Chromosomal abnormalities

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Chromosomal abnormalities

Sex chromosome abnormalities

  • a significant cause of genetic disease
  • chromosome abnormalities are found in 0.6% of live births, 50% of spontaneous abortions

Numerical chromosomal abnormalities

Polyploidy

  • multiple sets of chromosomes (extra full set)
  • triploidy (3n) and tetraploidy (4n)
  • rare births, short lived

Aneuploidy

  • the condition of having an abnormal number of chromosomes (not a multiple of haploid number)
  • nullisomy - 2n - 2 (missing chromosome)
  • monosomy - 2n - 1 (one missing copy of chromosome)
  • trisomy - 2n + 1 (three copies of one chromosome)

Arises due to:

Non-disjunction

  • failure of homologous chromosome (meiosis 1) or sister chromatids (meiosis 2) to separate at anaphase

Numerical sex chromosome abnormalities

Klienfelter syndrome

  • 47, XXY karyotype (or more rarely 48, XXYY, 48 XXXY, 49 XXXXY), male
  • form of trisomy
  • 1 in 1000 male births
  • common cause of male infertility
  • tall stature
  • mildly impaired IQ
  • slightly feminised physique
  • frontal baldness absent
  • little chest hair
  • some breast development
  • testicular atrophy
  • female type pubic hair pattern
  • testosterone can improve secondary sexual characteristics

Turner syndrome

  • XO, 45 X karyotype, female
  • form of monosomy
  • 1 in 5000 female births but >90% of cases are spontaneously aborted
  • defects more prominent in puberty
  • short stature
  • characteristic facial features
  • skin fold
  • shield-shaped thorax
  • constriction of aorta
  • poor breast development
  • widely spaced *******
  • elbow deformity
  • shortened metacarpals
  • low hairline
  • small fingernails
  • rudimentary ovaries
  • no menstruation
  • brown spots
  • mainly infertile

Autosomal abnormalities

  • extra copy of chromosome
  • all autosomal monosomies and most autosomal trisomies are lethal before birth
  • only autosomal trisomies to survive to birth are Patau syndrome (chromosome 13) and Edwards syndrome (chromosome 18)

Down syndrome

  • form of trisomy
  • affects chromosome 21
  • 15 in…

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