).
• Aneuploidy occurs during cell division when the
chromosomes do not separate properly between the two
cells (nondisjunction). Chromosome abnormalities occur
in 1 of 160 live births.
• Most cases of aneuploidy result in termination of the
developing fetus, but there can be cases of live birth; the
most common extra chromosomes among live births are
21, 18 and 13.
Nondisjunction
• Nondisjunction can occur in the meiosis I
or meiosis II, phases of cellular
reproduction, or during mitosis.
• This results in the production of gametes
which have either too many or too few of a
particular chromosome, and is a common
mechanism for monosomy or trisomy .
Nondisjunction
• Loss of a single chromosome (2n-1), in
which the daughter cell(s) with the defect
will have one chromosome missing from
one of its pairs, is referred to as a
monosomy.
• Gaining a single chromosome, in which
the daughter cell(s) with the defect will
have one chromosome in addition to its
pairs is referred to as a trisomy.
Nondisjunction
This is a cause of several medical conditions in humans,
including but not limited to:
•
Patau Syndrome - trisomy of chromosome 13
•
Edward Syndrome - trisomy of chromosome 18
•
Down Syndrome - trisomy of chromosome 21
•
Klinefelter Syndrome - extra X chromosomes in males
- i.e. XXY, XXXY, XXXXY, etc.
•
Turner Syndrome - lacking of one X chromosome in
females - i.e. X0
•
Triple X syndrome - an extra X chromosome in
females
•
XYY Syndrome - an extra Y chromosome in males.
Prenatal Diagnosis
Amniocentesis
• A small sample of the amniotic fluid
surrounding the baby is removed using a
syringe.
• The fluid contains skin cells from the baby.
• The skin cells are grown in the lab.
• The chromosomes from the cells are
magnified under a microscope and a
picture is taken.
• The chromosomes are cut out and
arranged in homologous pairs in
decreasing size order.
• This is called a karyotype.
Down Syndrome
trisomy of chromosome 21
Places to find out more information from the U.S. National Library of
Medicine :
Genetics Home Reference
http://ghr.nlm.nih.gov/condition/down-syndrome
MedlinePlus
http://www.nlm.nih.gov/medlineplus/downsyndrome.html
PubMed Health
http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0001992/
Down Syndrome (Trisomy 21)
• Extra chromosome 21 in
every cell of the body
• Karyotype = 47,XX+21 or 47,XY+21
• As a woman gets older, her chances of
having a baby with a chromosome
abnormality increases
***remember, a woman is born with all of her
egg cells, but meiosis is not yet complete
(egg development stops in prophase I until
the follicle matures prior to ovulation)
Copy this address to visit an animation
showing normal meiosis and nondisjunction
in mothers of different ages.
http://www.mhhe.com/biosci/genbio/biolink/j
_explorations/ch10expl.htm
Turner Syndrome
lacking of one X chromosome
in females - i.e. X0
Places to find out more information from the U.S. National Library of
Medicine :
Genetics Home Reference
http://ghr.nlm.nih.gov/condition/turner-syndrome
MedlinePlus
http://www.nlm.nih.gov/medlineplus/turnersyndrome.html
PubMed Health
http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0001417/
Turner Syndrome
Triple X syndrome
an extra X chromosome in females
Places to find out more information from the U.S. National Library of
Medicine :
Genetics Home Reference
http://ghr.nlm.nih.gov/condition/triple-x-syndrome
Klinefelter Syndrome
extra X chromosomes in males
- i.e. XXY, XXXY, XXXXY, etc.
Places to find out more information from the U.S. National Library of Medicine :
Genetics Home Reference
http://ghr.nlm.nih.gov/condition/klinefelter-syndrome
MedlinePlus
http://www.nlm.nih.gov/medlineplus/klinefelterssyndrome.html
PubMed Health
http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0001420/
Klinefelter Syndrome
XYY Syndrome
an extra Y chromosome in males.
Places to find out more information from the U.S. National Library of Medicine :
Genetics Home Reference
http://ghr.nlm.nih.gov/condition/47xyy-syndrome
Patau Syndrome
trisomy of chromosome 13
Places to find out more information from the U.S. National Library of Medicine :
Genetics Home Reference
http://ghr.nlm.nih.gov/condition/trisomy-13
PubMed Health
http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0002625/
Trisomy 13
Edward Syndrome
trisomy of chromosome 18
Places to find out more information from the U.S. National Library of Medicine :
Genetics Home Reference
http://ghr.nlm.nih.gov/condition/trisomy-18
PubMed Health
http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0002626/
Trisomy 18
Nondisjuction
• Surprisingly, in
plants, extra
chromosomes can
actually be helpful.
• Sometimes it
makes larger
flowers and fruits!
Polyploidy
Gene linkage and maps
• Genes on the same chromosome are usually linked and
inherited together instead of independently.
• It is the chromosomes that follow Mendel’s law of
independent assortment, not the genes.
• Linked genes can be separated as a result of crossing
over.
• Scientists have found that genes that are farther apart on
a chromosome tend to cross over more often than genes
that are close together.
• Using this information, scientist can make chromosome
maps that show the sequence of genes on a
chromosome.