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Essential Medical Science - GiM > Lecture 6 GiM > Flashcards

Lecture 6 GiM Flashcards

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USMLE® Step 1 flashcards Biology 101 flashcards
1
Q

What is cytogenetic?

A

Study of chromosome

Anything more than a single gene

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2
Q

How many pairs of chromosome are there in a nucleus?

A

23

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3
Q

What is chromosome 1-22 called?

A

Autosomes

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4
Q

List the different types of cytogenetic analysis

A

Conventional cytogenetics
- G-band metaphase chromosome analysis

Molecular cytogenetics
-FISH
-microarray chromosomal genomic hybridisation
-Next generation sequencing
-Multiplex ligation-dependent probe amplification (MLPA)
-QF-PCR
qPCR

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5
Q

How many genes and bands are there in a karyotype sample?

A

30000 genes

550 bands

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6
Q

What are the terminal part of a chromosome called?

A

Telomere

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7
Q

How do cytogenetic abnormalities produce an abnormal phenotype?

A
  • Dosage effect
  • Disruption of a gene
  • Effect due to parental origin
  • Position effect
  • Unmasking of recessive disorder
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8
Q

Which are more deleterious, gain or loss of genes?

A

Loss are more deleterious than excess

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9
Q

What can disrupt genes in a chromosome?

A

breakpoint and inappropriate activation/inactivation

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10
Q

What is breakpoints?

A

breakpoints in the genome are locations on a chromosome where DNA might get deleted, inverted, or swapped around.

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11
Q

What is genomic imprinting?

A

Genomic imprinting is the epigenetic phenomenon by which certain genes are expressed in a parent-of-origin-specific manner. If the allele inherited from the father is imprinted, it is thereby silenced, and only the allele from the mother is expressed. If the allele from the mother is imprinted, then only the allele from the father is expressed

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12
Q

What are the two types of cytogenetic abnormalities?

A

Structural and numerical

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13
Q

If a baby survive from a cytogenetic abnormalities, what might be the potential phenotypic imbalances experienced by her?

A
  • Facial dysmorphism
  • organ malformation
  • compromised mental/intelectual functioning
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14
Q

What are the three different types of numerical chromosome abnormality?

A
  1. Aneuploidy = gain (trisomy) or loss (monosomy)
  2. Polyploidy = gain whole sets (triploidy or tetraploidy)
  3. Mosaicism = diploidy & aneuploidy
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15
Q

What are the origins/causes of numerical chromosome abnormality?

A
  1. Aneuploidy - Gametogenesis/meiosis
  2. Polyploidy - Fertilisation
  3. Mosaicism - Post-zygotic non-disjunction (Early cleavage)
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16
Q

What is the factor that increase the susceptibility to aneuploidy?

A

Increase maternal age

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17
Q

During the meiotic error, what is the probability that the error happens in meiosis I rather than meiosis II ?

A

80% - 90%

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18
Q

What are the examples of autosomal aneuploidy

A 
Trisomy 18 (Edwards syndrome)
Trisomy 13 (Patau syndrome)
Trisomy 21 (Down syndrome)
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19
Q

All aneuploidies are possible, why only three chromosomes involved?

A

s

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20
Q

What are the epidemiology of trisomy 21?

A

1/700

75% spontaneously abort

21
Q

What are the characteristics of a baby with trisomy 21?

A 
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
22
Q

What are the characteristics of an adult with trisomy 21?

A 
Fertility
Females – normal, males usually infertile
Average life 55-68y
Medical problems
-increase risk (mainly leukaemia) cancer
-Alzheimer’s
-Hypothyroid
-Obesity/coeliac, arthritis, diabetes, hearing loss, seizures
23
Q

What do you know about Edwards syndrome?

A

Trisomy 18
~ 1 in 6000 livebirths (95% spontaneously abort)
10% survive >1y

24
Q

What are the clinical pictures of someone with Edwards syndrome?

A

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

25
Q

What are the examples of organs malformations happened in Edwards syndrome?

A

Umbilical or inguinal hernia
Congenital heart disease (90%)
Congenital kidney disease
Eye abnormalities eg: cataracts, micropthalmia

26
Q

What is micropthalmia?

A

In this condition, one or both eyeballs are abnormally small. In some affected individuals, the eyeball may appear to be completely missing; however, even in these cases some remaining eye tissue is generally present

27
Q

What is patau syndrome?

A

Trisomy 13

~ 1/12 000 livebirths (95% spont ab)

Small at birth

Mental retardation severe

Microcephaly/ sloping forehead

Defects of brain - holoprosencephaly

28
Q

What is holoprosencephaly?

A

Holoprosencephaly (HPE, once known as arhinencephaly) is a cephalic disorder in which the prosencephalon (the forebrain of the embryo) fails to develop into two hemispheres. Normally, the forebrain is formed and the face begins to develop in the fifth and sixth weeks of human pregnancy. The condition also occurs in other species.

The condition can be mild or severe. According to the National Institute of Neurological Disorders and Stroke (NINDS), “in most cases of holoprosencephaly, the malformations are so severe that babies die before birth.”[1]

When the embryo’s forebrain does not divide to form bilateral cerebral hemispheres (the left and right halves of the brain), it causes defects in the development of the face and in brain structure and function.

In less severe cases, babies are born with normal or near-normal brain development and facial deformities that may affect the eyes, nose, and upper lip.

29
Q

What are the phenotypes of someone with trisomy 13 (Patau syndromes)?

A

Eyes – microphthalmia, coloboma, retinal dysplasia, palpebral fissures slanted

Cleft lip and/or palate

Ears abnormal and low

Polydactyly & fingers flexed

Heart defect

Abnormal genitalia

30
Q

What are the maternal age factors that lead to autosomal aneuploidy?

A
  1. Unfavourable chiasmata distribution (foetus)
  2. Age-dependent deterioration of meiotic structures (10-40y later) - hormonal imbalance, irradiation, oral contraceptives, alcohol, etc
31
Q

What are the characteristics of sex chromosome aneuploidy?

A
  • No age related risk
  • Phenotype less severe than autosomal
  • Sexual orientation not affected
32
Q

What is Turner’s syndrome?

A 
(45,X) - 1/2500
       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
33
Q

What is Klinefelter Syndrome?

A

(47,XXY) - 1/1000

34
Q

How can diagnosis someone with Klinefelter syndrome?

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

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
35
Q

What are the possible error that may happen during fertilisation?

A
  1. Polyploidy (usually triploidy)

2. Molar pregnancy (double paternal, no maternal)

36
Q

What are the characteristics of someone with triploidy?

A

69,XXY or 69,XYY or 69,XXX

2% all pregnancies

99.9% spontaneously abort

1/57000 livebirths

37
Q

What is digyny?

A

1 sperm (n) + 1 ovum (2n)

38
Q

What is diplospermy?

A

1 sperm (2n) + 1 ovum (n)

39
Q

What is dispermy ?

A

2 sperm + 1 ovum

40
Q

What are 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

41
Q

What is mosaicism?

A

A mosaic or mosaicism denotes the presence of two or more populations of cells with different genotypes in one individual who has developed from a single fertilized egg.

42
Q

What is the consequences of mosaicism?

A

Variable phenotype

Variable lethality foetal vs extraembryonic

Non-identical twin

Tissue-specificity – lateral asymmetry

May generate Uniparental disomy (UPD)

Recurrence risk (if gonadal)

“All females are mosaic”

43
Q

What is reciprocal translation?

A

Reciprocal translocations are usually an exchange of material between nonhomologous chromosomes. Estimates of incidence range from about 1 in 500 to 1 in 625 human newborns. Such translocations are usually harmless and may be found through prenatal diagnosis.

44
Q

What is robertsonian translocation?

A

Robertsonian translocation (ROB) is a rare form of chromosomal rearrangement that in humans occurs in the five acrocentric chromosome pairs, namely 13, 14, 15, 21, and 22.

Whole arm fusion

45
Q

What is paracentric inversion?

A

Paracentric inversions do not include the centromere and both breaks occur in one arm of the chromosome

46
Q

What is pericentric inversion?

A

An inversion in which the breakpoints occur on both arms of a chromosome.

47
Q

What is copy number variation(CNV)?

A

A copy number variation (CNV) is when the number of copies of a particular gene varies from one individual to the next. Following the completion of the Human Genome Project, it became apparent that the genome experiences gains and losses of genetic material.

48
Q

What is the commonest unbalanced rearrangements?

A

Deletions and duplications

49
Q

What is acrocentric chromosome?

A

A chromosome in which the centromere is located quite near one end of the chromosome. Humans normally have five pairs of acrocentric chromosomes.

Essential Medical Science - GiM (5 decks)

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