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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
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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
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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
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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
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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
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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
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