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No significant effect of monosomy for distal 21q22.3 on the Down syndrome phenotype in "mirror" duplications of chromosome 21.
[monosomy 21]
Three
Down
syndrome
patients
for
whom
karyotypic
analysis
showed
a
"
mirror
"
(
reverse
tandem
)
duplication
of
chromosome
21
were
studied
by
phenotypic
,
cytogenetic
,
and
molecular
methods
.
On
high
-resolution
R-
banding
analysis
performed
in
two
cases
,
the
size
of
the
fusion
21
q
22
.
3
band
was
apparently
less
than
twice
the
size
of
the
normal
21
q
22
.
3
,
suggesting
a
partial
deletion
of
distal
21
q
.
The
evaluation
of
eight
chromosome
21
single
-copy
sequences
of
the
21
q
22
region--namely
,
SOD
1
,
D
21
S
15
,
D
21
S
42
,
CRYA
1
,
PFKL
,
CD
18
,
COL
6
A
1
,
and
S
100
B
--
by
a
slot
blot
method
showed
in
all
three
cases
a
partial
deletion
of
21
q
22
.
3
and
partial
monosomy
.
The
translocation
breakpoints
were
different
in
each
patient
,
and
in
two
cases
the
rearranged
chromosome
was
found
to
be
asymmetrical
.
The
molecular
definition
of
the
monosomy
21
in
each
patient
was
,
respectively
,
COL
6
A
1
-
S
100
B
,
CD
18
-
S
100
B
,
and
PFKL
-
S
100
B
.
DNA
polymorphism
analysis
indicated
in
all
cases
a
homozygosity
of
the
duplicated
material
.
The
duplicated
region
was
maternal
in
two
patients
and
paternal
in
one
patient
.
These
data
suggest
that
the
reverse
tandem
chromosomes
did
not
result
from
a
telomeric
fusion
between
chromosomes
21
but
from
a
translocation
between
sister
chromatids
.
The
phenotypes
of
these
patients
did
not
differ
significantly
from
that
of
individuals
with
full
trisomy
21
,
except
in
one
case
with
large
ears
with
an
unfolded
helix
.
The
fact
that
monosomy
of
distal
21
q
22
.
3
in
these
patients
resulted
in
a
phenotype
very
similar
to
Down
syndrome
suggests
that
the
duplication
of
the
genes
located
in
this
part
of
chromosome
21
is
not
necessary
for
the
pathogenesis
of
the
Down
syndrome
features
observed
in
these
patients
,
including
most
of
the
facial
and
hand
features
,
muscular
hypotonia
,
cardiopathy
of
the
Fallot
tetralogy
type
,
and
part
of
the
mental
retardation
.
Diseases
Validation
Diseases presenting
"hand features"
symptom
monosomy 21
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