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EMS - Genetics in disease > Cytogenetic basis of disease > Flashcards

Cytogenetic basis of disease Flashcards

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1
Q

What techniques are used in conventional cytogenetics?

A

G banding

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2
Q

What techniques are used in molecular cytogenetics?

A

FISH

Microarray CGH

Next generation sequencing (NGS)

MLPA

QF-PCR

qPCR

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3
Q

What is the advantage of molecular cytogenetics over conventional?

A

Molecular can be used at any stage of the cell cycle.

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4
Q

How to cytogenetic abnormalities produce an abnormal phenotype?

A

Dosage effect (gain or loss)

  • Loss > deleterious than excess

Disruption of a gene

  • Breakpoint
  • Inappropriate activation/inactivation

Effect due to the parental origin

  • Genomic imprinting

Position effect

  • A gene in a new chromosomal environment functions inappropriately

Unmasking of recessive disorder

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5
Q

What are the possible origins of numerical abnormality?

A
  1. Gametogenesis - meiosis
  2. Fertilisation
  3. Early cleavage (post-zygotic non-disjunction)
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6
Q

What are the risk factors for errors at gametogenesis?

A

Increased maternal age = increased risk of aneuploidy

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7
Q

Is meiotic non-disjunction more likely to occur at meiosis I or II?

A

80-90% occurs at meiosis I

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8
Q

What is the other name given to trisomy 18?

A

Edward’s syndrome

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9
Q

What is the name given to trisomy 13

A

Patau’s syndrome

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10
Q

Why are trisomies 21, 18 and 13 the only autosomal polyploidies we see in life?

A

They are relatively small chromosomes with fewer genes.

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11
Q

What are the clinical features of trisomy 21?

A

1/700 live births

75% spontaneously abort

Head

  • Eyes: upward slanting; brushfield spots
  • Nose: Small
  • Ears: abnormally shaped/low set
  • Tongue: protruding
  • General – flat face, brachycephalic, short neck

Neurological

  • Learning disabilities (mild to moderate IQ 30-60)

Hands and feet

  • single palmar crease
  • short broad hands
  • 5th finger clinodactyly
  • wide gap (sandal gap) between the 1st & 2nd toes
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12
Q

What are the particular features of trisomy 21 in adults?

A

Fertility

  • Females – normal, males usually infertile
  • *Average life 55-68y**
  • *Medical problems**
  • risk (mainly leukaemia) cancer
  • Alzheimer’s
  • Hypothyroid
  • Obesity/coeliac, arthritis, diabetes, hearing loss, seizures
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13
Q

What are the clinical features of Edward’s syndrome?

A

1 in 6000 livebirths (95% spontaneously abort)

10% survive >1y

Head: microcephaly; low set ears; micrognathia; ears low set; cleft lip and palate
Hands & feet: Clenched hands, overlapping fingers; Rockerbottom feet
Low birth weight
Short sternum
Severe mental retardation

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14
Q

What are the clinical features of patau syndrome?

A

1/12 000 livebirths (95% spont ab)
Small at birth
Mental retardation severe
Microcephaly/ sloping forehead
Defects of brain - holoprosencephaly

Eyes – microphthalmia, coloboma, retinal dysplasia, palpebral fissures slanted
Cleft lip and/or palate
Ears abnormal and low
Polydactyly & fingers flexed
Heart defect
Abnormal genitalia

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15
Q

Is there an age-related risk associated with sex chromosome aneuploidy?

A

No

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16
Q

What are the features of Turner’s syndrome?

A

Reproductive

  • Loss of ovarian function
  • No puberty
  • Infertility

Lymphatic (obstruction)

  • Webbed neck
  • Swelling of hands &/or feet

Others

  • Skeletal Abnormalities – short stature
  • Coarctation of aorta
  • IQ generally normal/reduced compared to sibs
17
Q

What are the features of Klinefelter syndrome?

A

Diagnosis:

  • Most undiagnosed (~64%)
  • Identified through infertility &/or hypogonadism

Cytogenetics:

  • 80% 47,XXY; 20% mosaic or variant

Infertility

  • May lack secondary sexual characteristics
  • Testicular dysgenesis
  • 30-50% gynaecomastia (20x risk breast cancer)

Growth

  • Normal in infants, then accelerates
  • Adults long legs and arms

IQ normal

  • Family background IQ important
  • IQ may decrease with increased Xs
18
Q

What are the possible origins of triploidy?

A

Digyny, diplospermy (one diploid sperm) and dispermy

19
Q

What is the significance of the parental origin of triploidy?

A

•Double paternal = large placenta

= some growth delay

• Double maternal = tiny placenta

= significant growth delay

= head-saving macrocephaly

Conclusions: Maternal genome for foetus

Paternal genome for placenta

20
Q

What is a molar pregnancy?

A

On haploid sperm fuses with an empty egg. Result is a massive cystic placenta = ‘conceptus without an embryo’

21
Q

Where does mosiacism occur?

A

An error at early cleavage - mitotic non-disjunction.

22
Q

What are the potential balanced chromosomal rearrangements that may occur?

A

1.Translocation

  • Reciprocal
  • Robersonian

2.Inversion

  • Pericentric
  • Paracentric

3.Insertion

23
Q

What is a reciprocal translocation?

A

An exchange of material between nonhomologous chromosomes. They swap reciprocal segments of the chromosome and the result is two altered chromosomes with the same genes carried between them.

24
Q

What is a Robertsonian translocation?

A

Fusion of the long arms of two acrocentric chromosomes. The short arms are usually lost. Heterozygotes are balanced carriers; homozygotes are trisomies.

25
Q

What is a chromosomal inversion?

A

An inversion is achromosome rearrangement in which a segment of a chromosome is reversed end to end. An inversion occurs when a single chromosomeundergoes breakage and rearrangement within itself. Inversions are of two types: paracentric and pericentric.

26
Q

What is the difference between paracentric and pericentric inversions?

A

Paracentric inversions do not include the centromere and both breaks occur in one arm of the chromosome. Pericentric inversions include the centromere and there is a break point in each arm.

27
Q

What is an interstitial deletion?

A

A segment of chromosome not involving the telomere is lost.

28
Q

What is a terminal deletion?

A

A segment including the telomere is lost.

29
Q

What is duplication?

A

Gain of a segment - may be direct or inverted.

EMS - Genetics in disease (13 decks)

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