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PoM Block 3 > Chromosomal abnormalities (21) > Flashcards

Chromosomal abnormalities (21) Flashcards

1
Q

What is the normal human karyotype?

A
  • normal human karyotypes contain 22 pairs of autosomal chromosomes
  • and one pair of sex chromosomes (allosomes)
  • normal karyotypes for females contain two X chromosomes and are denoted 46,XX; males have both an X and a Y chromosome denoted 46,XY
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2
Q

At what stage of division can chromosomes be karyotyped?

A

metaphase

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3
Q

How do we show a translocation in cytogenetic nomenclature?

A

46, XX, t(5;10)(q34.1;q11.2)
means translocation between - chromosomes 5 and 10
- break points shown in 2nd brackets

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4
Q

How do we show a deletion in cytogenetic nomenclature?

A

46, XX, del(5)(q21q23)

- missing between bands q21 and q23 in chromosome 5

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5
Q

What is the purpose of mitosis?

A
  • to create two identical daughter cells
  • for growth and repair
  • to replace exhausted cells
  • 2n–> 2n
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6
Q

What is added to cultures in order to arrest cells in metaphase?

A

colcemid

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7
Q

What is the purpose of meiosis?

A
  • to achieve reduction from diploid (2n=46) to haploid (n=23)
  • to ensure genetic variation in the gametes
  • enables random assortment of homologues and recombination
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8
Q

What is non-disjunction at meiosis 1?

A

chromosomes haven’t separated, so both chromosomes have gone to 1 daughter cell
so 2 trisomy and 2 monosomy zygotes

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9
Q

What is non-disjunction at meiosis 2?

A

1st division is normal, so each cell has 1 chromosome each, each containing 2 chromatids- but in 2nd division, sister chromatids fail to separate–> causes 2 normal gametes and 1 w/ trisomy, 1 w/monosomy

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10
Q

Why is female meiosis more vulnerable? (most aneuploidy is caused by non-disjunction in oogenesis)

A
  • paused in utero until puberty
  • 1 primary oocyte yields only 1 ovum bc polar bodies
  • finite number of primary oocytes
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11
Q

How can we detect chromosomal aneuploidy prenatally?

A

Quantitative Fluorescence (QF-PCR)

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12
Q

Why is sex chromosome imbalance tolerated?

A
  • X-inactivation of excess X chromosomes (e.g. XXX is not that bad bc only 1 X is active)
  • very low gene content on Y chromosome (so XYY is often unnoticed)
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13
Q

Why is there an increased genetic risk associated with advanced maternal age?

A

over time the chromosomes in the egg are less likely to divide properly leading to the egg having an extra or missing chromosome

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14
Q

What technology do we use now to detect chromosomal copy number changes instead of karyotyping?

A

Array Comparative Genome Hybridisation (aCGH)

  • test DNA in red
  • reference DNA in green
  • hybridise together
  • look for regions of yellow, red and green… yellow means equal
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15
Q

What is a contiguous gene syndrome?

A

a clinical phenotype caused by a chromosomal abnormality- deletion or duplication- that removes several genes lying in close proximity to one another on the chromosome

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16
Q

What is the vulnerability of male meiosis?

A
  • primary spermatocytes undergo around 23 mitotic divisions per year–> and potentially accumulate defects
  • no equivalent to menopausal limit on reproductive span
17
Q

How many chromatids are in each chromosome at metaphase?

A

each chromosome comprises 2 chromatids

4n prior to cell division

18
Q

What features are shown on ideograms?

A
  • ends are capped w/ a telomere to stop unravelling
  • centromere (if in middle, metacentric)
  • p-arm= small arm
  • q-arm= long
  • G-dark bands where DNA tightly bound, less accessible, gene-poor
  • vs. G-light bands- more accessible
  • Giesma staining
19
Q

How do we show a monosomy (loss) in cytogenetic nomenclature?

A

45,XX,-7

missing chromosome 7

20
Q

How do we show a trisomy in cytogenetic nomenclature?

A

47,XX,+8

21
Q

How do we show an inversion in cytogenetic nomenclature?

A

46,XX,inv(3)(q21q26)

22
Q

What is haploinsufficiency?

A

you need expression from both alleles to get a normal phenotype

23
Q

What is imprinting?

A

methylation pattern that passes in germline of parent and are maintained during divisions

24
Q

What is haplosufficiency?

A

only need expression from 1 of the alleles

25
Q

What are reciprocal chromosomal abnormalities?

A
  • translocation caused by exchange of parts between non-homologous chromosomes
  • 2 detached fragments of two different chromosomes are switched
26
Q

What are examples of unbalanced chromosomal abnormalities?

A
  • terminal deletion
  • interstitial deletion
  • inversion
  • duplication
  • ring chromosome
27
Q

What are point mutations?

A
  • missense: changes AA
  • nonsense: intro stop codon
  • frameshift: changes lots of AAs/ intro stop codon
  • can completely alter function of a gene
28
Q

What is ‘selfish spermatogonial selection’?

A
  • results from a selective advantage due to a mutation over neighbouring wild type cells
  • enhances paternal age effect–> sperm w/ mutation can accumulate–> causes subset of single gene disorders e.g. Apert syndrome, Crouzon syndrome and Pfeiffer syndrome

PoM Block 3 (8 decks)

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