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Segregation Of Structural Abnormalities Flashcards

1
Q

Alternate segregation

A

Alternate centromeres segregate together producing balanced gametes.

Either normal pair or two balanced homologues

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2
Q

Categories of combined imbalance

A

Mal segregation of translocations

Recombination of inversions

Segregation of insertional translocations

Ring chromosomes

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3
Q

Adjacent 1 segregation

A

Adjacent non-homologous centromeres segregate together

1 of each centromere involved in translocation

Produces unbalanced gametes, which are viable when both segments of translocation are small

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4
Q

Adjacent 2 segregation

A

Adjacent homologous centromeres segregate together

Produces only unbalanced gametes

Viable is translocation large and centric segment small

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5
Q

3:1 segregation

A

3 centromeres segregate to 1 pole and 1 to the other

8 possible segregations

Produces only unbalanced gametes

Viable if derivative chromosome is small

Two forms

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6
Q

Tertiary 3:1 segregation

A

2 normal chromosomes and 1 segregated

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7
Q

Interchange 3:1 segregation

A

2 translocation and 1 normal

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8
Q

Name the most common recurrent translocation

A

t(11;22)(q23;q11)

Carriers have an increased risk of pregnancy loss

1 viable segregation product 47,XX,+der(22)t(11;22) Emanuel syndrome = tertiary trisomy

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9
Q

Emanuel syndrome

A 
Severe ID
Facial dysmorphism (deep set eyes low ears and longer upper lip)
Microcephaly
Micrognathia 
Kidney abnormalities
Congenital here defects
Males may show genital abnormalities
Developmental delay
Central hypotonia
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10
Q

Familial implications after diagnosis of Emanuel syndrome

A

Parents should be offered Chr analysis to establish if balanced translocation carrier

Genetic counselling if carrier

Increased risk of miscarriage (23-37%)

Further adult relatives maybe offered testing

Affected ed individuals unlikely to reproduce

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11
Q

Name a viable adjacent 1 segregation

A

t(4;8)(p16;p23) is a recurrent reciprocal translocation

Both adjacent 1 segregants viable

46,XX,der(4)t(4;8) Wolf Hirschhorn phenotype

46,XX,der(8)t(4;8) Less specific features

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12
Q

Segregation analysis

A

Allows an estimate of the risk to any future pregnancy

If malsegregation has resulted in a viable unbalanced child then recurrence risk is significant

Most reciprocal translocations specific to family

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13
Q

Adjacent 1 segregation

A

Adjacent non-homologous centromeres segregate together

1 of each centromere involved in translocation

Produces unbalanced gametes, which are viable when both segments of translocation are small

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14
Q

Adjacent 2 segregation

A

Adjacent homologous centromeres segregate together

Produces only unbalanced gametes

Viable is translocation large and centric segment small

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15
Q

3:1 segregation

A

3 centromeres segregate to 1 pole and 1 to the other

8 possible segregations

Produces only unbalanced gametes

Viable if derivative chromosome is small

Two forms

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16
Q

Tertiary 3:1 segregation

A

2 normal chromosomes and 1 segregated

17
Q

Interchange 3:1 segregation

A

2 translocation and 1 normal

18
Q

Name the most common recurrent translocation

A

t(11;22)(q23;q11)

Carriers have an increased risk of pregnancy loss

1 viable segregation product 47,XX,+der(22)t(11;22) Emanuel syndrome = tertiary trisomy

19
Q

Emanuel syndrome

A 
Severe ID
Facial dysmorphism (deep set eyes low ears and longer upper lip)
Microcephaly
Micrognathia 
Kidney abnormalities
Congenital here defects
Males may show genital abnormalities
Developmental delay
Central hypotonia
20
Q

Familial implications after diagnosis of Emanuel syndrome

A

Parents should be offered Chr analysis to establish if balanced translocation carrier

Genetic counselling if carrier

Increased risk of miscarriage (23-37%)

Further adult relatives maybe offered testing

Affected ed individuals unlikely to reproduce

21
Q

Name a viable adjacent 1 segregation

A

t(4;8)(p16;p23) is a recurrent reciprocal translocation

Both adjacent 1 segregants viable

46,XX,der(4)t(4;8) Wolf Hirschhorn phenotype

46,XX,der(8)t(4;8) Less specific features

22
Q

Segregation analysis

A

Allows an estimate of the risk to any future pregnancy

If malsegregation has resulted in a viable unbalanced child then recurrence risk is significant

Most reciprocal translocations specific to family

23
Q

Peri centric inversion

A

Including centromere

24
Q

Paracentric inversion

A

Excluding centromere

25
Q

Inversions

A

Balanced carriers usually normal

Could have reproductive problems

Only have risk of abnormal offspring if inverted segment large, includes the centromere and the end sections are small

26
Q

Risk for peri centric inversion carriers

A

Risk of unbalanced offspring highest if non-inverted segment small or non-coding

27
Q

Risk for paracentric inversion carriers

A

Risk of unbalanced offspring as a result of recombination in the inversion is negligible as recombinant a are not viable

Pregnancies not usually noticed as miscarry early

28
Q

Recombination of pericentric inversions

A

Recombination within the inversion loop gives:

Normal chromosome
Inverted chromosome
Recombinant chromosome dup p distal
Recombinant chromosome dup q distal

29
Q

Recombination of paracentric inversion

A

Recombination gives:

Normal chromosome
Inverted chromosome
Dicentric recombinant chromosome (fail to segregate lost in mitosis)
Acentric recombinant chromosome (lost in division)

Unbalanced Chr would unlikely to be viable

30
Q

Names two forms of insertional translocation

A

Interchromosomal: segment from one chromosome inserted into a different chromosome

Intrachromosomal: segment from one chromosome inserted into different place within same chromosome

31
Q

Risk of imbalance for balanced insertion carriers

A

Depends on size and gene content of insertion

Risk is usually higher than for a translocation carrier as the unbalanced form is either a gain or loss

32
Q

Ring chromosomes

A

Formed from a single chromosome with loss of distal material from the p and q arms, followed by fusion.

Two categories:

  1. Large ring chromosomes, little material lost (in place of normal chromosome)
  2. additional ring chromosome (often small or markers) +r
33
Q

Ring chromosome behaviour

A

subject to frequent errors at mitotic division, which can result in:

  • Loss
  • Additional copies
  • Double sized rings
34
Q

Generalised ring chromosome syndrome

A

Associated with large rings replacing the normal homologue

Growth restriction/short stature with or without LD

Other features depend on genetic content

Results from dynamic mosaicism due to errors in mitosis

Any cells generated from a mis-segregation resulting in monosomy are unlikely to be viable

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