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GSSE > Genetics > Flashcards

Genetics Flashcards

1
Q

What are the four classified genetic disorder categories

A
  1. Mendelian disorders
  2. Multifactorial disorders
  3. Chromosomal (cytogenetic) disorders
  4. Single-gene disorders with non-classic inheritance
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2
Q

Define penetrance

A

The amount of people that carry an autosomal dominant gene and expressing the trait.

Penetrance refers to the probability of a gene or trait being expressed.

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3
Q

Define variable expressivity

A

Expressivity refers to variation in phenotypic expression when an allele is penetrant

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4
Q

Define co-dominance

A

full expression of both alleles of a given gene pair in a heterozygote

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5
Q

Define polymorphism

A

multiple allelic forms of a single gene
Each varient confers increased disease risk, no single gene is necessary or sufficient to produce disease

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6
Q

Define pleiotropism

A

multiple end effects of a single mutant gene

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7
Q

Define genetic heterogeneity

A

production of a given trait by different mutations at multiple loci

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8
Q

In what way ways can our genetic code mutate

#####

A
  • loss of a whole gene

Deletions and insertions (loss or gain of a whole nucleotide base)
- Either produces a stop codon and

Mutations in non-coding sequences

Point mutation/ missense mutation
- Substitution of one nucleotide base for another
- If amino acid is biochemically similar to the original = conservative
- If amino acid is different = nonconservative = sickle cell
- If it makes a stop codon = nonsense mutation (beta thalassemia)

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9
Q

What do point mutations usually result in

A

An abnormally short protein chain

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10
Q

What causes familial hypercholesterolemia, how is it inhereted

A

mutation in the gene for a membrane receptor (chromosome 19)
Autosomal dominant

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11
Q

How are mutations in structural genes normally transmitted

A

Autosomal dominant

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12
Q

Characteristic features of autosomal dominant disorders

A
  1. in many patients, structural proteins, rather than enzymes, are affected ! so it’s a problem of STRUCTURE
  2. age of onset usually delayed, clinical features variable, so high variable expressibility, low penetrance
  3. affects both men & women, and both can transmit to their kids
  4. Risk to child of affected person is 50%
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13
Q

Examples of autosomal dominant disorders

A
  1. CNS
    - neurofibromatosis, - Huntington’s disease
  2. Urinary
    - polycystic kidney disease
  3. GIT
    - Familial polyposis coli
  4. Skeletal
    - Marfan’s syndrome
    - Elhers-Danlos syndrome - osteogenesis imperfecta
  5. Metabolic
    - familial hypercholesterolemia
  6. blood
    - von Willebrand disease
    - hereditary spherocytosis
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14
Q

Examples of autosomal recessive disorders

A

Metabolic
- cystic fibrosis, - phenylketonuria
- alpha1-antitrypsin deficiency

  1. Storage problems
    - Wilson disease,
    - Haemochromatosis
    - Glycogen storage disease
    - lysosomal storage disease
  2. endo
    - congenital adrenal hyperplasia
  3. blood
    - thalassemias
    - sickle cell anaemia
  4. CNS
    - Friedreich ataxia
    - spinal muscular atrophy
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15
Q

Characteristics of autosomal recessive disorders

A
  1. in many patients, enzyme proteins, rather than structural proteins, are affected, So the problem is now FUNCTION
  2. age of onset usually at birth or early in life, clinical features more uniform, so low variable expressibility, high penetrance

Points to note
- Affected person usually has unaffected parents, these parents are usually carriers, so usually appear normal
- all children of affected parent will be carriers
- the birth of an affected child is usually the first indication of the disease in a family
- both the parents have transmitted the disease
- affects either sex
- 25% risk if a prior child is affected
- 2/3 of healthy offspring will be carriers

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16
Q

A sickle cell trait heterozygote will have what

A
  • An abnormal Hb
  • Mild likelihood of vascular occlusions
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17
Q

Key characteristics of X linked recessive disorders

A
  1. affects almost exclusively males
  2. affected males born to unaffected parents, mother is asymptomatic carrier
  3. females may be affected if father is affected and mother is carrier, or if X-inactivation if female is skewed
  4. no male-to-male transmission
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18
Q

Key characteristics of X linked dominant disorders

A
  1. affects either sex
  2. females often more mildly and more variably affected (males hemizygous, females heterozygous)
  3. child of affected female has a 50% risk, regardless of sex
  4. for affected male, all his daughters will be affected, but non of his sons will
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19
Q

Examples of X linked recessive disorders

A

Skeletal
- Duchenne muscular dystrophy

Blood
- haemophilia A and B, chronic granulomatous disease
G6P dehydrogenase deficiency

Immune
- agammaglobulinemia

Metabolic
- diabetes insipidus

CNS
- fragile X syndrome

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20
Q

Clinical expression of a female who is heterozygous for a recessive x linked disorder

A

The disorder may be partially expressed

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21
Q

Is an extra X chromosome more or less harmful than the possession of an extra autosomal chromosome and why

A

The possession of an X chromosome in excess of the normal complement is less harmful than the possession of an extra autosome BECAUSE
Only one X chromosome in a cell is functional

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22
Q

Inheritance patterns of multifactorial inheritance

A
  1. Mutations may be present in more than one gene
  2. The risk of recurrence in subsequent pregnancies is less than 10%
  3. Identical twins will show 20-40% concordance

4.The risk of recurrence of the disorder in first degree relatives is 2-7%

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23
Q

Characteristics suggesting a disorder is inherited as a multifactorial trait

A
  1. The disease occurs more frequently in the children of an affected person than among the grandchildren
  2. The risk of developing the disease is greater if both parents are affected than if only one parent is affected
  3. The disease occurs more frequently in women than in men
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24
Q

Malignant conditions usually associated with chromosomal translocation include

A
  1. chronic myeloid leukemia
  2. Burkitt’s lymphoma
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25
Q

Outline chromosomal number changes in cytogenic disorder

A
  • Monosomy
  • Trisomy
  • Mosaicism > a/w a group of cells, some with which is normal, others express extra or missing chromosomes
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26
Q

Outline chromosomal structural changes in cytogenic disorders

A
  • Deletion > loss of a segment of a chromosome
  • Translocation > transfer of a segment of one chromosome to another
  • Isochromosome > when the short or long arm duplicate to make up for its missing partner
  • inversion
  • Ring chromosome > deletion affecting both ends, followed by fusion of the damaged ends
27
Q

Are chromosomal/karyotype abnormalities implicated in neoplasms and if so why.

A

Karyotype/chromosomal abnormalities are thought to be a primary event in development of many human neoplasms

BECAUSE
In certain types of human neoplasia, karyotype abnormality is non-random and common in that tumour type

28
Q

What are the most common types of non-random structural abnormalities in tumour cells.

A
  1. balanced translocations
  2. chromosomal deletions, and
  3. cytogenetic manifestations of gene amplification
29
Q

What is aneuploidy

A
  • It refers to a chromosome number different from the normal or euploid number
  • Aneuploid cells may have more or fewer chromosomes than normal
30
Q

Common disorders associated with aneuploidy

A
  • Most patients with Down syndrome
  • All trisomy syndrome
  • Turner’s syndrome
  • Poly-X females
31
Q

How can Aneuploidy be detected

A
  • May be detected by karyotyping
  • In tumour cells may be detected by flow cytometry
32
Q

What causes Aneuploidy

A

Non-disjunction
- Failure of homologous chromosomes to separate in anaphase 1
- Failure of sister chromatids to separate at meiosis II
- gives rise to nullisomic and disomic gametes

33
Q

Are all cells in the body affected by aneuploidy

A

Meiotic non-disjunction means an aneuploid

34
Q

Examples of trisomies and monosomy

A

Trisomies only survival for
> Sex chromosomes: XXY, XYY, XXXY, etc…
> 21 (down syndrome) 1/700 live births
> 13 (Patau syndrome) 1/5000 live births
> 18 (Edwards syndrome) 1/3000 live births

Monosomy X only
> Turner’s syndrome

35
Q

What are multigenic disorders

A

Caused by interactions between multiple variant forms of genes and environmental factors.
Occurs when several polymorphisms are present
Small effect and low penetrance

36
Q

What are 2 distincttive features of mitochondrial disorders

A
  • Matrilineal inheritance
  • Heteroplasmy (some mitochondria acquire duplications, deletions etc and others don’t. Leads to variation in expression of mutations in various genomes in the mitochondria).
37
Q

What tissues are affected by mitochondrial diosders

A

Neural
Muscular

38
Q

Inheritance pattern of mitochondrial disorders

A

Mostly autosomal recessive (in paediatrics)
Around 25% obey mitochondrial inheritance

  • Both sexes affected
  • Transmission only by females (mitochondria are maternally derived only)
  • No transmission by males
39
Q

What is incomplete penetrance

A
  • An individual carrying mutation who does not express the phenotype
  • A feature of dominantly inherited disorders
40
Q

What is variable expressivity

A
  • Variation between affected individuals in clinical manifestation
  • Most common in dominant disorders
41
Q

What are genes composed of

A

Exons: Specify the amino acids in the protein chain and also contain untranslated regulatory regions

Introns: Seperate exons

Promoters: Direct transcription

42
Q

Where can mutations occur

A

Exons
- After the coding sequence (missense - incorrect amino acid = malfunctioning protein)
- Insert a truncating codon (nonsense stop codon added)
- After spllicing (splice site mutation)
- Cause a frameshift ( insertion or deletion of nucleotide bases in numbers that are not multiples of three)

Introns
Promoters
Enhancers

43
Q

What chromosomes contain redundant DNA on their short arms

A

13,14,15, 21,22

44
Q

Is familial polyposis coli x linked

A

No

45
Q

Sensitive technique for HIV detection

A

PCR

46
Q

What does FISH enhance and what can it indentify

A
  1. enhances karyotyping
  2. can identify intragene deletion
47
Q

What can the southern blot analysis detect and what does the technique involve

A
  1. involves separation of DNA sequences according to size
  2. may be used to detect abnormalities due to point mutations
48
Q

Genetic abnormality associated with Angelman syndrome

A

Genomic imprinting

49
Q

Genetic abnormality associated with CF

A

Deletion

50
Q

Genetic abnormality associated with Osteogenesis imperfecta

A

Gonadal moscism

51
Q

Genetic abnormality associated with Beta thalassemia

A

Deletions

52
Q

Genetic abnormality associated with Fragile X

A

Triple repeat mutation

53
Q

4 categories of single gene disorders that dont follow non-classic inheritance

A

These are classified into 4 categories

  1. Triplet repeat mutations Fragile X syndrome
  2. mutations in mitochondrial genes
    Leber hereditary optic neuropathy
  3. genomic imprinting Prader-Willi and Angelman syndromes
  4. Gonadal Mosaicism
54
Q

What causes Turners syndrome

A

Non-dysjunction (failure of chromosomes to seperate)

55
Q

Does Turners exhibit mosacism

A

Yes

56
Q

Is spontaneous abortion common in Turners syndrome

A

Yes, occurs in 99%

57
Q

Clinical features of Turners syndrome

A

Neck skin folds
Lymphodema
short stature
amenorrhoea
broad chest
low hairline
increased carrying angle of arm
coarctation of aorta 20% “ hypertension
IG and life span normal

58
Q

Frequency of Turners

A

1:5000 female births

59
Q

Most common Karyoptype of Klinefelters and other possible karyotypes

A

47,XXY is most common (80% of cases)

others are mosaics, e.g. 46,XY/47XXY

60
Q

Clinical features of Klinefelters (7)

A

leading cause of male infertility

Eunuchoid body habitus

minimal or no mental retardation

failure of male secondary sexual characteristics

gynecomastia; female distribution of hair

atrophic testis with hyperplasia of Leydig cells

plasma follicle-stimulating hormone and estrogen levels elevated; testosterone levels low

61
Q

Clinical features of Trisomy 21 (6)

A

flat facies with oblique palpebral fissures and epicanthic folds

severe mental retardation IQ <50

congenital heart malformation 40%, esp. septal defect! death during infancy, otherwise lifespan is not reduced

10 to 20 fold increased risk of developing acute leukemia

serious infection resulting from abnormal immune responses

premature Alzheimer’s disease in those who survive after 35 yrs

62
Q

Is down syndrome associated with mosaiscm

A

Yes, in milder cases

63
Q

Most common cause of trisomy 21

A

95% free trisomy 21 > 47,XY (or XX) + 21

  • Maternal non-disjunction > meiosis I 65%, meiosis II 23%
  • Paternal non-disjunction > meiosis I 3%, II 5%
64
Q

Less common cause of trisomy 21

A

Translocation ####

GSSE (16 decks)

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