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EMS Genetics > Chromosome Pathology > Flashcards

Chromosome Pathology Flashcards

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1
Q

what is cytogenetics?

A

study of chromosomes - anything more than a single gene

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2
Q

what is the improved name for cytogenetics and why?

A

molecular cytogenetics and genome disorders as there is a blurred boundary between cytogenetics and molecular genetics

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3
Q

why are we diploid and what are the exceptions?

A

all cells have 23 pairs of chromosomes per cell, but gametes are haploid - they only gave 23 chromosomes

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4
Q

what are the autosomes?

A

chromosomes 1-22, X/Y are sex chromosomes

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5
Q

what percentage of births is cytogenetics implicated in?

A

0.7% live births, 5% of stillbirths and 50% of miscarriages - major contributor to congenital malformation and learning difficulties

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6
Q

what do most cytogenetic changes result in?

A

copy number variations - this is a DNA sequence with a variable copy number compared to the reference genome - can be 1Kb to several Mb

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7
Q

what is a continguous gene?

A

it is a disorder that results from a chromosomal abnormality resulting in the duplication/deletion etc of numerous genes next to each other

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8
Q

what was the earliest cytogenetic technique still used now occasionally to detect copy number variation?

A

G banding

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9
Q

what are some whole genome techniques for CNV detection?

A

G banding, next generation sequencing or microarrays

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10
Q

what are some techniques for targeted testing and what do they look at?

A

they look for CNVs in particular parts on the genome and examples are FISH, QF-PCR and microarrays

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11
Q

what types of CNV are there?

A

numerical (polyploidy, aneuploidy or mosaicism)

structural - the gain or loss of one or more genes - duplication or deletion

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12
Q

what is more severe loss or gain?

A

loss is more deleterious than gain - dosage effect describes the loss or gain of part of or a whole chromosomes

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13
Q

what is the position effect?

A

this is a cytogenetic abnormality that will cause an abnormal phenotype - a gene in a new chromosomal environment will function inappropriately

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14
Q

what is aneuploidy?

A

it is trisomy (the gain of just one chromosome) or monosomy (the loss) of a chromosome

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15
Q

what is polyploidy?

A

when you gain whole sets (triploidy - 3 of every chromosome or tetraploidy)

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16
Q

what can increase the change of aneuploidy occurring during meiosis/gametogenesis?

A

the increase in maternal age - paternal age increase has no significant effect

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17
Q

what occurs when there are errors in meiosis?

A

non-disjunction - failure of chromosome or chromatid separation - majority happen in meiosis I

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18
Q

what is the result of non disjunction in meiosis I?

A

it is nullisomic (no chromosomes in a gamete therefore resulting in monosomy when fertilised) or disomic (2 copies of a chromosome in each gamete) gametes

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19
Q

why do we only see trisomy 1, 18 and 13?

A

most aneuploidys do not make it past full term - trisomy 21 only has a 75% chance of making it past full term

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20
Q

when do the errors resulting in polypoidy occur?

A

most are triploidy and occur at fertilisation

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21
Q

what is the difference between diplospermy and dispermy?

A

in diplospermy the sperm does not reduce it’s chromosome number, in dispermy two sperm will fertilise the same egg. Digymy is when the egg doe not reduce it’s chromosome number

22
Q

what is the difference in presentation between a maternal and paternal triploidy?

A

double maternal - underdeveloped placenta and a disproportionately large head
double paternal - large cystic placenta

23
Q

what is a hydatidiform mole?

A

when a sperm fertilises and egg that is empty and therefore there is no foetus but a placenta develops

24
Q

when do the errors resulting gin mosaicism occur?

A

these occur at early cleavage and result in normal and abnormal cells - mitotic nondisjunction

25
Q

what is trisomic rescue?

A

when cells kick out the extra copy of the chromosome during early stages of division

26
Q

why is there variable clinical expression in structural rearrangements?

A

theses can be deletions or duplications, and there can be a variable size of imbalance, and other genetic and environmental effects

27
Q

what is inverted duplication?

A

a duplication that is followed downstream by it’s inverted component

28
Q

what techniques are used in conventional cytogenetics?

A

metaphase chromosome analysis and G banding

29
Q

what techniques are used in molecular cytogenetics?

A

FISH, QF-PCR and microarray

30
Q

what is the karyotype for G banding?

A

there are 3000 genes and 550 bands

31
Q

what is the difference in duplication and deletion results in array CGH?

A

deletion will appear to left and duplication to the right

32
Q

what is a decipher track?

A

it is used to detect deletion - may show haploinsufficiency - low score indicates higher levels of pathogenicity

33
Q

what techniques do you use in parental studies?

A

targeted arrays or FISH

34
Q

what are the advantages of aCGH?

A

early diagnosis - reduced need for other tests, can show the location and size of imbalances, high resolution and therefore high diagnostic hit rate, and information only on relevant genes

35
Q

what are the disadvantages of aCGH?

A

low level mosaics are not detected, less than 10% are not detected, only dosage is detected not structural rearrangement and mutations, needs good quality and fresh DNA, non pathogenic and uncertain pathogenic changes are also detected

36
Q

what is QF-PCR?

A

quantitative fluorescence polymerase chain reaction - amplification of short tandems repeat (chromosome specific repeated DNA sequences) using fluorescent primers

37
Q

what are STRs?

A

they are short tandem repeats - they are highly repetitive regions of genome with four particular bases that are duplicated many times in a sequence

38
Q

in aneuploidy detection what shows positive result?

A

2 or more markers with abnormal dosage (up to 4/5)

39
Q

what is the size of the product related to?

A

it is directly related to the number of ATTT repeats

40
Q

what is the process for non-invasive prenatal testing?

A

take mothers blood sample and extract foetal DNA (<10%) - assess aneuploidy (if there is a risk then invasive test needed)

41
Q

what is the process for spontaneous abortion testing?

A

extract tissue (skin, lung, cord, cartilage, placenta), macerate and extract then do QF-PCR or aCGH

42
Q

what is neuroblastoma and how can we identify it?

A

FISH to identify it - it is a rare fast growing cancer in the abdomen of children

43
Q

what will FISH show in neuroblastoma?

A

multiple double minutes and paired episomal structures - MYCN gene is massively overproduces and amplified within gene - poor prognosis and needs high dose chemo

44
Q

what is a balanced chromosomal translocation?

A

can be reciprocal or robertsonian - there is no net loss or gain of chromosomal material

45
Q

what is a reciprocal translocation and what is the risk of this?

A

it is when two or more chromosomes break and exchange material, there is no loss or gain of genetic material and is usually of no issue to the individual however there is a 5% risk of hitting an important gene every time a chromosome breaks

46
Q

what is the result of pairing after reciprocal translocation?

A

quadrivalent structure - pachytene cross - 50% of gametes abnormal

47
Q

what chromosomes are acrocentric and what does this mean?

A

13,14,15,21 and 22 are acrocentric - where the centromere is quite near one end of the chromosome - usually long q and short p

48
Q

what is a robertsonian translocation?

A

when the chromosomes break close to the centromere, the p parts are lost and q pair up - not deleterious because the p part does not contain any coding DNA - not phenotypic risk but there is a fertility risk

49
Q

what is the prevalence of robertsonian and reciprocal?

A

1 in 500 = reciprocal and 1 in 1000 for robertsonian

50
Q

what is next generation sequencing?

A

sequence based analysis of chromosome changes

EMS Genetics (19 decks)

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