Numerical Chromosomal Abnormalities

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  1. Turner Syndrome
    Frequency ~1/5000 female live births

    Newborns are fully viable, but 99% of 45,X spontaniously aborted

    • ~50% of cases involve 45,X:
    • • The monosomy is usually due to a paternal meiotic error (X present is from the mother)
    • • Remaining cases also lack chromosomal material from one X
  2. Down Syndrome
    Frequency: 1/800 live births (but majority spontaneously abort)

    • ~95% of cases involve Trisomy 21 (47,XY,+21 or 47,XX,+21)
    • • Usually due to non-disjunction in maternal meiosis
    • • Associated with AMA

    • Remaining cases are not related to
    • AMA and may have higher
    • recurrence rates for future pregnancies: • Rob translocation involving 21q [e.g., der(14;21)]
    • • i(21q)
    • • Trisomy for only part of 21q
    • • Must karyotype affected child to estimate recurrence risk
  3. Trisomy 13 (Patau Syndrome)
    Incidence: 1:20,000- 1:25,000

    • Phenotype:
    • Congenital heart defects,
    • holoprosencephaly, micropthalmia,
    • cleft lip/plate, polydactyly, urogenital
    • defects, clenched fists, rocker-bottom
    • feet, severe MR, growth retardation
    • (50% survive < 1 mo)

    About 1/5 cases involve a translocation of chromosome 13
  4. Trisomy 18 (Edward Syndrome)
    Incidence: 1:6000- 1:8000

    • Phenotype:
    • Congenital heart defects, failure to
    • thrive, malformed & low-set ears,
    • prominent occiput, receding jaw, short sternum, characteristic clenched fist
    • (2nd finger overlaps 3rd, 5th overlaps 4th finger), rockerbottom
    • feet, MR (few survive to 1 yr)

    About 1/5 of cases involve a translocation of chromosome 18
  5. Complete Hydatidiform Mole
    • Complete moles are disorders of imprinting!
    • Phenotype results because gene expression from the maternal and
    • paternal genomes is not equivalent!
    • (46,XX 90%)

    No fetal tissue present

    • Endoreduplication of haploid sperm after fertilizing an ovum lacking
    • a nucleus (only paternal
    • genome present)

    Risk of progression to choriocarcinoma
  6. Partial Hydatidiform Mole
    Triploidy: 69 XXX or 69,XXY

    Fetal tissue present

    • Fertilization of a haploid ovum and duplication of the paternal haploid
    • chromosomes or from dispermy

    No risk of progression to choriocarcinoma
Card Set
Numerical Chromosomal Abnormalities
cytogenetics, molecular, numerical abnormality syndromes
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