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Cytogenetics > Final > Flashcards

Final Flashcards

1
Q

Divergence of X and Y

A

Four inversions, development of SRY gene (moved from q to p), Y shrinks d/t no homologous recombination

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2
Q

What are the PAR regions?

A

pseudoautosomal regions on tips of sex chromosomes

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3
Q

PAR1 genes

A

p arm- all escape inactivation in women

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4
Q

SHOX

A

PAR1 gene. missing one- short. missing two-dysplasia. Can be recombination to Y, leading to male-male transmission (inherited from mother, recombination happens, pass on to son)

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5
Q

when does sexual differentiation start?

A

around 6 weeks

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6
Q

CAH cause and frequency

A

1/12,000 live births. Most frequently due to 21-hydroxylase deficiency (21-OH) encoded by CYP21A2 gene. Enzyme deficiency leads to excess testosterone production by adrenal glands and lack of cortisol and aldosterone production. impaired synthesis of cortisol from cholesterol by the adrenal cortex

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7
Q

classic CAH forms

A
  1. Simple virilizing 21-OHD CAH

2. Salt-wasting 21-OHD (with/without virilization) – 75% of all individ

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8
Q

dexamethasone

A

Off label to prevent genital virilization in females. Does not impact salt-wasting. Long term effects still being studied.

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9
Q

AIS frequency

A

1/20,000 live births

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10
Q

AIS management

A

To prevent testicular malignancy, individuals with complete AIS may remove testes after puberty or have prepubertal gonadectomy accompanied by estrogen replacement therapy

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11
Q

5-ARD cause

A

Mutations in the SRD5A2 gene (on 2p23) prevent steroid 5-alpha reductase 2 from converting testosterone to DHT. DHT critical for formation of external genitalia before birth. During puberty, testes produce more testosterone, thus can develop secondary male sex characteristics (testes descends, micropenis or clitoris grows larger, deepening voice. Do not develop much facial or body hair)

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12
Q

Why does extra X’s still have an effect after X inactivation?

A

Not all genes inactivated. Abnormal dosage in embryogenesis before X inactivation. X-inactivation occurs at ~3 days post fertilization and completed by the end of first week of gestation.

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13
Q

Most frequent sex chromosome abnormality observed at birth in females

A

XXX (from maternal nondisjunction in meiosis I)

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14
Q

Phenotype of XXX

A

Very variable. Seizures, DD, subfertility

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15
Q

XXY phenotype

A

taller than average, gynecomastia, small testes in adulthood, inadequate testosterone production thus requiring testosterone supplement, slightly lower IQ. Most are infertile. Some psychological problems.

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16
Q

4 phenotypic categories of X-autosome translocations

A
  • normal phenotype with or without recurrent miscarriages
    • gonadal dysfunction with primary amenorrhea or
      premature ovarian failure
    • known X-linked disorder
    • congenital abnormalities and/or developmental delays
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17
Q

Fragile X associated with which gene?

A

FMR1

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18
Q

Where is fragile X mutation?

A

5’ UTR promoter region. 6-54 CGG repeats with two AGG interruptions at positions 11 & 22.

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19
Q

Fragile X premutation carrier

A

55 to 200 CGG repeats without methylation of the FMR1 promoter

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20
Q

Fragile X full mutation

A

> 200 CGG repeats with methylation of the FMR1 promoter (methylation shuts down the gene)

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21
Q

Molecular test for fragile X

A 
PCR (fast, pretty good repeat counting).
Southern blot (less accurate for size, but can see expansion, can measure methylation, slow)
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22
Q

most common single gene cause of autism

A

fragile x

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23
Q

fragile x common presentation

A

2-3 y.o. boy, multiple ear infections, speech delay, global developmental delay

24
Q

conditions of expression of fragile x premutation

A

FXPOI and FXTAS

25
Q

FXPOI

A

Amenorrhea, elevated FSH, estrogen defeciency, subfertility (20-30% of premutation carriers). Most seen at 80-100 repeats

26
Q

FXTAS

A

tremor, ataxia, cognitive deficits, peripheral neuropathy, anxiety. About 33% of male carriers and 5-8% of female carriers

27
Q

Intermediate zone for fragile x repeats

A

45-54 (loss of AGG interruption leads to instability)

28
Q

Fragile X expands through

A

mom

29
Q

FMRP absence

A

impairs maturation and pruning of neuron synapses

30
Q

Are Mendelian or chromosomal abnormalities more common?

A

chromosomal

31
Q

Average heterozygocity upon SNP array

A

25-28%

32
Q

What does SNP array detect that aCGH does not?

A

ROH, polyploidy, UPD

33
Q

what can a microarray NOT tell you?

A

location of indels, presence of balanced rearrangements

34
Q

typical resolution of karyotypes

A

5 Mb

35
Q

typical array resolution

A

20-50 kb

36
Q

CNVs

A

Stretches of DNA larger than 1000 bp. Use CMA to detect these. dels or dups undetectable by conventional sequencing analysis (opposite end of spectrum from karyotype). If sequencing is negative, del/dup by aCGH recommended

37
Q

when is chromosomal microarray recommended?

A

ID/DD, multiple congenital anomalies, when differential includes more than one condition

38
Q

indels

A

Insertion/deletionsless than 1000 bp. Use sequencing to detect these

39
Q

on a microarray, one extra copy is numerically where?

A

0.5

40
Q

on a microarray, one missing copy is numerically where?

A

-1

41
Q

what is most recent genome build?

A

hg38 or build 38, released in December 2013

42
Q

estimated SNVs in genome

A

10 million

43
Q

what type of array has the 3 tracks?

A

SNP

44
Q

For a DD indication, what is diagnostic yield upon karyotyping? Upon aCGH after normal karyotype? Upon microarray as first line?

A

5%; 10-15%; 15-20%. So can be used as first tier

45
Q

when is highest prenatal microarray yield?

A

With abnormal US and normal karyotype

46
Q

Is microarray or karyotype better on stillbirth?

A

microarray- 87% (karyotype- 70%)

47
Q

CAH inheritance

A

auto rec

48
Q

5-ARD inheritance

A

auto rec

49
Q

Can females have 5-ARD?

A

No. They can be homozygous for the mutation, but they don’t require DHT, so are not affected.

50
Q

AIS inheritance

A

X linked (recessive)

51
Q

When does meiosis occur in females?

A

initiated once at 4 months gestation, remain arrested in prophase I (dictyotene), first division at ovulation, stops at metaphase II, second division at fertilization

52
Q

When does meiosis occur in males?

A

ongoing initiation/production beginning at puberty, complete at 20(?) days. Entire process takes ~64 d.

53
Q

Parts of interphase

A

G1, S, G2

54
Q

Phases of mitosis

A

Prophase, prometaphase, metaphase, anaphase, telophase

55
Q

With a balanced translocation, which kind of segregation in meiosis can lead to balanced or normal gametes?

A

Alternate

56
Q

What type of inversion is most likely to lead to abnormal liveborn?

A

Pericentric- Unbalanced gametes likely to have one centromere and can therefore are more viable and can produce abnormal phenotypes.

57
Q

Which chromosomes have known imprinting disorders?

A

6, 7, 11, 14, 15, 20

Cytogenetics (2 decks)

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