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Genetics, Cancer and Development Emily > 012 cytogenetics > Flashcards

012 cytogenetics Flashcards

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1
Q

at what stage is it easiest to see chromosomes?

A
  • during metaphase of mitosis as this is when they condense into sister chromatids to make an X shape
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2
Q

what is the centromere?

A

handle of the chromosome
- where cellular machinery grabs to move chromosome
- where sister chromatids are joined

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3
Q

what is the telomere?

A
  • ends of chromosomes with specific sequences
  • they get shorter during lifetime and when they are gone the chromosome becomes unstable could lead to mutations/chromosomal rearrangement
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4
Q

what is the P arm of a chromosome?

A
  • the shorter arm of the chromosome (measured from the centromere)
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5
Q

what is the q arm of a chromosome?

A
  • the longer arm of the chromosome (measured from the centromere)
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6
Q

what is heterochromatin?

A
  • very condensed DNA found at centromeres and telomeres
  • gene poor
  • drawn as cross-hatched due to variable length under staining
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7
Q

what does 9q24.1 mean in terms of chromosomes?

A

9 = chromosome 9
q = long arm
2 = region 2 on q arm
4 = band 4 of region 2 on q arm
1 = subband (most specific gene loci)

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8
Q

what is an ideogram?

A
  • cartoon representation of banding pattern in chromosomes
  • helps identify certain chromosomal abnormalities e.g. extra chromosomes, deletions, translocations
  • nowadays a human karyotype can be used instead with each chromosome hybridized with a different colour chromophore
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9
Q

how is a karyotype obtained?

A
  • each chromosome can be hybridized with a different colour chromophore to show all the individual and pairs of chromosomes in a karyotype (at metaphase)
  • Multifluor FISH generates a karyotype in which each chromosome appears to be painted with a different color. Each “paint” is actually a collection of hybridization probes for sequences that span the length of a particular chromosome
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10
Q

what is fluorescent in situ hybridisation?

A
  • FISH = using DNA probes labelled with fluorescent dyes that can bind to specific genes on DNA to visually understand the chromosome and gentic pattern in someones DNA
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11
Q

what is haploidy?

A
  • only 1 of each chromosome
  • e.g. gametes
    (23 chromosomes)
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12
Q

what is triploidy?

A
  • having 1 complete extra set of chromosomes
  • usually caused by polyspermy
  • embryos usually self abort
    (69 chromosomes)
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13
Q

what is tetraploidy?

A
  • having 2 complete extra sets of chromosomes ( 92 chromosomes in total)
  • usually the result of a failure of the first zygotic division
  • lethal to the embryo
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14
Q

what is aneuploidy?

A
  • the category of chromosomes that are not whole sets
  • having an abnormal number of chromosomes in a haploid state (e.g. not having 23 chromosomes)
  • usually the consequence of a failure of a single chromosome to complete division/mitosis
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15
Q

what are monosomies?

A
  • having only 1 member in a pair of chromosome
  • all autosomal monosomies are lethal
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16
Q

what is a trisomy?

A
  • having 3 copies of a chromosome, rather than 2
  • e.g. Down’s syndrome of chromosome 21
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17
Q

what is the relationship with Down’s syndrome and mother’s age?

A
  • the incidence of Down’s syndrome increases rapidly with mother’s age, especially over 35
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18
Q

what is meiotic nondisjunction?

A
  • failure to seperate chromosomes properly during meiosis
  • e.g. in meiosis 1 = the homologous chromosomes do not separate, so 1 daughter cell gets both and the other gets none
  • meiosis 2 = the sister chromatids fail to separate, so 1 cell gets both sister chromatids and the other gets neither
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19
Q

describe how Down’s syndrome is caused

A
  • most cases arise from non-disjunction in the first meiotic division
  • the father contributes the extra chromosome 15% of cases
  • a small proportion of cases are mosaic ( not in all cells) and these probably arise in an early zygotic division
  • about 4% of cases arise by inheritance of a translocation chromosome from apparent who is a balanced carrier (Robertsonian translocation) this is inheritable
  • causes characteristic facial features (rounded and flat face) and an IQ of less than 50 and a shorter statue
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20
Q

describe features of trisomy 13, Patau syndrome

A
  • 1/5000 births
  • 50% die in first month
  • few survive beyond a year
  • most involve maternal meiotic non-disjunction
  • a significant fraction have a parent who is balanced translocation carrier
  • can cause microcephaly, malformations of part of the brain, structural defects of the eyes, cleft lip or cleft palate, polydactyly, congenital heart disorders,
    neural tube defect, malformations of the sex organs
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21
Q

describe features of trisomy 18, Edwards syndrome

A
  • 1/3000 births
  • most babies die within first year
  • can cause irregularity of the kidneys, ureters, heart, lungs and diaphragm, cleft lip or cleft palate, microcephaly, malformations of the hands and feet – including missing thumbs, club feet and syndactyly,
    neural tube defect, malformations of the sex organs
22
Q

what are the 3 most common trisomies?

A
  • 21 = Down’s syndrome
  • 18 = Edward’s syndrome
  • 13 = Patau syndrome
23
Q

how can you detect trisomy without karyotyping?

A
  • PCR of highly polymorphic regions on chromosome of interest: for binary alleles, 1 allele will produce 2x PCR product
  • Look for a systematically stronger signal of hybridisation on genomic microarray (many markers for each chromosome)
  • Fluorescent in situ hybridization (FISH) of non-dividing cells using a single region as a probe: will give 3 points of fluorescence in each cell, one for each chromosome
24
Q

what is the difference between sex chromosome aneuploidies and autosomal aneuploidies?

A
  • because of X inactivation and the small amount of genes on the Y chromosome, symptoms of sex chromosome aneuploidies are much less severe
  • but this means that more people are affected by is (as more people survive with them)
25
Q

what is the syndrome involving having karyotype 45 X?

A
  • Turner’s syndrome
  • 1/500 female births
  • but 99% of embryos are aborted so don’t see full picture
  • causes short stature, webbed neck, infertility, broad chest with widely spaced nipples, delayed or absent puberty, heart defects
26
Q

what is the syndrome of having 47 XXY karyotype

A
  • Klinefelter’s syndrome
  • 1/1000 male births
  • causes small testes that fail to produce enough testosterone, leading to breast growth (gynecomastia) and delayed/incomplete puberty = undescended testes (cryptorchidism), micropenis, and urethra opening on the underside of the penis (hypospadias)
  • taller, thinner (low bone and muscle density) and slight reduction in IQ, but can usually live a normal life
  • very rarely can also have 48 XXXY and 49 XXXXY, which are slightly more severe
27
Q

describe the characteristics of 47 XYY syndrome

A
  • 1/1000 male births
  • many may have no symptoms
  • usually tall but normally proportioned
  • a slight reduction in IQ?
  • Jacob’s syndrome
28
Q

describe characteristics of 47 XXX syndrome

A
  • 1/1000 female births
  • has little effect and they do not pass it on to their children
  • slight reduction in IQ
  • may be slightly taller
29
Q

what are chromosomal inversions?

A
  • when a piece of a chromosome has been lifted out, turned around and reinserted
  • pericentric = includes centromere
  • paracentric = excludes centromere
30
Q

what is a pericentric inversion?

A
  • when a piece of chromosome including the centromere is lifted out, turned around and reinserted
31
Q

what is a paracentric inversion?

A
  • when a piece of chromosome not including the centromere is lifted out, turned around and reinserted
32
Q

what is a translocation?

A
  • when part of a chromosome is removed and moved and added to another chromosome
  • in a balanced translocation there is no gain or loss of chromosomal material
  • could cause aneuploidy
33
Q

describe how deletions can cause unbalanced rearrangements

A
  • deletions may be either interstitial (within the main part of the chromosome) or terminal (at the end of the chromosome)
  • if the deletion is big enough to be visible, it will likely be removing many phenotypes and causing a severe phenotype
34
Q

give an example of interstitial deletion syndrome

A
  • Angelman sydrome/Prader-Willi syndrome
  • where there is a deletion on chromosome 15, if it is the maternal chromosome = Angelman’s, if it is the paternal chromosome = Prader-Willi’s
35
Q

give an example of a terminal-end deletion syndrome

A
  • Cri du Chat syndrome
  • where the end of p arm of chromosome 5 is lost
  • called this as baby’s cry sounds like a cat cry
36
Q

what are Robertsonian translocations?

A
  • translocations involving acrocentric chromosomes
    (where the centromere is located near one end of the chromosome) (e.g. chromosomes 13,14,15,21,22)
  • the fusion of 2 chromosomes with the loss of the 2 short p arms
  • people carrying Robertsonian translocations are usually unaffected (as lost short arms often don’t carry many genes on them)
  • however if their children inherit the Robertsonian chromosome, they may get trisomies
37
Q

describe the possible outcomes of children of 1 parent being a Robertsonian translocation carrier and the other being normal

A
  • could be completely normal genotype ( 46 and correct pairing)
  • could be balanced (no missing or extra genetic material), but there has been a translocation of one of the chromosomes (46 but 22 correct pairs)
  • trisomy of either of the chromosomes e.g. 14 or 21 (47)
  • could loose one of either chromosome and end up with monosomy (45)
38
Q

what are isochromosomes?

A
  • when a chromosome can split ‘ the wrong way’ in mitosis or meiosis 2, so instead of splitting at the centromere lengthways (split sister chromatids), it is split horizontally to give 2 p arms connected and 2 q arms connected
  • this forms an isochromosome
  • these are simultaneously duplicated for genes in the retained arm and deleted for the genes in the other
  • the prognosis is poor except for if it iXq (isochromosome of the q/long arm of the X chromosome)
39
Q

what are ring chromosomes?

A
  • when a mutation event which removes both telomeres can then be repaired by sealing the ends together forming a ring chromosome
  • symptoms depend on how many genes at the end were deleted
  • surprisingly they are mitotically stable and can usually replicate okay, but do sometimes become entangled
40
Q

what does cen mean in a karyotype?

A
  • centromere
41
Q

what does del mean in a karyotype?

A
  • deletion
  • e.g. 46 XX, del(5)(p15.3) = Cri du chat syndrome
42
Q

what does der mean in a karyotype?

A
  • a chromosome derived from chromosome 22 and containing the chromosome 22 centromere = der(22)
43
Q

what does dup mean in a karyotype?

A
  • duplication
  • 46 XY, dup(21)(q21)
    = male with a duplication of band q21 on chromosome 21
44
Q

what does inv mean in a karyotype?

A
  • inverstion
  • 46 XX, inv (9)(p12q13)
    = a female with a pericentric inversion of chromosome 9 (section is from p12 to q13 = includes centromere)
45
Q

what does mar mean on a karyotype?

A
  • marker
  • 47 XX, +mar = a female with an unidentified extra chromosome
46
Q

what does mat and pat mean on a karyotype?

A
  • mat = of maternal origin
  • pat = of paternal origin
  • 46 XY, del (15)(q12)(mat) = a boy with Angelman’s syndrome
  • 46 XX, del(15)(q12)(pat) = a girl with Prader-Willi syndrome
47
Q

what does t mean on a karyotype?

A
  • translocation
  • 46 XY, t(1;2)(p21;p24) = a boy with a reciprocal translocation between band p21 on chromosome 1 and band p24 on chromosome 2
48
Q

what does ter mean on a karyotype?

A
  • then end of chromosome arm
  • pter = end of short arm
  • qter = end of long arm
49
Q

what does + or - mean on a karyotype?

A

+ = gain of chromosome material e.g. +21
- = loss of chromosome material e.g. 5p-

50
Q

what does / mean on a karytotype?

A
  • mosaic
  • e.g. 46 XX / 45 X = mosaic for Tuner’s syndrome

Genetics, Cancer and Development Emily (8 decks)

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