Chromosomes and Chromosomal Abnormalities - Genetics 2 Flashcards

1
Q

How is a chromosome recognised?

A
  • Banding pattern with specific stains
  • Length
  • Position of the centromere
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2
Q

What can the short arm of the chromosome be written as?

A

p

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3
Q

What can the long arm of the chromosome be written as?

A

q

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4
Q

What is an afrocentric chromosome?

A
  • Doesn’t really have a short arm (little bit that encodes satellite ribosomal genes, tRNAs etc)
  • Has a centromere and a long arm
  • Only chromosomes 13, 14, 15, 21 and 22
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5
Q

What is a balanced chromosomal rearrangement?

A

• All chromosomal material is present

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6
Q

What is an unbalanced chromosomal rearrangement?

A
  • Extra or missing chromosomal material, usually one or three copies of the same gene
  • Developmentally ‘bad news’
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7
Q

What is aneuploidy

A

When there is a whole extra or missing chromosome

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8
Q

What is Translocation?

A

Rearrangement of chromosomes

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9
Q

What is Aneuploidy usually caused by?

A

Nondisjunction in meiosis in either meiosis 1 (resulting in two gametes having 3 chromosomes and two having one) or in meiosis 2 (resulting in two gametes being normal and one having 3 and the final having only one)

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10
Q

What is Down syndrome?

A

Trisomy 21 (3 copies of chromosome 21)

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11
Q

What is Edward syndrome?

A

Trisomy 18

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12
Q

What is Patau syndrome?

A

Trisomy 13

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13
Q

How can chromosomal defects be tested before birth?

A
  • Interphase chromosome counting using FISH (fluorescence In situ Hybridization)
  • Chromosome will fluoresce and then can be counted, for example: a trisomy will show 3 fluorescent chromosomes instead of 2
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14
Q

How can chromosomal defects be tested (not before birth)

A

Array CGH (comparative genome hybridisation)
• Patient DNA and Control DNA of an equal amount and normal chromosome complement are mixed
• Hybridise them into a gene chip and if the control DNA and the patients DNA match, it should produce an even graph with equal amounts of the control and patient DNa
• If there is less or more of the patient DNA/chromosome this will be shown as a slight dip or a peak in the graph

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15
Q

What is Turner syndrome?

A

45X: only one sex chromosome

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16
Q

Why is X chromosome aneuploidy better tolerated?

A

Because of X inactivation

17
Q

What are the X chromosome aneuploidies?

A
  • Turner syndrome: 45 X
  • Triple X: 47 XXX
  • Klinefelter syndrome 47 XXY (male)
18
Q

What is chromosomal insertion?

A
  • Area of one chromosome is inserted into another
  • Usually not known or recognised until they go on to have children (as long as the insertion doesn’t cut through a gene)
19
Q

What is Robertsonian translocation?

A
  • Only affect the acrocentric chromosomes (13, 14, 15, 21 and 22)
  • 2 Acrocentric chromosomes get stuck end to end
  • An example would be 45 XY t(14:21)
  • Prior to testing they may not be aware of this translocation
20
Q

What are the different chromosomal results for the children of a mother with a roberstonian translocation (45 XX t(14:21) and a father with normal chromosomes

A
  • Normal - inherits the normal versions of each gene from both parents
  • Balanced translocation - inherits the normal versions from the father and the translocated version from the mother: 45XX/XY t(14:21)
  • Trisomy 14 - inherits the normal versions from the father and one normal 14 from the mother and the translocated chromosome (results in miscarriage)
  • Trisomy 21 (Down syndrome) - inherits the normal version from the father and the translocated chromosome and a normal 21 chromosome
21
Q

How can the chromosomal pattern for someone with Down syndrome that has been caused by an inherited Roberstonian translocation be written (boy)?

A

46 XY der(14:21) + 21

22
Q

What is a reciprocal translocation?

A
  • Can happen with any of the autosomes
  • Exchange of 2 eccentric fragments of a chromosome (stable)
  • Exchange of eccentric and centric fragments- dicentric and not stable in mitosis
23
Q

What are the different chromosomal results for the children of a mother with normal chromosomes (1 and 9) and a father with one set of reciprocal translocated chromosomes (one normal 1, one normal 9, one pair of reciprocally translocated 1 and 9)?

A
  1. Normal - inherit normal chromosomes from both parents
  2. Balanced 1;9 translocation - normal set from mother, both reciprocally translocated chromosomes from father
  3. Partial Trisomy 9 and monosomy 1 - normal from mother, one reciprocally translocated from father and one normal chromosome 9
  4. Partial Trisomy 1 and monosomy 9 - normal set from the mother, one translocated chromosome from father and one normal chromosome 1
24
Q

If a mothers chromosomal pattern is written as 46, XX, t(12:17) (p13;p13), what is the sones if he has inherited the translocation?

A

46, XY, der(17) t(12;17) (p13;p13)mat

25
Q

What are the reproductive risks of reciprocal translocations?

A
  • 50% chance no effect: either balanced translocation or normal
  • 50% unbalanced: if large segment, miscarriage and if a small segment, dysmorphic delayed child
26
Q

What is a Philadelphia chromosome?

A
  • Reciprocal translocation between Ch 9 and Ch 22
  • Forms a bcr-abl gene (an oncogene)
  • 46, XY, t(9:22) (q34;q11.2)
  • 95% of patients with chronic myeloid leukaemia (CML) have this gene
27
Q

What is the Charcot Marie Tooth Disease?

A
  • Duplication (happens in 39 genes, 70-80% of the time on Ch 17 on p)
  • Results in damage to the myelin sheath