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CSF block 6 > Mendelian Inheritance II > Flashcards

Mendelian Inheritance II Flashcards

1
Q

what is hemizygosity?

A

having only one member of a chromosome pair or segment, rather than 2 (males are homozygous for the X chromosome)

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2
Q

describe X-linked recessive d/o (5)

A
  • single mutant allele for males and two mutant alleles for females needed to cause disease
  • never passed father to son
  • incidence of trait higher in males
  • males more severely effected
  • affected man will transmit to all of his daughters
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3
Q

when will females be affected by x-linked recessive d/o?

A

1- homozygous female- 45,X (turner syndrome)
2- translocation of X and autosome with preferential inactivation of the normal x (only express mutant X)
3- skewed x inactivation- mutant X expressed more
4- females are homozygous for the mutation (two affected Xs)

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4
Q

discuss male-male inheritance of X-linked d/o?

A

father will not pass on to son bc he gives son Y, not X

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5
Q

why is the phenotype for x linked recessive more severe for males than females?

A

due to hemizygosity of the X in males, the mutant allele is fully expressed

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6
Q

what is X inactivation?

A

only one of the X chromosome’s genes are expressed in each cell so that males an females express the same dosage of X-linked genes

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7
Q

describe x-linked dominant d/o

A

lethal in the homozygous state (no affected males)

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8
Q

what are the recurrence risks for an affected mother with an X-linked dominant d/o?

A

33% unaffected daughter
33% affected daughter
33% unaffected son
affected son will to survive

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9
Q

what are examples of X linked dominant d/o?

A

goltz syndrome- usually de novo
incontinentia pigmenti- inherited or de novo
rhett syndrome

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10
Q

what is a trinucleotide repeat d/o?

A

described by the expansion (within affected gene) of a segment of DNA that contains a 3 nt repeat

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11
Q

what is “expansion”?

A

increase in the number of repeats that is passed to subsequent generations, eventually leading to expression of the d/o

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12
Q

what d/o was trinucleotide repeat first discovered?

A

fragile X syndrome

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13
Q

what is repeat size in trinucleotide repeat d/o?

A

of repeats, allows for range of phenotype from mild to severe

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14
Q

what is anticipation in trinucleotide repeat d/o?

A

the occurrence of a disease at an earlier age of onset or with increasing severity in successive generations, basis for anticipation is expansion of the repeats

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15
Q

what are 4 common trinucleotide repeat d/o?

A

Huntington’s disease
fragile X
Myotonic dystrophy
Friedrich ataxia

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16
Q

what are “long expansions” in trinucleotide repeat d/o?

A

repeat usually more than 10x normal size, present outside the coding region, CCG/CGG or CTG, assoc with fragile sites in some cases

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17
Q

examples of long expansion trinucleotide repeat d/o

A

fragile X, FXTAS, FRAXE, myotonic dystrophy, spinocerebellar ataxia type 8

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18
Q

what is the age of onset of myotonic dystrophy type 1?

A

20-40

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19
Q

what are the sx of myotonic dystrophy type 1?

A

muscular weakness, wasting, cardiac arrhythmia, cataracts, male balding/infertility

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20
Q

what is the inheritance pattern for myotonic dystrophy type 1?

A

autosomal dominant

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21
Q

when is anticipation a factor with myotonic dystrophy type 1??

A

when it is inherited from the mother

10% risk of congenital myotonic dystrophy type 1

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22
Q

what gene is affected in myotonic dystrophy type 1?

A

19q13, myotonin protein kinase

23
Q

what/where is the repeat in myotonic dystrophy type 1?

A

CTG, 3’-UTR

24
Q

what are the normal/asymptomatic/affected/congenital repeat ranges for myotonic dystrophy type 1?

A

normal- 5-35
asymptomatic- 35-49 *risk for expansion
affected- over 50
congenital- large # (1000s)

25
Q

what are the phenotypic features of myotonic dystrophy type 1?

A

hypotonia, tented mouth, absent suck/swallow, dev delay, club feet

26
Q

what gene is affected in fragile X syndrome?

A

FMR1 on X chromosome

27
Q

what is the repeat in FXS?

A

CGG with AGG interrupting at every 9-10 repeats

28
Q

what is the purpose of AGG repeats?

A

anchor sequences that protect against expansion

29
Q

what will increase the risk for instability of maternal alleles and expansion with FXS?

A

uninterrupted CGG repeats beyond the last AGG triplet, greater than 33-39

30
Q

what causes the FXS phenotype?

A

loss of protein product

31
Q

what is the consequence of methylation of CGG expansion?

A

results in decreased or absent FMR1 transcription and loss of protein

32
Q

when does expansion of FXS repeats occur?

A

oogenesis

33
Q

what are the repeat ranges for FXS?

normal/intermediate/premutation/ full mutation

A

normal- 5-44
intermediate- 45-54
premutation- 55-200
full mutation- over 200

34
Q

what is the phenotype of someone with intermediate # of repeats in FXS?

A

learning/behavioral problems

35
Q

what is the phenotype of someone with premutation # of repeats in FXS?

A

increased risk of FXTAS, POF

possible subtle intellectual problems- social anxiety, learning d/o

36
Q

what are some implications for women with premutation # of repeats in FXS?

A

increased risk for having children with FXS due to repeat instability, also increased risk of POF and may also develop tremor and ataxia

37
Q

what is the phenotype of someone with full mutation # of repeats in FXS?

A

mental retardation, characteristic physical features (long face, big head, joint laxity, large testes), possibly autism

38
Q

what is FXTAS?

A

fragile X assoc tremor/ataxia syndrome
occurs in males with FXS pre-mutation
late-onset/progressive cerebellar ataxia and intention tremor

39
Q

what is POF?

A

premature ovarian failure

40
Q

who will males with FXTAS transmit their pre-muations to?

A

all daughters, no sons

41
Q

what is the penetrance of FXTAS?

A

age related, increases with age

42
Q

what is the phenotype of a female with full mutation in FXS?

A

mental retardation, although can be mild secondary to lyonization

43
Q

what are “short expansions” in trinucleotide repeat d/o?

A

less than 100 copies
repeats present in protein coding region
CAG sequence forms polyglutamine tracts = gain of function

44
Q

2 examples of “short expansions” trinucleotide repeat d/o?

A

Huntington’s disease

Friedrich ataxia

45
Q

what inheritance pattern is Huntington’s disease?

A

autosomal dominant

46
Q

what is the basis for age of onset with Huntington’s disease?

A

35-44 years, based on size of repeats

47
Q

what are the trinucleotide repeat ranges in Huntington’s disease? normal/risk for expansion/affected

A

normal- under 26
risk for expansion- 27-35
affected- over 36

48
Q

when is there an increased risk of expansion in huntington’s disease?

A

if inherited from the father

49
Q

what is Friedreich ataxia?

A

characterized by slowly progressive ataxia with average onset between ages 10-15 and usually before 25

50
Q

sx of Friedreich ataxia?

A

dysarthria, mm weakness, spasticity in lower limbs, scoliosis, bladder dysfunction, absent limb reflexes, loss of position/vibratory sense

51
Q

what is the inheritance pattern of Friedreich’s ataxia?

A

autosomal recessive

52
Q

what/where is the repeat?

A

GAA in intron 1 of FXN

53
Q

what are the repeat number in Friedreich’s ataxia?

normal/pre-mutation/borderline/full

A

normal- 5-33
Pre-mutation- 34-65
borderline- 44-66
full disease- 66-1700

54
Q

what is “non-penetrance”?

A

the expected disease state is not expressed

CSF block 6 (5 decks)

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