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Genetics Flashcards

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1
Q

What are chromosomes?

A
  • are structures found within the cell nucleus
  • composed of
    • DNA
    • RNA
    • polysaccharides
    • histone
    • non histone proteins
  • ​thin thread like structures that have a
    • ​short p arm
    • long q arm
    • separated by a centromere
  • ​only seen under light microscope during cell division when condensed x1000 magnifcation
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2
Q

How many chromsomes does the normal human have?

A
  • karotype of normal human is 46 chromosomes= diploid number
  • this includes 22 autosomal pairs and **2 sex **chomosomes
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3
Q

What are the chromsomes basic shape?

A
  • All depend on centromere location
  • Metacentric
    • chromosomes have p and q arms of approx **equal length **
    • e.g. chromosome 1
  • Submetacentric
    • Chromosomes have a p and q arms of different length
    • e.g. chromosome 6
  • Acrocentric
    • centromere is near one end
    • so very short p and q long
    • e.g chromosome 13
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4
Q

What is DNA?

A
  • Polymer of deoxyribonucleotides
  • composed of
    • nitrogenous base
      • ​purine or pyridamine
      • contain genetic information
    • a sugar
    • phosphate group
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5
Q

What are the purine bases?

A
  • Adenine
  • Guanine
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6
Q

What are the pyrimidine bases?

A
  • Thymine
  • Cytosine
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7
Q

What is the structure of DNA?

A
  • Watson and Crick deduced the Double helical structure of DNA with paired bases at the centre
  • Hydrogen bonds between the bases hold them together
  • Adenine always paired w Thyamine = ‘AT’
  • Guanine always paired with Cytosine = “GC”
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8
Q

What process allow somatic cells to grow and divide?

A
  • Mitosis
  • replication of DNA involves unwinding of DNA
  • allowing a complementary DNA daughter strand to be formed using the original strand as a template
  • semi conserative process as each new helix contains one of the original chromosomal strands
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9
Q

What do genes code for?

A
  • Protein synthesis
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10
Q

What happens in transcription?

A
  • Templates are produced from the DNA in the form of messenger ribonucleic acid ( mRNA)
  • the mRNA exits nucleus into the cellular cytoplasm
  • By combining with other RNA moelcules such as tRNA ( transfer RNA) and ribosomal RNA ( rRNA) protein synthesis takes place at the ribosomes
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11
Q

What is the role of proteins?

A
  • Mechanical tissue strength seen in skeletal tissue due to collagen
  • muscle contraction occurs with interaction of actin and myosin protein filaments
  • proteins act as transport molecules and enzymes during biological reactions
  • immune system in the form of antibodies
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12
Q

What is the process of meisosis?

A
  • in sexual reproduction the chromosomes separate to form a haploid number, including one copy of each autosome and a single sex chromosome
  • a child inherits one half of his or her autosomal pair and one sex chromosome from each parent
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13
Q

Name some whole chromosome abnormalities?

A
  • Monosomy = Turner’s syndrome
  • Trisomy= Down’s syndrome/ Kleinefelter
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14
Q

What is the name given to loss or gain of whole chromosomes?

A
  • Aneuploidy
  • can effect autosomes or sex chromosomes
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15
Q

What is Turner’s syndrome?

A
  • Loss of one X chromosome
    • ie XO ( paternal X chromsome absent- sporadic event)
  • incidence 1 in 2000 live births
  • exclusively females
  • short female
  • skeletal abnormalities
    • cubitus valgus
    • medial tibial exostosis
    • short 4th MC/ MT
  • cranoifacial
  • renal tract
  • cardiovascular anomalies
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16
Q

What is trisomy ?

A
  • Cells have an extra chromosome
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17
Q

What is down’s syndrome?

A
  • Trisomy 21 ( extrachromosome 21)
  • Incidence 1 in 660 live births
  • Risk assoc with increase maternal age > 35 yrs
  • 95% aetiology is failure of separation of the autosomal pair during meisosis
  • 3.5 % due to translocation of one autosomes
  • 2.5 % due to mosaicism (where some cells have 47 chromosomes)
  • orthopaedic manifestations
    • atlantoaxial instability
    • joint laxity with recurrent dislocation of patella
    • increased risk of SUFE
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18
Q

What is Kleinfelter’s syndrome?

A
  • An addition of an extra X chromsome
  • so XXY
  • incidence is 1 in 1000 male births
  • Tall , thin male with infertility and hypogonadism
19
Q

What is the genotype ?

A
  • The inherited genetic code that produces the **physical appearance= ** phenotype
20
Q

What is an allele?

A
  • One or two alternative forms of the gene that can have the same locus on the homologous chromosome and are responsible for alternative traits
  • if alleles similar = homozygous trait
  • if alleles different = heterpzygous trait
21
Q

Describe how the structure abnormalites of chromosomes can occur?

A
  • Point mutations
    • specific nucleotide bases
  • Deletions
    • loss of part of chromosome
  • Inversions
    • inversion of chromosome by 180 degrees-> misreading of genetic code
  • Translocations
    • movement of genetic material from one chromosome to naother
22
Q

What is penetrance?

A
  • relates to the probability that the phenotype will be expressed
23
Q

What is incomplete penetrance?

A
  • defined as inheritance of the mutant gene without expression of the phenotype of the disorder
24
Q

What is variable expressivity?

A
  • In which a patient always expresses some of the symptoms of the mutant gene
25
Q

What is autosomal dominant inheritance?

A
  • Affected individuals are heterozygous for the mutation
  • homozygous individuals often don’t survive
  • 50% of inheritance from one heterozygous parent
  • M= F affected
26
Q

Name some examples of autosomal dominant inheritance?

A
  • Achondroplasia
  • Osteogenesis imperfecta
  • Neurofibromatosis type 1
  • Marfan’s syndrome
  • multiple epiphyseal dysplasia
  • mutliple hereditary exostoses
27
Q

What is achondroplasia?

A
  • Most common form of disporportionate rhizomelic dwarfism ( short limb)
  • inherited as complete penetrance
  • 87% result of new mutation
  • gene encoding on chromosome 4, p16.3
  • mutation affects Fibroblast growth factor receptor 3 ( FGFR3)
28
Q

What is Osteogenesis imperfecta?

A
  • A generalised disease of connective tissue due to a quantitative or qualitative defect of type 1 collagen
  • Sillence classified
    • 1-IV
    • with subgroups in 1 and IV of A /B
    • where B = dentinogenesis imperfecta
  • ​Autosomal dominant condition- cases from unaffected parents is mosaicism
  • pro-alpha 1 chain is encoded by COL1A1 gene on chromosome 17 and the pro-alpha 2 chain is encoded by COL1A2 gene on chromosome 7
29
Q

What is neurofibromatosis inherited as?

A
  • Autosomal dominance
  • Complete penetrance but high variable expressivity
  • 50% of pts have a fresh gene mutation
  • gene encoded on chromosome 17 locus q11.2
  • the gene encodes the protein Neurofibromin which acts as a tumour suppressor
30
Q

What is the defect in marfan’s syndrome?

A
  • Mutation in fibrillin 1 gene
  • connective tissue disorder
  • arachnodactyly
  • scoliosis (50%)
  • protrusio acetabuli (15-25%)
  • ligamentous laxity
  • recurrent dislocations (patella, shoulder, fingers)
  • pes planovalgus

nonorthopaedic conditions

  • cardiac abnormalities
  • aortic root dilatation
  • possible aortic dissection in future
  • mitral valve prolapse
  • superior lens dislocations (60%)
  • pectus excavatum
  • spontaneous pneumonthoraces
  • dural ectasia (>60%)
  • meningocele
31
Q

What is the genetic mutation in mutliple hereditary exostoses?

A
  • EXT 1 and EXt2 gene
  • autosomal dominant
  • individuals with the EXT1 mutation have a more severe presentation compared to patients with the EXT2 mutation including
    • higher rate of chondrosarcoma 5-10% of transformation, proximal lesions more likely to undergo transformation than distal ones
    • more exostoses
    • more limb malalignment with less forearm and knee range of motion
    • more pelvic and flatbone involvement
32
Q

What is autosomal recessive inheritance?

A
  • Affected individuals are homozygous for the genetic mutation
  • Heterozgyous individuals are carriers
  • 25% of producing an affected individual from 2 parent carriers
  • M= F affected
  • autosomal recessive conditions often produce errors of metabolism due to deficiency of specific enzymes
33
Q

Can you name some examples of autosomal recessive conditions?

A
  • Mucopolysaccharidoses ( except Hunters)
  • Sickle cell anaemia
  • Diastrophic dysplasia
34
Q

What is type 1 mucopolysaccharidosis?

A
  • Hurler syndrome
    • mental retardation
    • disporportinate dwarfism
    • diaphyseal bordening of short misshapen boness
    • flaring of rib cage
    • widening medial end of clavicle
    • anterior wedging of verterbral bodies-> kyphosis/ thoracolumbar gibbus
    • odontoid hypoplasia
    • cloudy cornea
    • caused by alpha 1 iduronidase deficiency
    • Dermatan sulphate in urine
    • death in first decade of life
35
Q

What is type IV mucopolysaccharidosis?

A
  • Morquio
    • Type A= deficiency in galactosamine 6 sulphatase
      • chromosome 16 q24.3
    • Type B= deficiency in beta -galactosidases
      • ​chromosome 3 p21.33
  • Proportinate dwarfism
  • Normal mental IQ
  • both need enzyme to break down glycoaminoglycans- esp keratan sulphate interferes with cartilage at growth plate
  • Keratan sulphate in urine
  • type A more severe
  • both live into adulthood
  • skeletal
    • platyspondyly​
    • ovoid vertebra & odontoid hypoplasia
    • flaring of rib cage
    • flattening of femoral heads
    • Genu valgum
    • Joint laxity
36
Q

What other form of mucoploysaccharidosis is inherited by autosomal recessive?

A

San filipio

  • Multiple enzyme defiencies
  • Proportinate dwarfism
  • Mental retardation
  • clear cornea
  • Urine- Heparan sulphate in urine
  • death in 2nd decade of life
37
Q

What is the inhertance of sickle cell disease?

A
  • Autosomal recessive of single gene leading to the subsitution of glutamine for valine at position 6 of the beta-globulin chain of haemaglobin
38
Q

What is the genetic defect in disatrophic dysplasia?

A
  • Sulphate transporter
  • DTDST gene (SLC26A2) on chromosome 5
  • failure of formation of the secondary centres of ossification ( epiphysis)
  • rhizomeliz dwarfism
  • hitcher’s thumb
  • cauliflower ears
  • autosomal recessive
39
Q

What is the genetic defect in multiple epiphyseal dysplasia?

A
  • Cartilage oligomeric matrix protein (COMP) or type IX collagen ( **COL9A2) **
  • causes type 2 collagen dysfunction as type IX collagen acts as a link protein for type 2 collagen
  • autosomal dominant
  • A form of dwarfism ( short limbed) characterized by irregular, delayed ossification at multiple epiphyses
40
Q

What is x linked dominant inheritance?

A
  • Either sex affected
  • phenotype is dominant when a heterozygous female expresses the phenotype
  • 50% sons and 50% daughters risk inheritance from an affected mother
41
Q

Name an example of x linked dominant inheritance?

A
  • Hypophosphataemic rickets
    • males express disease fully but females have expressivity in the heterozygous genotype
    • PEX ( cellular endopeptidases)
42
Q

What is X linked recessive inheritance?

A
  • Phenotype is recessive when expressed by a homozygous female
  • All males are affected
  • Heterozygous females are carriers
  • 25% of producing a normal male/female
  • an affected male transmits the affected gene thru all his daughters
43
Q

Name some x linked recessive disorders?

A
  • Duchenne muscular dystrophy
    • deficiency of protein **dystrophin **
  • Haemophilia A
    • deficiency in clotting factor VIII
  • ​Hunter’s syndrome - mucopolysaccharidosis
    • ​sulpho-iduronate-sulphatase deficiency
    • -> accumulation of dermatan and sulphate in urine
    • proportionate dwardfism
    • mental retardation
    • clear cornea
44
Q

What is the genetic mutation in spondyloepiphyseal dysplasia?

A
  • caused by abnormal synthesis of Type II collagen
  • **A form of short-trunk dwarfism caused by a defect in the secondary ossification center (epiphysis) **
  • **mutation = COL2A1 on chromosome 12 **
  • inheritance
    * **autosomal dominant (SED congenita)
 X linked recessive (SED tarda)
 random mutation (50% of cases)**

Basic Science (31 decks)

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