Download Karyotyping

Title: Chromosomal mutations
05 July 2017
Learning question: what can go wrong in meiosis and
how can this be detected?
Homework: STUDY!!!
Aims from specification
(h) explain the use of fetal ultrasonography, amniocentesis and
chorionic villus sampling (CVS) for detecting named disorders and
assessing fetal development. The advantages and disadvantages of
each technique should be outlined (HSW6a, 6b);
(i) outline how a karyotype is produced and used to determine fetal
sex and to diagnose chromosomal mutations, with reference to
Turner’s and Klinefelter’s syndromes;
(j) explain how chromosome mutations such as Turner’s and
Klinefelter’s may occur during meiosis (with reference to nondisjunction only).
Problems in meiosis
• Meiosis produces gametes that are haploid – this
means that they only contain half the genetic
information needed
• On rare occasions, sex chromosome fail to
separate during anaphase 1 or 2
• Results in a gamete without a sex chromosome and
another with 2 sex chromosomes
• When chromosomes do not separate properly,
this is called non-disjunction
Turner’s syndrome
• The result of a gamete with no sex
chromosome being fertilised by a gamete with
an X sex chromosome
• XO
• The resulting child is a female with Turner’s
syndrome
Turner’s syndrome
• 1 in 6000 females are born
lacking another X
chromosome
• Females are short, have a
webbed neck, shieldshaped chest,
underdeveloped breasts,
may not menstruate and
have cardiovascular and
renal problems
Klinefelter’s syndrome
• Gamete with an X
chromosome is
fertilised by a gamete
with both an X and Y
chromosome
• Baby will be XXY and
have Klinefelter’s
syndrome
Karyotyping
• A karyotype is a photomicrograph of chromosomes from
metaphase of mitosis, with the chromosomes arranged in a
standard sequence, so that doctors can see if there is an
abnormality of the number of chromosomes in the fetus
• To prepare a karyotype, fetal cells have to be obtained, by
amniocentesis or chorionic villus sampling (CVS)
Test
Procedure to obtain cells
Amniocentesis
Hollow needle inserted into
amnion in pregnant uterus;
amniotic fluid with sloughed
off fetal skin cells withdrawn
Ultrasound scan
(ultrasonography) carried out
so needle does not damage
the fetus or the placenta
Chorionic villus sampling
Tube inserted into vagina,
through cervix and into
uterus.
Ultrasound scan used to show
where placenta is
Cells from fetal part of
placenta obtained
Advantages
Enables parents to prepare
for a child with an
abnormality or to terminate
the pregnancy
Can be done from 10 weeks,
so if parents decide to
terminate the pregnancy it
may be less traumatic
Disadvantages
Cannot be done until the 16th
week as before that the
amniotic sac is not large
enough
2% risk of causing a
miscarriage
1% risk of causing a
miscarriage
Making a Karyotype
1.
Cells are cultured, under aseptic conditions, for about 3 weeks, so they
are dividing by mitosis
2.
Cells are arrested at the beginning of metaphase by adding colchicine,
which inhibits spindle formation
3.
The cells are placed in a solution of very high water potential, so that
they swell and burst and chromosomes spread out from each other
4.
A stain is added to make the chromosomes easily seen
5.
These cells are observed under the microscope and a photograph is
taken
Questions
1. Sometimes, a zygote forms with three sets (69) of chromosomes. This is
not viable and it spontaneously aborts. Suggest how a zygote with 69
chromosomes can occur.
2. Some patients with Turner’s syndrome are found to have some cells with
XO and some with XX or XY chromosomes. Such patients are called
mosaics as they have two cell lines in their bodies. What does this suggest
about when the non-disjunction of chromosomes occurred in these
patients?
3. Karyotyping could show a chromosome abnormality and prospective
parents may decide to terminate the pregnancy. What are the risks to be
considered before opting for such as test?
4. Explain why human cells placed in very dilute salt solution will swell and
burst
Answers
1.
An egg is fertilised by two sperms
2.
It occurred after fertilisation, in one of the cells in an early fetus, during the early
mitotic division of the zygote. If the zygote was originally XY but divided
unevenly so that one resulting cell had no Y chromosome, then half the cells will
be XO and half XY. This well develop as Turner’s syndrome female. if the zygote
was XX but divided so that one cell lost an X chromosome, the descendants of
that cell will also be XO.
3.
Could have a healthy fetus to be aborted. Chorionic Villus Sampling can be
carried out earlier in the pregnancy and allow an earlier termination, but carries
a greater risk than amniocentesis of causing a miscarriage.
4.
Water enters by osmosis, down the water potential gradient, through the
partially permeable membrane. There is no cell wall, so the cell swells and
eventually bursts as the cell surface membrane ruptures.
Diagnostic screening for genetic diseases can involve examination of chromosomes or
examination of the base sequence of specific genes. Some genetic diseases are caused by
chromosomal mutations.
(a) State what is meant by a chromosomal mutation.
.....................................................................................................................................
Karyotypes may be used to diagnose chromosomal mutations.
1
2
6
13
19
7
3
8
14
20
4
9
15
21
10
16
22
5
11
17
X
12
18
Y
(i)Name the condition shown by this foetus.
........................................................................................................................... [1]
(ii)State how the karyotype shown in the diagram above differs from a normal female
karyotype.
..........................................................................................................................
...........................................................................................................................[1]
(iii) Name one procedure that could have been used to obtain the foetal cells.
........................................................................................................................... [1]
(iv) Outline the stages in the production of a karyotype from foetal cells.
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........................................................................................................................... [3]
Chorionic villus sampling (CVS) is a technique that can be used to test for the presence of
genetic disorders in a foetus. A doctor inserts a narrow tube through the cervix into the uterus
and removes a sample of foetal cells from the chorionic villi of the placenta.
(i) Name one genetic disorder that could be diagnosed using this test. [1]
.....................................................................................................................................
(ii)
Amniocentesis is another test used to screen for disorders in the foetus.
Suggest one advantage and one disadvantage of using amniocentesis rather than
CVS.
advantage ......................................................................................................
disadvantage .................................................................................................. [2]
(iii)
A karyotype may be produced using chromosomes from cells collected during these
tests.
Describe how the karyotype from a foetus with Turner’s syndrome would differ from
that of a normal female karyotype.
.....................................................................................................................................
.....................................................................................................................................
.....................................................................................................................................[2]