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MCD Year 1 > Genetics > Flashcards

Genetics Flashcards

1
Q

What is a malformation and give one example?

A

A primary structural defect, usually involving a single organ showing multifactorial inheritance
e.g. cleft lip

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2
Q

What is a disruption and give one example?

A

Secondary abnormal structure of an organ or tissue caused by ichaemia, infection or trauma. Not genetic but genetic factors can predispose
e.g. amniotic band causing digital amputation

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3
Q

What is a deformation and give one example?

A

Abnormal mechanical force distorting a naturally formed structure. Occurs late in pregnancy and has good prognosis as the organ is normal in structure, just physically malformed
e.g. club foot, hip dislocation

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4
Q

What is a syndrome and give one example?

A

A consistent pattern of abnormalities with a specific underlying cause, usually genetic
e.g. Down’s syndrome (chromosomal abnormality)

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5
Q

What is a sequence and give one example?

A

Multiple abnormalities initiated by a primary factor. Could have genetic component as initial factor
e.g. Potter sequence
Oligohydramnios= reduced amniotic fluid volume due to a failure to produce urine which is classically due to bilateral renal agenesis

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6
Q

What is a dysplasia and give one example?

A

An abnormal organisation of cells into tissues
e.g. Thanatophoric dysplasia= severe skeletal disorder characterised by extremely short limbs and folds of extra skin on the arms and legs as well as a large head and a small thorax. Caused by a single gene defect in the FGFR3 gene and carries a high recurrance in siblings and offspring of the affected person.

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7
Q

What is an association and give one example?

A

A non-random occurrence of abnormalities not explained by a syndrome. Cause is typically unknown
e.g. VATER association (Vertebral Anal Tracheal Oesophageal Renal)

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8
Q

What is a dysmorphism and give one example?

A

An unusual or abnormal physical feature (sometimes as part of a genetic syndrome)
e.g. Hypertelorism- an abnormality which results in an increased distance between two organs (e.g. eyes)

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9
Q

What risks are associated with advanced maternal age? What age is this?

A

Women over the age of 35 at the time of delivery have an increased risk of having an infant with a chromosomal abnormality.
This is classed as the advanced maternal age because at this age the risk of a chromosomal abnormality exceed the risk of a complication from amniocentesis

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10
Q

What are the risks associate with advanced paternal age?

A

Advanced paternal age is associated with higher risk of certain single gene disorders eg achondroplasia

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11
Q

Describe the human karyotype?

A

Diploid number= 22 autosomes and 1 sex chromosome from each parent

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12
Q

What are the three types of chromosome?

A

Metacentric- Two equally long arms
Submetacentric- One set of “short arms”
Acrocentric- One set of chromatids virtually non-exisent, but rather seen as “satellites”

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13
Q

What is a numerical chromosomal abnormality?

A

Aneuploidy, loss or gain (change in total number)

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14
Q

What is a structural chromosomal abnormality?

A

Translocation, deletions, insertions, inversions or rings

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15
Q

What is a mosaicism chromosomal abnormality?

A

Different cell lines

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16
Q

What is monosomy?

A

The loss of a single chromosome. It is almost always lethal

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17
Q

What is trisomy?

A

Gain of one chromosome. It can be tolerated

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18
Q

What is tetrasomy?

A

Gain of two chromosomes. It can be tolerated

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19
Q

What is a balanced translocation?

A

It is the “swap of areas” on a chromosome. They are still normal length so it is unlikely to have a significant effect.

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20
Q

What is an unbalanced translocation?

A

The swap results in the translocation result in chromosomes that are not of normal length. The total DNA on each chromosome is not equal so more likely to have disease

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21
Q

What mutation causes Down Syndrome?

A

Trisomy of chromosome 21

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22
Q

What are the clinical features of Down Syndrome?

A

Macroglossia (enlarged tongue), small ears, epicanthic folds, upward sloping palpebral fissures, Brushfield spots, single palmar crease, wide gap between first and second toes, A and V septal defects, short stature and duodenal atresia.

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23
Q

What causes trisomy 21 in Down Syndrome

A

95% of cases caused by a nondisjunction in meiosis (not splitting properly)
4% of cases caused by translocations; the breakage of acrocentric chromosomes and fusion of their long arms
1% of cases caused by mosaicism; occurs after the zygote is formed and depending on when is has occurred you can determine the proportion of affected cells in the body

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24
Q

Give two examples of common sex chromosome aneuploidies?

A

Turner’s syndrome (monosomy X)

Polysomy X in females (47, XXX)

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25
Q

Give two examples of common autosomal chromosome aneuploidies?

A 
Down syndrome (trisomy 21)
Edward's syndrome (trisomy 18)
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26
Q

What is dosage compensation with relation to chromosomes?

A

The equalisation of gene expression between the males and females of a species. Females silence the transcription of one X chromosomes and transcribe all the information from the other

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27
Q

What causes an individual to be chromosomally one gender but phenotypically another?

A

An abnormality involving the SRY gene

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28
Q

Explain what occurs to result in XX males?

A

Translocation of the SRY male determining gene from the Y chromosome to an X chromosome. Phenotypically male, testes develop, but sterile because some genes on Y chromosome needed for spematogenesis

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29
Q

Explain what occurs to result in XY females?

A

Mutations or deletions of SRY gene leads to phenotypically female who is infertile

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30
Q

What possible meiotic products can result from someone with a balanced translocation?

A

50% normal or balanced

50% unbalanced

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31
Q

Give two examples of genomic disorders?

A

Di George Syndrome

Cri du Chat Syndrome

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32
Q

What is a genomic disorder?

A

The loss of gain of genetic material

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33
Q

What is recurrent microdeletion disorder?

A

A very small deletion in a DNA sequence

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34
Q

What is a monogenic disorder, how are they passed on, are they rare?

A

They are familial, have a specific mode of inheritance and can be common or rare

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35
Q

What is a complex disorder, how are they passed on, are they rare?

A

The effects of multiple genes in combination with lifestyle and environmental factors. No clear cut pattern of inheritance. They can be sporadic. Cause of many common disorders such as type II diabetes, obesity, Parkinson’s

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36
Q

What is Mendelian inheritance?

A

The process whereby individuals inherit and transmit to their offspring one out of the two alleles present in homologous chromosomes

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37
Q

What is an allele?

A

The alternate forms of a gene or DNA sequence at the same locus

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38
Q

What are homologous chromosomes? Do they contain the same alleles?

A

They are a matching, non-identical pair of chromosomes. One is inherited from each parent. They can have different alleles

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39
Q

What is a polymorphism?

A

The occurrence of a chromosome or genetic characteristic in more than one form, which results in the coexistence of different phenotypes within a population eg different hair colours.

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40
Q

What is a mutation?

A

Heritable change in DNA sequence. This is a change in the genotype which has a definite effect on the phenotype

41
Q

What is a missense mutation?

A

A point mutation (a change in a single base pair) which codes for a different amino acid. Does not definitely affect the protein function

42
Q

What is a nonsense mutation?

A

A point mutation which results in the formation of a stop codon which leads to premature termination of the polypeptide chain; this can have a significant effect of protein function

43
Q

What is an insertion?

A

Insertion of a nucleotide can create a frame shift as the genetic code is read in triplets. Can result in a completely different protein being made

44
Q

What is a deletion?

A

Can also create a frame shift- but if 3 base pairs were deleted it may have no effect

45
Q

What is dominance?

A

The effect of an allele which will control the phenotype

46
Q

What is codominance?

A

Where both alleles are equally strong and are both visible in the genotype

47
Q

What is dominant anticipation?

A

The increase in severity and/or earlier onset of symptoms in each generation eg Huntington Disease (unstable CAG which may expand with each generation)

48
Q

What is an autosomal dominant disorder? Who does it affect, how is it transmitted, what probability will the disease be passed on to the next generation? Give one example.

A

At least one parent is affected, it does not skip generations. It is transmitted by males of females and both genders can be affected. It has verticle transmission. A child of an individual with an autosomal dominant disorder has a 50% chance of being affected
e.g. Huntington disease

49
Q

What is an autosomal recessive disorder? Who does it affect, how is it transmitted, what is the probability the disease will be passed on to the next generation? Give one example.

A

No affected parent. Transmitted by both males and females and both genders can be affected. Usually no family history. If both parents are carriers there is a 25% chance their child will have the disease, a 50% chance their child will be a carrier and a 25% chance they will be normal.
e.g. Cystic fibrosis

50
Q

What is an X-Linked disorder? Who does it affect, how is it transmitted, what is the probability the disease will be passed on to the next generation? Give one example.

A

No affected parents. Males are affected but it is transmitted by carrier females. 50% of sons are affected if the mother is a carrier and 50% of daughters will be a carrier. Caused by a mutation on the X chromosome. Majority are recessive but there is no homologous section on the Y chromosome for men.
e.g. Haemophilia A

51
Q

What is genetic heterogenity?

A

Single phenotype or genetic disorder may be caused by a multiple number of alleles or non-alleles (locus) mutations

52
Q

What is penetrance?

A

The frequency with which symptoms are present in an individual who inherits a disease causing mutation
The frequency of phenotype associated with a particular genotype

53
Q

What is variable expressivity?

A

The degree of severity in an individual who inherits a disease causing mutation

54
Q

What is a phenocopy?

A

Disease with the same phenotype as a genetic disease, but non-genetic eg Autoimmune haemophilia

55
Q

What is epistasis?

A

Interaction between disease gene mutations and other modifier genes can affect phenotype

56
Q

What is consanguinity?

A

Two individuals are related

57
Q

What is genetic imprinting? Give two examples.

A

Where genes are expressed differently according to whether they are inherited from the mother or the father.
Paternal- Prader-Willi
Maternal- Angelman
Occurring on the same chromosomal region on Chr15

58
Q

What is Ovarian Teratoma?

A

Where the genome of a child has only come from the mother

59
Q

What is Hydatidiform Mole?

A

Where the genome of a child has only come from the father

60
Q

What non-genetic factor can lead to congenital abnormalities?

A

Mitochondrial disorders (inheritance)

61
Q

What is a mitochondrial disorder? Who does it affect, how is it transmitted? Give two examples.

A

Distinct inheritance pattern. Transmission through females. Affects both males and females. Only certain cells will be affected as mitochondria are present in different numbers in different cells
e.g. MELAS (Mitochondrial myopathy, Encephalopathy, Lactic acidosis and Stroke)- progressive neurodegenerative disorder
LHON (Leber’s Hereditary Optic Neuropathy)- More common in males? Blindness

62
Q

What does the UK newborn screening programme screen for?

A 
PKU (PhenylKetonUria)
Congenital hypothyroidism
Sickle cell disorders
Cystic fibrosis
MCAD Deficiency (Medium-Chain Acyl-coA Dehydrogenase)
63
Q

What is a somatic mutation?

A

Alterations in DNA that occur after conception. Occurs in any of the body cells except the germ cells so is not passed on to children

64
Q

What is a germline mutation?

A

Any detectable and heritable variation in the lineage of germ cells. Mutations are transmitted to offspring.

65
Q

What is an oncogene? Where do they arise?

A

An oncogene is a gene that causes cancer. It arises from the mutation of a normal gene (proto-oncogene). It produces proteins (e.g. growth factors, transcription factors, tyrosine kinases) that transform a normal cell into a malignant cell.

66
Q

How does a tumour suppressor gene cause cancer?

A

Downregulation of tumour suppressor genes can cause cancer. Most TS genes require two mutations for tumorigenic effect. Hit 1 and hit 2 cause loss of transcription to cause a malignant phenotype. Some genes require only 1 hit

67
Q

What genes are responsible for an inherited predisposition to breast and ovarian cancer?

A

BRCA1 and BRCA2 (60% risk of inheriting cancer)

68
Q

What is the genetic mechanism for the increased risk of cancer associated with BRCA1 and BRCA2?

A

BRCA1 and BRCA2 repair double strand breaks in cooperation with Rad15, called homologous recombination.
Point mutations, several base pair deletions, whole exon deletions and amplifications of the BRCA1/2 gene are associated with increased risk of cancer as they result in a truncated non-functional protein

69
Q

What genes are associated with an increased risk of colorectal cancer?

A

FAP and HNPCC

70
Q

How is FAP responsible for increased risk of bowel cancer?

A

FAP (familial adenomatous polyposis) is the growth of thousands of intestinal polyps, one or more of which is likely to become cancerous. Virtually 100% risk of cancer

71
Q

What is HNPCC?

A

HNPCC (hereditary non-polyposis colorectal cancer) is commonly a mutation in MLH1 and MLH2 repair genes

72
Q

What is MYH polyposis?

A

A less common bowel cancer syndrome. It is autosomal recessive characterised by two mutated alleles of the MYH gene (mutY Homologous/MUTYH)

73
Q

What chromosome translocation is associated with chronic myeloid leukaemia?

A

Blood cells in patients contain a reciprocal translocation between chromosome 9 and 22, which leads to a foreshortened long arm of chromosome 22

74
Q

What treatment is used for chronic myeloid leukaemia?

A

Imatinib- a tyrosine kinase inhibitor (20-30% of patients are immune to Imatinib)

75
Q

Why is cytogenetics important?

A

i.e. quantify outstanding disease
Defines response to treatment and helps clinical management, absence of cytogenetic response after 12 months indicates change of therapy, change in cytogenetics predictive of survival, loss of major molecular resistance indicator of therapy resistance and imminent relapse.

76
Q

What is a pharmacogenomic marker? Give 3 examples.

A

Is the drug response based on genetic influences, showing the effects of genes on drug response
e.g. p53- deletions in chronic lymphocytic leukaemia and multiple myeloma predict for aggressive disease and may change patient management (FISH)
HER2 (ERB2) amplification in breast cancer is a marker of aggressive disease but also predicts efficacy of herceptin (FISH)
T315I mutation in chronic myeloid leukaemia predicts resistance to tyrosine kinase inhibitor therapy (direct sequencing)

77
Q

What is G-banding?

A

A technique used in cytogenetics to produce a visible karyotype by staining condensed chromosomes. Metaphase chromosomes are treated with trypsin and stained with Giemsa. Dark strands that take up the stain are A and T rich. Useful to identify differences between chromosomes

78
Q

When are routine ultrasound scans conducted during pregnancy?

A

11-14 weeks (Nuchal scan) and 20-22 weeks

79
Q

What is nuchal translucency? What does it screen for?

A

Increased thickness of fluid at the back of foetal neck. Indicates increased risk of genetic syndromes such as Down, Edwards, Patau, Turners

80
Q

What indications would require referral to Genetic Services regarding prenatal testing?

A

Abnormal findings at nuchal scan or mid-trimester scan, results of combined test which give increased risk of Down syndrome, previous pregnancy affected with condition (cystic fibrosis or down syndrome) or if parents are a carrier of chromosome rearrangement or genetic condition

81
Q

What is ultrasound used for in pregnancy?

A

Early/dating scan, nuchal translucency and nasal bone, high-level anomaly scan

82
Q

When is a foetal MRI scan used in pregnancy?

A

Usually around 20 weeks onwards

83
Q

What is HCG?

A

Human Chorionic Gonadotrophin- The main pregnancy hormone. It is made in the placenta and acts to maintain a pregnancy

84
Q

What biochemical markers would you expect to see in a maternal blood test during pregnancy? (4)

A 
Foetal AFP (α-foetalprotein) (leaks into maternal blood)
HCG (human chorionic gonadotrophin) (major pregnancy hormone)
E3 (breakdown product of active oestrogen)
PAPP (Pregnancy-associated plasma protein)
85
Q

What are the two types of minimally invasive testing conducted during pregnancy?

A

Maternal blood test

Cell-free DNA

86
Q

What are the two types of invasive testing conducted during pregnancy?

A

Chorionic villus sampling (CVS)

Amniocentesis

87
Q

In the first trimester what biochemical markers are measured in maternal serum testing?

A

hCG

PAPP

88
Q

In the second trimester what biochemical markers are measured in maternal serum testing?

A

AFP
uE3
hCG

89
Q

What is cfDNA testing? When is it performed?

A

Cell-free DNA testing tests the foetal DNA circulating in the mother’s blood. It is not accurately detected until 9 weeks into the pregnancy and is cleared within the first hour following birth

90
Q

How is cfDNA testing used to sex the pregnancy?

A

Tests the SRY gene on the Y chromosome

91
Q

What is CVS testing? When is it conducted? What are the risks?

A

Chorionic Villus Sampling is conducted at 11-14 weeks. It is transabdominal or transvaginal. There is a 1-2% chance of miscarriage. It takes part of the chorionic villi, part of the developing placenta which has the same DNA as the foetus. Allows patients to get results earlier than amniocentesis

92
Q

What is Amniocentesis? When is it conducted? What are the risks?

A

Conducted from 16 weeks onwards. It takes a sample of the amniotic fluid which contains foetal cells. Up to 1% risk of miscarriage. Needle passed through the abdomen and amniotic fluid is aspirated.

93
Q

What are the reproductive options available to couples? (6)

A 
Conceive naturally, no prenatal testing
Conceive naturally, have prenatal testing
Use of egg/sperm donors
Adoption
Choose not to have children
Pre-implantation genetic diagnosis (PGD)
94
Q

What is PGD?

A

Pre-implantation genetic diagnosis

IVF with an additional step to genetically test the embryo before implantation

95
Q

What is IVF

A

In-vitro fertilisation

Sperm and egg placed together in a culture dish

96
Q

What is ICSI

A

Intra-cytoplasmic sperm injection

Injection of a single sperm into each egg

97
Q

How can we estimate the heritability of a common complex disease?

A

Twin studies of monozygotic and dizygotic (identicle and fraternal) twins

98
Q

What is the most common single-gene cause of obesity?

A

MC4R

99
Q

How does the leptin-melanocortin pathway contribute to obesity?

A

Causes leptin deficiency which leads to hunger, obesity, no puberty, poor growth, low thyroid and immune problems

MCD Year 1 (10 decks)

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