liveonearth (liveonearth) wrote,

Genetics (week 2 part 1) Trisomy 21 and other Xsome ABN

*designates something to know for the test

OMIM 190685
better known as Down's Syndrome, aka "mongoloid idiocy"
some Down's synd pts have other trisomy
47,XX,+21 or 47,XY,+21
INCIDENCE: most common trisomy, 1 in 800 live births
higher incidence with older mothers, in utero survival is minimal
est: 25% of all miscarriages dt trisomy 21
350,000 living people with it in US
accounts for ~33% of all moderate and severe mental retardation in children
abstract reasoning in 3-5yo range, performs poorly on IQ exams
maternal age effect: older mom-->more chromosomal defects
more risk if already had one down's syndrome child
recommend amniocentesis over age 35 because then rate of fetal loss is greater from dz than from procedure
for younger woman: can screen in utero less invasively via alpha fetoprotein
SX: distinctive facies*, oblique palpebral fissures*, epicanthic folds, low nasal bridge, protruding tongue, small round head with flat occiput, low set ears, speckling iris, short stature/hands, broad neck, congenital heart dz (40-50%, serious in fewer who often die young), "simian" line in dermatoglyphics (single palmar crease, not double), premature aging, high incidence of leukemia (10-30x normal risk, about 2% of all child leuk), alz in almost 25% esp early onset
NEURO SX: neonatal hypotonia, mental retardation (IQ 20-50), hearing/visual impairment in >50%
TX: needs intensive care, nurturance
customary practice to mainstream these kids
PROG: can live relatively normal life, expectancy is about 55 years but variable

very serious, fatal, dies w/in 6 mo, at most lasts a few years
Edwards syndrome
live birth incidence 1/7,000
2nd mc trisomy
severe mental retardation, seizures, microcephaly, heart defects, growth retardation, GI probss
death ww/in 6 mo

Patau's syndrome
live birth incidence 1/5,000
like Ed's synd plus others: low ears, cleft palat, polydactyly, death within a few mo


aneuploidy (classic trisomies and monosomies)
arise from failures during meiotic chromosome separation
*non-disjunction during anaphase II or III

4 other major kinds of xsome probs
deletion, duplication, inversion, translocation


Prader-willi & Angelman syndrome
both very uncommon
spontaneous *deletion from xsome *15
15q11 to 15q13 (70% of cases)

Prader-Willi when deletion on paternal homolog
-->mental retardation, learning disability, hypotonia, short, incomplete sexual development,
feeding difficulties as infants followed by serious over-eating and obesity

Angelman syndrome
when deletion on maternal homolog
aka "happy puppet syndrome", smiles plenty, funny dance
-->appears normal at birth, then developmental delay, feeding probs, serious motor disturbances,
seizures, mental retardation/learning disabilities

Cri Du Chat syndrome
rare *deletion on xsome *5p14-15
"cat's cry syndrome"
born with abnormal larynx, really does sound like a cat when they are crying as infants
goes away in a few weeks

skipping duplications
not enough diseases to worry about
skipping inversions

swapping bits
mb normal or not

Translocation Down's Syndrome
%5 of Down's cases, identical to trisomy 21 except in karyotype
usu a 14/21 translocation
lots of monosomy 21 and translocation downs cases die in utero, some are "carriers" of transloc genes
risk of Down's is 15% of mother is carrier, 5% if father is carrier
much higher than the trisomy 21 risk

Chronic Myelogenous Leukemia (*t9:22)
dt *transloc btw *9 and *22 to form the Philadelphia chromosome: t(9:22)(q23:q11)
**created leukemia tag, previous entries with leukemia may be located by using the "blood" tag
elevated WBC's, malaise, low grade fever, incr susceptibility to infx

*~15% of recognized pregs abort, at least 50% are chromosomally abn
(mc abort: in utero trisomy 16-->100,000 miscarriages/year)
*1/2 of these miscarriages are conceptuses w/ apparently normal karyotype
*xsome defects are common, 1/200 live births overall
(0.6% of live births are chromosomally abn)
*incidence of xsome dz is fx of conceptus production rate and likelihood of in utero survival
*sig xsome disturbances generally not compatible with life
*repeat miscarriages suggestive of xsome prob in one or both partners

*see sample questions on page 23 of paper notes
Tags: fertility, genetics, leukemia, nd4, parenting, pregnancy

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