Anomalies of Chromosome Number & Structure*

Question 1

define chromosome

Answer 1

a single molecule of DNA with a set order of genes

Question 2

what is the P and Q arm of a chromosome?

Answer 2

P arm = short
Q arm = long

Question 3

what is the difference between the light and dark bands on a chromosome?

Answer 3

Light bands: replicate early in S phase, less condensed chromatin, transcriptionally active. Gene and GC rich.
Dark Bands: Replicate later, contain condensed chromatin, AT rich.

Question 4

how do we examine chromosomes?

Answer 4

ordered lowest to highest resolution:
1. Karyotype
2. Florescent in situ Hybridisation (FISH)
3. Array CGH

Question 5

what is a karyotype test used for?

Answer 5

Karyotype test identifies and evaluates the size, shape, and number of chromosomes in the nuclei of a sample of body cells.

Question 6

what is colchicine?

Answer 6

a drug that specifically inhibits microtubule formation

Question 7

how is the karyotype test carried out?

Answer 7

1) isolate metaphase chromosomes
2) Colchicine is added- cells enter mitosis, but chromosomes remain at metaphase plate
3) Chromosomes are recognised based on their banding pattern
4) Chromosomes are arranged in order of size (1 biggest, 22 smallest)
We can then examine:
- whether the correct number of chromosomes are present
- whether there are copy number variants (duplications or deletions)
- whether there are changes in the position of genetic material etc.

Question 8

what is the fluorescent in situ hybridisation (FISH) test used for?

Answer 8

uses hybridisation
provides a way for researchers to see the location of a gene and number of a specific chromosome present

Question 9

how is a FISH test carried out?

Answer 9

1) The first step is to prepare short sequences of single-stranded DNA that matches a portion of the gene the researcher is looking for. These are called probes.
2) The next step is to label these probes by attaching different colours of fluorescent dye to each of them.
3) Since the researchers’ probes are single-stranded, they are able to bind to the complementary strand of DNA, wherever it may reside on a person’s chromosomes.
When a probe binds to a chromosome, its fluorescent tag is highlighted

Question 10

what is array CGH used for?

Answer 10

Uses hybridisation
Allows us to look at all the chromosome material (not targeted like FISH) and at a higher resolution.

Question 11

how is the array CGH carried out?

Answer 11

1) You take a DNA microarray containing probes representing genomic regions of interest
2) You then add test and reference DNA (tagged with different colours (i.e. red & green)
3) The two sets of DNA compete for same probe sites of microarray:
- More green dots if there are more copies of the target sequence on the test DNA (duplication)
- Yellow if there are equal copies of the target sequence on both reference and test DNA
- More red dots if there is a deletion of sequences of the test DNA (more probes bind to the normal DNA)

Question 12

what type of chromosome abnormalities are there?

Answer 12

• Abnormalities of chromosome number
• Abnormalities of chromosome structure
• Chromosome abnormalities are also classified according to which cells of the body they are distributed in:
  Constitutional: all cells of the body
  Somatic: only in certain cells/tissues of the body

Question 13

define trisomy

Answer 13

an extra copy of a chromosome is present in the cell nuclei, causing developmental abnormalities
e.g trisomy 21 - extra copy of chromosome 21

Question 14

what order is the karyotype method of description of chromosomes abnormalities written in?

Answer 14

Written in the format: total no. of chromosomes, sex chromosome constitution, & finally anomalies/variants
examples:
46, XY = normal

47, XX, +21 = trisomy 21 (Down’s syndrome) (+ 21 means additional 21st chromosome so there are three in total)

47, XXX = Triple X syndrome

69, XXY = triploidy

Question 15

define Aneuploidy

Answer 15

changes in a single chromosome number

Question 16

give examples of aneuploidy

Answer 16

Trisomy – additional chromosome (47)
E.g. 47 XX +21
Monosomy – missing chromosome (45)

Question 17

define polyploidy

Answer 17

duplication in all chromosomes

Question 18

give examples of polyploidy

Answer 18

Triploidy (69)
Tetraploidy (92)

Question 19

where do abnormalities in chromosomes come from?
give examples

Answer 19

errors in cell division
e.g. non - disjunction

Question 20

what is non- disjunction and use trisomy to explain this?

Answer 20

failure of homologous chromosomes or sister chromatids to separate properly during cell division. (Meiosis 1, Meiosis 2 or mitosis).
Non-disjunction during meiosis result in the production of disomic gametes => trisomy

Trisomy can occur due to non-disjunction in meiosis I (homologous chromosomes fail to separate) or meiosis II (sister chromatids fail to). There are now two of e.g. chromosome 21 in the cells which then become 3 copies when fertilised by the other gamete.

Question 21

when do the chances of getting trisomy increase?

Answer 21

Incidence of trisomy 21 increases with maternal age, this is because of the way the eggs are produced.
All oocytes are ready by 5 months gestation. Each remains in maturation arrest at the crossing over stage of meiosis I.
There is a lengthy interval between onset of meiosis in an oocyte and completion (up to 50 years).
The accumulating effects on the primary oocyte during this phase may damage the cell’s spindle formation and repair mechanism predisposing to non-disjunction.

Question 22

what are the clinical consequences of abnormalities in chromosome number?

Answer 22

often lethal consequences
the only type of abnormalities in chromosome number that will lead to life are difference in number of sex chromosomes or trisomy of chromosome 21, 18 and 13.
Trisomy or monosomy of other chromosomes are not compatible with life.
Significant development abnormalities occur due to the imbalance of gene products.
The effect of reducing the gene copy usually has more severe consequences than increasing the gene copy

Question 23

what are the 3 known causes of Down syndrome?

Answer 23

Trisomy 21: 95% of people with Down Syndrome have three separate copies of chromosome 21. This is occurring via non-disjunction.

Robertsonian translocation: 4% gain the extra copy of chromosome 21 because of Robertsonian translocation.

Mosaicism causes the last 1%:
This is where the genetic trisomy occurs after fertilisation – not present in gametes.
Non-dysfunction occurs during mitosis so you get two different cell lines, one with trisomy 21 and one with a normal number of chromosomes.
They usually have much milder features because of the presence of the normal cells

Question 24

Why do chromosome conditions cause problems?

Answer 24

“gene dosage effect” - features of the syndromes are caused by the 1.5 x amount of specific gene products being produced from e.g. (chromosome 21 – DS)
“amplified developmental instability” - overall effect of imbalance on development of human.

Question 25

what is Edwards syndrome?

Answer 25

47 XX +18 (trisomy 18)
flexed fingers
Effects on the heart and kidneys
affects 1 in 3000 births

Question 26

what is Klinefelter Syndrome?

Answer 26

47, XXY (male with additional copy of X)
1 in 1000 males
Tall stature
causes mild learning problems
Causes infertility
Poorly developed secondary sexual characteristics

Question 27

what is Turner Syndrome?

Answer 27

45, X (female one copy of X)
Interfere with development of ovaries – infertile
Lymphedema – swelling of feet
Short
Lack of puberty
Abnormalities of the sex chromosomes are usually less severe

Question 28

what is genetic counselling?

Answer 28

refers to the process of giving people info of genetic risk in their family so they can make informed decisions about management and reproductive choices

Question 29

what info would be given in genetic counselling to someone with aneuploidy?

Answer 29

Aneuploidy is usually the results of meiotic non-disjunction related to maternal age
no family history
The risk of recurrence is therefore low (bc they are not inherited) and will be related to the age of the mother

Question 30

what info would be given in genetic counselling when dealing with a case of Down syndrome?

Answer 30

3 affected children with down syndrome in the family so the cause is most likely due to be an inherited chromosome condition.
This is further supported by miscarriage.
Therefore, it is very unlikely that non-disjunction, due to maternal age, is the cause.
It is due to a translocation.

Question 31

what are the 7 classifications of abnormalities of chromosome structure?

Answer 31

may be inherited from parents

Translocations: reciprocal/Robertsonian
Deletion
Duplication
Inversions: chromosome material that has inverted itself 180o
Ring chromosome
Marker chromosome
Complex rearrangements

Question 32

what is Robertsonian translocation?

Answer 32

Only involves acrocentric chromosomes (13, 14, 15, 21, & 22)
Acrocentric Chromosomes: have a high centromere so p-arm composed of only non-coding repeat sequences
The breakage of two acrocentric chromosomes at/close to centromeres and then the fusion of their long (q) arms results in new metacentric chromosomes.
The short arms are lost

Question 33

what is a balanced Robertsonian Translocation?

Answer 33

No genetic material is lost, so no change in phenotype and no effect on health. e.g. C21 fuses with C14. So you still have two C14 and two C21 (i.e. C14 and C21 [separate] and C14/21[together])
However, if you are a carrier (you have Balanced Robertsonian translocation) you have a chance of passing down unbalanced Robertsonian Translocation to your child.
E.g. down syndrome – extra copy of 21 in the form C14/21. E.g. the child has 2 x C21 and C14/21 so has three copies of chromosome 21 so has down syndrome. (for c14 you have one C14 and one C14/21)

Question 34

what is the mechanism of passing on Robertsonian Translocation?

Answer 34

Chromosome segregation
(refer to diagram)
Fertilised = normal & balanced carrier
Fertilised = trisomy (miscarriage) & monosomy 14 (miscarriage)
Fertilised = monosomy (miscarriage) & trisomy 21 (down syndrome)

LEARN THE DIAGRAM

Question 35

what dangers do most monosomies & trisomies cause?

Answer 35

most are lethal
Multiple miscarriages as genetic imbalance is too large
Miscarriages are therefore a clue to indicate monosomies/trisomies chromosomal abnormalities
This is the method in which down syndrome is caused if multiple people in the same family have it.

Question 36

what are the probabilities of reoccurrence for Robertsonian Translocation?

Answer 36

A carrier for a Robertsonian 14:21 translocation has a high theoretical probability of having a live child affected with Down syndrome. (1/3)
But the observed figure of having a liveborn child with Down syndrome is less than that predicted (Female carrier 10%, Male carrier 1%.) This is due to child miscarriage and preferential segregation of sperm not carrying the translocation.
The recurrence risk in regular trisomy 21 Down syndrome (due to maternal aging non-disfunction) is about 1 in 100.
This is why it is important to karyotype a person with Down syndrome to determine the chromosomal cause so we can determine the risk of having a second child also with down syndrome.

Question 37

what is reciprocal translocation?

Answer 37

Exchange of genetic material between any two non-homologous chromosomes
You name the chromosome based on what centromere it retains (see below)

Question 38

what is a balanced reciprocal chromosome translocation?

Answer 38

chromosome material swapped position
doesn’t affect phenotype
1/500 are carriers of a Balanced Reciprocal Chromosome Translocation

Question 39

discuss the outcomes of this balanced reciprocal chromosome translocation carrier during meiosis

Answer 39

Normal = no risk
Balanced carrier = normal, but risk of your children having unbalanced translocation
Partial tri + partial mono (unbalanced): miscarriage, congenital malformation, developmental delay, mental abnormality
Specific abnormalities dependent on which chromosomes & genes are affected and the size of imbalance (gene dosage)

Question 40

how are chromosomal abnormalities identifiable in family trees?

Answer 40

multiple congenital abnormalities
multiple miscarriages
multiple generations

Question 41

what are the probabilities of recurrence for reciprocal translocations?

Answer 41

The size and position of the chromosome segments involved in the translocation may have an effect on:
- the pairing of the chromosomes at future meiosis
- frequency of different forms of the translocation in the gametes
- the likelihood of the conceptus (embryo) with that abnormality developing to term.

Whether these occur are dependent on the effects of the genes on the translocated segments and the amount of chromosome imbalance.
It can therefore be complicated to determine a precise recurrence figure: most couples request prenatal diagnosis (test during pregnancy) you can also get a rough idea of reoccurrence based on outcome of previous children.

Question 42

where do large chromosome deletions and duplications occur?

Answer 42

occurs when chromosomes pair
they slightly misalign themselves causes unequal crossing over causing duplications and deletions (often at sites of repeated sequences) e.g 22Q11 - particular site on chromosome 21 prone to deletions and duplications due to repeated sequences

Question 43

define Mosaicism

Answer 43

two populations of cells with different genetic constitutions

Question 44

what are the types of Mosaicism?

Answer 44

Somatic mosaicism - somatic cells, two populations of cells in the body
Gonadal mosaicism - (germ line cells) two populations of gametes in the gonads
parents generally unaffected - recurrence occurs as the anomaly can being passed on to child

Question 45