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302 Scientific basis of medicine > Genetics 3) Chromosomes 1 > Flashcards

Genetics 3) Chromosomes 1 Flashcards

1
Q

What is the function of chromosomes?

A

Allow cellular DNA to be stably maintained and function as vessels of genetic inheritance

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2
Q

How is 2.17m of DNA contained in 40um nucleus?

A

Structures such as nucleosomes and chromatin fibre

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3
Q

Describe the structure of chromosomes

A

Short arm and long arm
Telomeres to protect ends of chromosomes
Centromeres to hold sister chromatids together

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4
Q

What is the function of telomeres?

A

Protects ends of chromosomes so they aren’t recognised as DNA breaks do won’t be susceptible to DNA repair

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5
Q

What does metacentric mean?

A

Centromere is in middle of chromosome

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6
Q

What does sub-metacentric mean?

A

Centromere is closer to one telomere

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7
Q

What does acrocentric mean?

A

Centromere is very near telomere so short arm is very short

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8
Q

What is mitosis?

A

Type of cell division in which each parent cell gives rise to 2 daughter cells each having the same number of chromosomes as the parent

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9
Q

What are the 2 divisions in meiosis?

A

Meiosis I - reductional division

Meiosis II - equational division

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10
Q

Describe the human karyotype

A

Normal human somatic cell has 46 chromosomes

44 autosomes and 2 sex chromosomes

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11
Q

How is a person’s karyotype determined?

A

Using 20 metaphase cells stained with giemsa satin which reveals a unique banding that helps identify each chromosome

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12
Q

Describe the cytogenetic notation

A

Total number of chromosomes, sex chromosomes, irregularity

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13
Q

What are the types of chromosomal abnormalities

A

Aneuploidy

Structural

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14
Q

How can aneuploid gametes be generated?

A

Meiotic nondisjunction
During maternal meiosis I
During meiosis II if chromatids fail to separate leading to disomic gametes and nullisomic gametes

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15
Q

What happens if mitotic nondisjunction occurs early in the development of an initial trisomy conceptus?

A

1 copy of trisomy chromosome is lost

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16
Q

What is trisomy 21?

A

Down’s syndrome

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17
Q

What is the prevalence of trisomy 21?

A

1 in 700-1000 live births

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18
Q

What is the maternal age effect of trisomy 21?

A

1 in 35 at 43 years old

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19
Q

What are the clinical features of trisomy 21?

A 
Learning defects
Dysmorphic features
Heart defects
Early onset Alzheimers 
Intestinal blockage 
Short and broad hands
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20
Q

What is the karyotype for trisomy 21?

A

95% have primary (full) trisomy 47,XX,+21 or 47,XY,+21
1-2% have mosaicism 46,XX / 47,XX,+21
4% have translocation 46,XX,der(14;21)(q10;q10),+21

21
Q

What is the incidence of trisomy 18?

A

1 in 3000

22
Q

What is the maternal age effect of trisomy 18?

A

1 in 500 at 43 years old

23
Q

What are the clinical features of trisomy 18?

A

Severe mental disability
Dysmorphic features - micrognathia, prominent occupant
Rocker-bottom feet, clenched fists with overlapping fingers
Congenital heart defects

24
Q

What is the karyotype of trisomy 18?

A

Primary trisomy: 47,XX,+18 or 47,XY,+18

5% have mosaic form

25
Q

What is the incidence of trisomy 13 (Patau’s syndrome)

A

1 in 5000 live births

26
Q

What is the maternal age effect of trisomy 13?

A

1 in 1100

27
Q

What is the maternal age effect?

A

Main risk factor for trisomy

Mitotic nondisjunction and recombination

28
Q

What is the incidence of Kleinfelter syndrome?

A

1 in 500 to 1 in 1000 newborn males

29
Q

What are the clinical features of Kleinfelter’s syndrome?

A 
Tall stature, long limbs
Gynaecomastic
Small testes or cryptorchidism
Infertility, azoospermia
Delayed or incomplete puberty
30
Q

What is the karyotype for Kleinfelter’s syndrome?

A

47,XXY
Mosaic: 47,XXY/46,XY
48,XXXY or 49,XXXXY

31
Q

What is the incidence of Turner syndrome?

A

1 in every 2000 live female births

32
Q

What are the clinical features of Turner’s syndrome?

A

Short stature
Infertility
Poor breast development
Constriction of aorta

33
Q

What is the karyotype for Turner’s syndrome?

A

45,X monosomy

Mosaicism: 45X/46XX, 45,X/46XiXq, 45X/46XY

34
Q

What is the incidence of XYY syndrome?

A

1 in 1000 newborn males

35
Q

What are the clinical features of XYY syndrome?

A 
Often asymptomatic
Learning disability, speech delay
Low muscle tone
Taller than expected
Normal sexual development and fertility but can sometimes develop testicular failure
36
Q

What is the karyotype of XYY syndrome?

A

47,XYY

47,XYY/46,XY

37
Q

What is the incidence of triple X syndrome?

A

1 in 1000 newborn females

38
Q

What are the clinical features of triple X syndrome?

A

Largely asymptomatic
Seizures or kidney abnormalities in 10%
Taller
Risk of learning disabilities, delayed development of speech or language skills
Doesn’t cause unusual physical features or infertility

39
Q

What is the karyotype of triple X syndrome?

A

47,XXX

47,XXX / 46,XX

40
Q

What is the mechanism of X-inactivation?

A

Randomly occurs in all cells
2 weeks post fertilisation
Begins at X-inactivation centre
X-inactive specific transcript (List) RNA is expressed on inactive X
Xist transcripts coat X chromosome leading to inactivation

41
Q

What is chromosome translocation rearrangement?

A

Chromosome segment is transferred to a non-homologous chromosome or to a new site on same chromosome

42
Q

What is a reciprocal translocation?

A

2 way exchange of material between any 2 homologous chromosomes
Incidence of 1 in 500 newborn
Balanced carriers are phenotypically normal but carry reproductive risks
Chromosome segregation in carrier of autosomal translocation leads to 5 possible pairs of daughter gametocytes

43
Q

What is a Robertsonian translocation?

A

Fusion of 2 acrocentric chromosomes
Incidence of 1 in 1000 newborns
Phenotype is normal in balanced carriers but has reproductive risk

44
Q

What is the karyotype of translocation Down’s?

A

Carrier: 45,XX,-14,-21,+t(14q21q)
Affected: 46,XX,-14,+t(14q21q)

45
Q

What is an isochromosome?

A

Centromere fission

2 identical arms - 1 duplicated and other is deleted

46
Q

Give an example of a deletion syndrome

A

Cri du chat

47
Q

What is Cri du chat syndrome?

A

Named for its high pitched cry
Incidence of 1 in 20,000-50,000 newborns
Most are de novo, 10% are inherited
Deletion on chromosome 5p

48
Q

What are the clinical features of cri du chat syndrome?

A

Slow growth
Microcephaly
Moderate-severe mental disability

49
Q

Give example of duplication and deletion of chromosome 17

A

Deletion - Smith magenis syndrome

Duplication - Potocki Lupski syndrome

302 Scientific basis of medicine (28 decks)

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