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Year 1 IMS > Genetic Diseases > Flashcards

Genetic Diseases Flashcards

1
Q

What are the 5 classifications of genetic disorders?

A

1) Multifactorial: common conditions, variants from the environment, not technically mutation but to do with some mutated allele
2) Single gene
3) Chromosomal
4) Mitochondrial
5) Somatic mutations (eg. cancer): mutations affecting somatic cells that can only affect those cells and their off spring so cannot be passed on to future generations

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2
Q

What is a mendelian disease?

A

A disease caused by a single gene mutation eg. sickle cell disease

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3
Q

In genetics what are fully penetrant genes?

A

environmental factors and other genes have no effect

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4
Q

In genetics what are low penetrant genes?

A

gene plays a part along with other genes and environmental factors in determining a persons susceptibility to a disease

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5
Q

Multiple sclerosis is a good example of a mendelian disease or a multifactorial disease?

A

Multifactorial disease - genetic factors play a part in determining susceptibility but each individual factor has very low penetrance

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6
Q

What is the genetic problem in chromosomal disorders?

A

Chromosomal imbalance (changes in chromosome number) causes alteration in gene dosage ie. the amount of genetic information and the amount of proteins produced

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7
Q

What is the genetic influence in multifactorial diseases?

A

Environmental influences act on a genetic predisposition to produce a liability to a disease, one organ system tends to be affected, person affected if liability is above the threshold

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8
Q

What is mendelian inheritance?

A

Autosomal dominant or recessive or sex linked inheritance

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9
Q

How many organ systems are affected in chromosomal genetic disorders?

A

Multiple organ systems affected

Thousands of genes may be involved

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10
Q

What are the 2 parts to a chromosome?

A 
Telomere = each end - dna and protein cap ensures replication of the tip, tethers to nuclear membrane
p arm (short)
q arm (long)
centromere joining the p and q arm - joins sister chromatids, essential for chromosome segregation at cell division
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11
Q

What are the clinical features of down syndrome?

A

1) Round face
2) Protruding tongue
3) Upslanting plapebral fissures
4) Epicantic folds
5) Developmental delay
6) Large sandal gap
7) Single palm crease

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12
Q

What are the ultrasound features of down syndrome?

A

1) Short femurs
2) Nuchal translucency (nape of the neck)
3) Echogenic bowel (bowel appears brighter than it should be)
4) CP cysts (cysts within the choroid plexus of the brain)

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13
Q

What are the 3 different patterns of chromosomes that can cause down syndrome, and which is more common?

A

1) 3 separate copies of chromosome 21 - trisomy 21 (95%)
2) Robertsonian translocation (4%)
3) Mosaicism - normal and trisomy 21 cell lines, often occurs post zygotically

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14
Q

What is non dysjunction?

A

Chromosomes dont segregate correctly during nuclear division, normally line up on the spindles and are pulled to opposite poles, but sometimes this doesnt occur correctly and 2 homolagous chromosomes get pulled to the same side, you end up with 2 of the same chromosome on one side and trisomy occurs - also have monosomy which is generally catastophic!

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15
Q

Which chromosomes can a robertsonian translocation happen in and what characteristic of these chromosomes allows that to occur?

A

13,14,15,21,22
They are accrocentric (centromere is postitioned so that one arm is much shorter than the other)
Translocations can occur in other chromosomes but dont lead to a viable foetus

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16
Q

What trisomy occurs in Edwards syndrome and what defects is it associated with?

A

Trisomy 18
Associated with multiple malformations (especially heart and kidneys)
Clenched hands with overlapping fingers

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17
Q

What trisomy occurs in Patau syndrome and what defects is it associated with?

A

Trisomy 13
Associated with multiple malformations
Affects midline structures, incomplete lobation of the brain, cleft lip, congenital heart defects

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18
Q

What genetic abnormality occurs in Klinefelter syndrome, what defects is it associated with?

A

47, XXY
Male phenotype
Infertility (atophic testes do not produce sperm)
Tall
Poorly developed secondary sexual characteristics in some (lack of testosterone)
Gynaecomastia (enlargement of male breasts) and osteoporosis
Hypogonadism and oligiospermia (deficiency of sperm in the semen)

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19
Q

What genetic abnormality occurs in Turner syndrome and what defects is it associated with?

A

45, X
Female phenotype
Many lost spontaneously in pregnancy
Primary Amenorrhoea
Short stature
Congenital heart disease (coarctation of the aorta)
Puffy feet
Redundant skin at the back of the neck
Low hair line, shield shaped chest, narrow hips
Histology of gonads: ovarian corticol strome devoid of some germ cell elements

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20
Q

What is a common cause of numerical chromosomal abnormalities?

A

Non dysjunction

In germ cells at meiosis, if occurs in somatic cells, leads to mosaicism

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21
Q

What happens in autosomal monosomies?

A

Foetus cant survive - fewer serious effects from sex chromosome anomalies

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22
Q

What is a microdeletion?

A

Bit of chromosome that is missing is too small to be seen down a microscope, identified using specific cytogenetic techniques

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23
Q

DiGeorge/Velocardiofacial syndrome (22, q11) is caused by what kind of genetic abnormality and gives rise to what genetic defects?

A

Microdeletion
Small mouth, prominent nose, heart defects including tetralogy of fallot, interrupted aortic arch, VSD, truncus arteriosus

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24
Q

Other than DiGeorge/Velocardiofacial syndrome, what 2 other syndromes are caused by microdeletions?

A

Cat Eye Syndrome
Prader-Willi syndrome
Williams beuren syndrome

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25
Q

What is the process of FISH (Fluorescence in situ hybridisation)?

A

1) DNA is denatured
2) Fluorescent probe is added which can be seen down a microscope
3) Absence of a fluoroscent spot indicates a missing or mutated gene

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26
Q

Which kind of genetic abnormalities would you use FISH to identify?

A

Microdeletions as they are too small to be seen down a microscope

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27
Q

Williams-Beuren syndrome is caused by a microdeletion, what defects does it give rise to?

A

1) Bright eyes, stellate irises
2) Upturned nose
3) Wide mouth
4) Heart defect

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28
Q

Huntingtons disease is caused by what kind of genetic abnormality, and what are its characteristics?

A

Single gene disorder, Mendelian inheritance
Advanced stage = involuntary movements of the head
Before this gradual neurodegradation as loss of grey and white matter in the brain
Early stages, forgetfullness, irritability, inability to make a decision

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29
Q

Familial hypercholesterolaemia is caused by what kind of genetic abnormality, and what is its clinical presentation?

A

Single gene disorder, mendelian inheritance, mutation in a large gene
Deposition of cholesterol in the joints, particularly the achilles tendon
Arch of cholesterol in the eye
Major problem is increased risk of heart disease

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30
Q

What test could you do for familial hypercholesterolaemia?

A

Biochemistry test - look at levels of protein or activity of an enzyme (in this case LDL cholesterol levels)

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31
Q

What kind of genetic abnormality causes CF?

A

Single gene mutation

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32
Q

Autosomal sensorineural recessive deafness is caused by what genetic abnormality?

A

Single gene
Deletion of a single guanine nucleotide on chromosome 21, common cause of deafness
Inherited deafness may be caused by a variety of different mutations though!

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33
Q

What kind of genetic abnormality is the cause of Duchenne muscular dystrophy and what is its clinical presentation?

A

Single gene - mutation in the gene that codes for dystrophin

Affected children cant stand up and have to use Gower’s manoeuvre (pushing self up on knees)

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34
Q

What is needed to confirm the initial diagnosis of Duchenne muscular dystrophy?

A

Muscle biopsy, dystrophin is lost and structure of muscle is disorganised, irregular arrangement and fibrous tissue also forms as muscle tissue is trying to repair itself into new myo tubes

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35
Q

What is mitochondrial inheritance?

A

Mutations in the circular chromosome of mitochondrial DNA

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36
Q

What is abnormal about mitochondrial inheritance?

A

No mitochondrial DNA come from the father, all comes from the mother, so mitochondrial mutations can only be inherited from the mother

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37
Q

What is the genetic abnormality in Leber Hereditary Optic Neuropathy?

A

Mitochondrial DNA mutation

Need mitochondria to make eye muscles work - with mutation they dont function properly

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38
Q

What is imprinting?

A

Process by which certain genes are expressed in a ‘parent of origin’ manner, if fathers allele is imprinted then only mother will be expressed and visa versa, appropriate expression of imprinted genes is important for normal development, when this goes wrong it can cause disease

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39
Q

How does imprinting occur?

A

Epigenetic process that involves DNA methylation and histone methylation in order to achieve the expression of only one allele without altering the gene sequence, these epigenetic markers are established in the germ line and maintained throughout mitotic divisions in the somatic cells of an organism

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40
Q

Other than caused by microdeletions what other genetic abnormality can cause Prader Willi syndrome and what are the defects associated with it?

A

Incorrect imprinting - 7 genes on chromosome 15 are deleted or unexpressed
Defects = low muscle tone, short stature, incomplete sexual development, cognitive disabilities, problem behaviours and a chronic feeling of hunger that can lead to excessive eating and life threatening obesity

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41
Q

What kind of mutations is cancer generally caused by?

A

Somatic mutations - can be mutations in genes which pre dispose to cancer syndromes and a small segment of familial hereditary cancer cells

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42
Q

What is the genetic basis of cancer and why is there generally an early onset of inhereted familial cancer in children?

A

1) Need 2 mutations to occur so a gene becomes inactivated leading to a loss of control of growth and unchecked cell proliferation
2) In dominantly inherited familial cancer syndromes an altered allele (of tumour suppressor or oncogene) is inherited so is in all body cells containing genetic material
3) When a second (previously normal allele) becomes inactivated you get out of control growth and unchecked cell proliferation

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43
Q

Mutation in autosomal dominant non-polyposis colon cancer?

A

Mutation in tumour suppressor (APC) gene

Tumour arises from a 2nd somatic mutation in APC gene

44
Q

What is multifactorial inheritance?

A

Inheritance controlled by many genes having an additive effect (polygenic) plus the effects of the environment
Clinical clue tends to be that there is only one organ system affected
Environmental influences act on a genetic predisopsition

45
Q

What are the 4 stages of nuclear division and what happens in each?

A

1) Prophase: genetic material has been duplicated and chromatin condensed into chromosomes
2) Metaphase: Nuclear envelope disappears and chromosomes align at the equatorial plate
3) Anaphase: Sister chromatids separated and pulled to opposite pole, centromeres divide
4) Telophase: 2 distinct nuclei formed

46
Q

What happens in each of the stages of the cell cycle (G1, S, G2, mitosis, cytokinesis, G0)?

A

1) G1: cell contents are duplicated
2) S: genetic material is duplicated, each chromosome forms 2 chromatids
3) G2: Checking point for any errors in the replicated DNA
4) Mitosis: nuclear division
5) Cytokinesis: cytoplasm, organelles and cell membrane are divided into 2 cells
6) G0: quiescent phase where there is a period of inactivity, cell is not dividing nor preparing to divide

47
Q

At what point in the cell cycle are distinct chromosomes visible?

A

During metaphase

48
Q

What is the function of meiosis?

A

To produce sperm and egg cells

49
Q

What cells are produced from meiosis compared to mitosis?

A

Haploid cells (23 chromosomes) instead of diploid cells

50
Q

In what 2 ways does the process of meiosis ensure that every gamete is genetically unique?

A

1) Independent segregation of homologous chromosomes

2) Crossing over

51
Q

What is the process of meiosis?

A

Meiosis 1:
1) Homologues (chromosomes) replicated to give 2 sister chromatids
2) The maternal and paternal homologues (now made up of 2 sister chromatids) line up on metaphase plate
3) The maternal homologue (still made up of 2 sister chromatids) is pulled to one side and the paternal homologue to the other
Meiosis 2: (this occurs in both cells that have been made in meiosis 1)
1) The homologues (containing 2 sister chromatids) line up on the metaphase plate
2) Sister chromatids are pulled to opposite poles and centromeres separate
3) Now have 4 daughter cells

52
Q

When and how does crossing over occur?

A

During meiosis 1, when maternal and paternal homologues line up next to eachother on the metphase plate, equivalent portions of non sister chromosomes are exchanged

53
Q

What is a chiasma?

A

A point of overlap between 2 homologous chromosomes during meiosis 1 at which genetic material can be exchanged

54
Q

What is the structure of a metaphase chromosome?

A

1) DNA double helix
2) Beads on a string, form of chromatin (DNA wrapped histones)
3) Chromatin fibre packed into nucleosomes
4) Chromosome in an extended form
5) Condensed chromosome in metaphase

55
Q

What is the locus of a gene?

A

Position on the chromosome

56
Q

What is a karyotype?

A

Chromosome complement of an individual

57
Q

What are light bands and dark bands of a chromosome and when do they replicate?

A

Light bands - replicate early in the S phase, less condensed chromatin, transcriptionally active gene and GC rich
Dark (G) bands - replicate late in the S phase, contain condensed chromatin, AT rich

58
Q

Other than DNA what other components do chromosomes contain?

A

1) RNA
2) Histone proteins
3) Non-histone proteins

59
Q

How is the specific banding of chromosomes visible and what can it be used for?

A

Visible using light microscopy (different resolutions can resolve bands, sub bands and sub sub bands)
Used to identify chromosomes

60
Q

What are epigenetic modifications?

A

Modifications not in DNA but on chromosome, can occur on the packaging part of DNA (histones)

61
Q

What is the difference between aneuploidy and polyploidy?

A

Aneuploidy - condition when the number of chromosomes in a nucleus is not an exact multiple of the monoploid number of a particular species eg. trisomy or monsomy
Polyploidy - condition where there are more than 2 paired homologous sets if chromosome eg. triploidy (extra set of chromosomes)

62
Q

What are translocations and what is the difference between robertsonian translocations and reciprocal translocations?

A

Re arrangement of parts between non homologous chromosomes
Reciprocal - exchange of material between non homologous chromosomes
Robertsonian - rare (occurs only in accrocentric chromosomes 13,14,15,21,22) fusion of the whole long arms of chromosomes can be balanced or unbalanced (you lose the p arm of the chromosomes that fuse but doesnt matter as they are accrocentric and p arm so small it doesnt contain significant DNA)

63
Q

What does the karyotype 46,XX,dup(2)(p13,p22) describe?

A

A female with 46 chromosomes who has a duplication on the short arm of chromosome 2 from band 12 to 22

64
Q

Other than translocations what are the other structural abnormalities that can occur in chromosomes?

A

1) Duplications
2) Deletions
3) Inversions

65
Q

What process occurs in the formation of Spermatozoa (Sperm cell)?

A

1) Spermatogonia is a cell produced in the early stages and is formed in the wall of seminiferous tubules
2) Spermatagonia undergoes mitosis to form Primary spermocyte
3) Meiosis 1 forms 2 secondary spermocytes
4) Meiosis 2 forms 4 spermatids
5) The 4 spermatids undergo differentiation to form 4 spermatozoa

66
Q

Why is there a large potential for error in the production of sperm?

A

To maintain the pool or spermatogonia from which spermatozoa are formed, the spermatogonia continue to divide, lots of divisions = lots of room for error

67
Q

When are the oocytes of a female produced?

A

During gestation of the foetus - born with all the oocytes you will ever have

68
Q

What happens during ovulation and fertilisation to give rise to a zygote in a female from an oocyte and why is there high potential for error?

A

Each month one of the big pool of oocytes is ovulated, Meiosis 1 is completed at ovulation
Meiosis 2 is completed during fertilisation
There may be a lengthy interval between onset and completion of meiosis 1
Damage to repair mechanisms of the primary oocyte over time leads to greater chance of non dysjunction

69
Q

What is non dysjunction and when can it occur?

A

Error of segregation of chromosomes on the metaphase plate, instead of going to opposite sides of the cell, they end up on the same side
When this occurs in meiosis when cells become haploid, some have 2 of a chromosome and some have none
Non dysjunction can occur during meiosis 1 or 2

70
Q

In terms of sperm divisions why does the chance of a child being born from Marfans syndrome and Achondroplasia increase with age of father?

A

Dominant conditions, chance of mutation of fathers DNA increases with age as more mutations to produce sperm so greater chance of a mutation occurring

71
Q

Are you more likely to inherit a syndrome to do with aneuploidy from your mother or father and why?

A

Due to the long lag in meiosis 1 of oocytes, there is a greater risk of non dysjunction in oogenesis
Thus aneuploidy is mainly caused by females

72
Q

What is polydactyly and what genetic disease is it characteristic of?

A

More than 5 fingers or toes on any one hand or foot

Characteristic of Patau syndrome which is trisomy 13

73
Q

What is mitotic non dysjunction and what does it lead to?

A

Non dysjunction that occurs during mitosis and results in mosaicism
Once cause of down syndrome, depends where the population of trisomic cells occurs as to whether it presents in the phenotype

74
Q

What is tetraploidy?

A

Having 92 chromosomes

75
Q

When are trisomys more likely to be fatal?

A

When they occur in a large chromosome

76
Q

What is a reciprocal translocation?

A

Breakage of 2 non homolagous chromosomes with exchange of the fragments

77
Q

In terms of a Robertsonian translocation what is a balanced carrier?

A

Person who carries the abnormal chromosome containing the Robertsonian translocation but has a balanced amount of genetic information

78
Q

What is an aquired cytogenetic abnormality?

A

Genetic change associated with neoplastic or cancer disease process

79
Q

What translocation is associated with CML (Chronic myelogenous leaukaemia?

A

Translocation between chromosome 9 and 22

Dubbed the philadelphia chromosome - lengthened chromosome 9 derived from the tip of chromosome 22

80
Q

Why does the translocation that occurs in CML lead to cancer?

A

Leads to the fusion of 2 proto oncogenes and thus the expression of bcr-abl protein in the derivative protein

81
Q

What is heteroploidy?

A

Having an abnormal chromosome number

82
Q

What is the translocation that occurs in Follicular lymphoma and why does it lead to cancer?

A

BCL-2 oncogene normally suppresses cell death by apoptosis but is not normally expressed in its position on Chromosome 18
BCL-2 oncogene is translocated to chromosome 14 into a cluster of Ab genes which are continually expressed in B cells
BCL-2 is thus continuously expressed an suppresses cell death by apoptosis in B cells, get neoplastic proliferation of B lymphocytes

83
Q

What is the genetic mutation and thus amino acid substitution in sickle cell anaemia?

A

T-A replaced with A-T

Glutamic acid replaced with Valine

84
Q

What is the purpose of PCR?

A

Used to amplify small amounts of target DNA for analysis

85
Q

What DNA polymerase is used in the PCR and why?

A

Thermostable DNA polymerase which can withstand the temperature of the PCR without being denatured
Can be Taq polymerase from the bacterium thermos aquaticus
Recombinant DNA polymerase is now mass produced from gene cloned and expressed in E.Coli

86
Q

What is an oligonucleotide primer?

A

Primer needed for the PCR
Specific sequence of the PCR is essential to ensure the specificity of the PCR reaction
Need to know the sequence
Usually ~20 nucleotides long
Region to be amplified is defined by the oligonucleotides

87
Q

What is the rough process of PCR?

A

1) DNA denatured by heat 94 degrees
2) Temperature reduced to 55 degrees for annealing of the primers
3) Temperature increased to 72 degrees, Taq polymerase adds nucleotides to the 3’ end of the primers
4) Now have templates for the next cycle

88
Q

What are the advantages and disadvantages of PCR?

A

Advantages:
1) Speedy and easy
2) sensitive (can use a tiny bit of DNA)
3) robust (Can amplify DNA from material which is badly degraded)
Disadvantages:
1) Requires target DNA sequence in order to make primers
2) Efficiency decreases with larger amplimer products
3) Infedelity - taq polymerase has no proof reading function and any mistake made will be amplified loads

89
Q

What are the clinical applications of PCR?

A

1) Genotyping genetic markers
- Linkage analysis of inherited disease
- Tissue typing for transplants
2) Detection of mutations
- diagnosis of genetic disease, cancer

90
Q

What is gel electrophoresis?

A

Way of separating DNA fragments according to size

91
Q

How does Gel electrophoresis work?

A

1) DNA is negatively charged so travels away from the cathode and towards the anode
2) Gel contains cross links, large pieces of DNA pass through the cross links with more difficulty so dont travel as far on gel
3) DNA fragments can be visualised using UV light

92
Q

What is used to detect unknown mutations?

A

Sanger sequencing - ‘chain termination chemistry’

93
Q

What type of cell are most observations of karyotype carried out on?

A

B lymphocyte

94
Q

What is the most widely used banding technique and what determines the banding pattern?

A

G banding or Giesma banding

AT rich areas stain darkly, CG areas stain more lightly

95
Q

What is the process if G banding?

A

DNA treated with trypsin
Giesma stain then added
G bands stain darkly with Giemsa

96
Q

What specimens are used for chromosome studies?

A 
Specimens containing rapidly proliferating cells:
1) Bone marrow
2) Lymph nodes
3) Solid tumours
4) Chorionic villi (CVS) from placenta
Specimens routinely cultured in the lab:
1) Blood lymphocytes
2) Tissue biopsies
3) Amniotic fluid samples
4) Long term CVS
97
Q

What are the 4 types of bands that come up in G banding?

A

1) G bands = giemsa stained
2) R bands = reverse giemsa stain
3) C bands = centromere
4) Non centromeric heterochromatin = chromosome material of a different density from normlly, in which the activity of genes in modified or suppressed which doesnt constitute to the centromere

98
Q

What is the topography of a chromosome?

A 
The distribution of parts or features
eg. size, centromere position:
telocentric = centromere terminally situated so there is only one arm
accrocentric = one really short arm
metacentric = arms equal
99
Q

What stage of the cell cycle can FISH be performed?

A

Dividing cells - metaphase

Or non dividing cells -interphase

100
Q

What are the 3 types of FISH probe?

A

1) Repetitive sequences, including those at the centromere of a chromosome
2) DNA segments that will cover the entire length of a chromosome (DNA paint)
3) DNA segments from specific genes or regions on a chromosome that have been previously mapped or identified

101
Q

What is multicoloured fish?

A

Several different probes of different colours each corresponding to a particular gene all used at once

102
Q

What is spectral karyotyping or SKY?

A

Paint each chromosome in 1 of 24 colours

In bladder cancer SKY shows interchromosomal translocations

103
Q

What is comparative genome hybridisation (CGH) and what is it used for?

A

Reveals the loss or gain of chromosomal regions in test samples compared to normal controls
DNA in test sample labelled with green fluorescence
DN in the control sample is labelled with red fluorescence
The 2 samples are then allowed to compete for hybirdisation sites on metaphase chromosomes or on a micro array
Areas that are more red than average show genes that are missing in the test sample

104
Q

What are the uses of cytogenetics?

A

1) Pregnant women over 35 = greater risk of aneuploidy
2) Pregnant women with screening results indicating risk of aneuploidy
3) Children with phenotypic problems and or developmental delay
4) Couples with reproductive problems
5) Microdeletion syndromes
6) Diagnosis, prognosis and monitoring of many cancers

105
Q

What is cytogenetics?

A

Branch of genetics concerned with the study of structure and function of the cell, especially chromosomes

Year 1 IMS (14 decks)

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