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Clinical genetics > Chromosomal Abnormalities II > Flashcards

Chromosomal Abnormalities II Flashcards

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1
Q

What are the two translocations?

A

➝ Reciprocal

➝ Robertsonian

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2
Q

What are the 7 structural abnormalities?

A 
➝ Translocation
➝  Inversion
➝  Deletion
➝  Duplication
➝  RIngs
➝  Isochromosomes
➝  microdeletions/microduplication
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3
Q

How do the structural abnormalities occur?

A

➝ Double strand DNA breaks
➝ occur throughout the cell cycle
➝ misrepair leads to structural abnormalities

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4
Q

How are double strand breaks usually repaired?

A

➝ DNA repair pathways

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5
Q

What is translocation?

A

➝ two double strand breaks each on a different chromosome

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6
Q

How do balanced translocations arise?

A

➝ DNA repair mechanisms stitch the chromosome in incorrect pairs
➝ there is no net loss or gain of genetic material it is just in a different place

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7
Q

What does der mean?

A

➝ Derivative

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8
Q

What is a balanced translocation?

A

➝ The right amount of each chromosome just in the wrong place

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9
Q

What is an unbalanced translocation?

A

➝ too much or too little of a particular chromosome

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10
Q

What is a Philadelphia chromosome?

A

➝ abnormal chromosome 22

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11
Q

What does the Philadelphia chromosome lead to?

A

➝ Chronic myeloid leukaemia

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12
Q

What is a BCR?

A

➝ Break point cluster region (function of normal protein is unknown)

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13
Q

What tends to happen in the BCR region?

A

➝ Tendency to have double stranded breaks

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14
Q

What is an ABL?

A

➝ a proto oncogene

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15
Q

What does fusion of genes in the Philadelphia chromosome lead to?

A

➝ An activated oncogene ABL

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16
Q

What is a reciprocal translation?

A

➝ exchange of two segments between non-homologous chromosomes

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17
Q

When does a deleterious phenotype occur?

A

➝ when the breakpoint affects the regulation of a gene

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18
Q

What is a carrier of a balanced translocation at risk of?

A

➝ producing unbalanced offspring

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19
Q

What are unbalanced individuals at significant risk of?

A

➝ Chromosomal disorder

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20
Q

Describe how an unbalanced individual with partial trisomy for chromosome 11 and partial monosomy for chromosome 22 arises?

A

➝ In the cell you have one copy of normal 11 and normal 22 and the two derivative chromosomes ( 22 with a piece of 11 and 11 with a piece of 22)
➝ In the gametes you want one chromosome 11 and one 22
➝ you can get the normal intact copies or the derivative copies
➝ if you inherit the normal copy of 11 (from both parents so you have 2 copies of 11) and the derivative copy of 22 with a piece of 11 on it during fertilisation
➝ there is an extra piece of chromosome 11 on the end of one of the 22 chromosome
➝ partially trisomic for chromosome 11

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21
Q

When do homologous chromosomes align?

A

➝ in metaphase 1

22
Q

How do derivative chromosomes align with their homologs?

A

➝ They form a quadrivalent structure
➝ chromosomes are trying to find their partners so the pieces align with each other
➝ pulling apart can occur in 4 orientation

23
Q

What can an unbalanced reciprocal translocation lead to?

A

➝ Miscarriage

➝ learning difficulties, physical disabilities

24
Q

What should you do if there’s a woman who has a high number of unexplained miscarriages?

A

➝ Screened for a balanced translocation

25
Q

What is a Robertsonian translocation?

A

➝ When two acrocentric chromosomes break at or near their centromere
➝ the fragments that are joined together are the long arms of both the acrocentric chromosomes and there is loss of the small arms

26
Q

What are the acrocentric chromosomes?

A

➝ 13,14,15,21&22

27
Q

How many chromosomes does a balanced carrier have?

A

➝ 45 chromosomes

28
Q

Why is it not deleterious to lose acrocentric chromosomes?

A

➝ the p arms that are lost encode rRNA (multiple copies)

29
Q

What are the two common Robertsonian translocations?

A

➝ 13:14

➝ 14:21

30
Q

What does a 21:21 translocation lead to?

A

➝ 100% risk of Downs syndrome

31
Q

How do you get Downs syndrome from a Robertsonian translocation?

A

➝ If you have a robertsonian copy of 21 and get an intact copy of 21 from the other parent with normal chromosomes
➝ you get 2 copies of 14 but 3 copies of 21

32
Q

What are the other outcomes apart from Downs with Robertsonian translocations?

A

➝ one normal copy of both (2)
➝ one robertsonian chromosome (1) - carrier
➝ other forms are not consistent with life

33
Q

How many trisomy 21 patients have Robertsonian translocations?

A

➝ 4%

34
Q

What are the three outcomes of translocations?

A

➝ some may lead to spontaneous abortion
➝ some may lead to miscarriage
➝ some result in live born baby with various problems

35
Q

What are 3 examples of an Interstitial deletion?

A

➝ Prader Willi
➝ Di George syndrom
➝ Cri du chat

36
Q

If the end of a chromosome is lost what is this called and how can it be stabilised again?

A

➝ Terminal deletion

➝ a telomere is added

37
Q

How many live births have deletions?

A

➝ 1 : 7000

38
Q

What can deletions be?

A

➝ terminal or interstitial

39
Q

What do deletions cause?

A

➝ a region of monosomy
➝ haploinsufficiency in some genes
➝ the monosomic region has phenotypic consequences
➝ phenotype is specific for size and place on the deletion

40
Q

How are gross deletions seen?

A

➝ on metaphase spread of G banded karyotype

41
Q

How is Cri du chat caused?

A

➝ terminal deletion on chromosome 5

42
Q

What does cri du chat present with?

A

➝ microcephaly

➝ developmental delay

43
Q

How can you visualize microdeletions/duplications?

A

➝ FISH
➝ high resolution banding
➝ CGH

44
Q

What does unequal crossing over lead to?

A

➝ unequal distribution of the loci of the gametes

➝ this can lead to microdeletions and duplications because the regions they affect are smaller

45
Q

How does unequal crossing over occur?

A

➝ homologous pairs misalign

46
Q

How does CGH work?

A

➝ If you take two individuals they should have the same number of chromosomes and copies
➝ you have a healthy persons DNA and a patients DNA which are both tagged with a color
➝ they should hybridise equally to the microarray
➝ if the patient and the control have two copies the color will be a mix of the two tage

47
Q

What happens if a patient has a microdeletion in CGH?

A

➝ the patient DNA binds to less probes than the control
➝ the DNA fights for binding to the probes
➝ the color shows up the tag of the control DNA

48
Q

How do double strand breaks and non homologous end joining result in translocations?

A

➝ Two chromosomes undergo a double strand break

➝ the non-homologous end joining repair system incorrectly sticks together two different chromosomes

49
Q

How does a balanced carrier of a translocation lead to an unbalanced offspring?

A

➝ Trivalent or quadrivalent formation in meiosis I results in gametes which are partially disomic and partially nullisomis
➝ this leads to zygotes that are partially trisomic and partially monosomic

50
Q

How can abnormal karyotypes be detected using stained metaphase chromosomes?

A

➝ Large structural abnormalities can be seen with G banding and FISH
➝ microdeletions and microduplications can only be seen with CGH

Clinical genetics (5 decks)

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