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1. Lecture: 3
CHROMOSOMAL DISORDERS
Dr. Noor Barraq
Medical college/ university of Mosul
Fifth grade
11/12/2023
2 p.m.

2. CHROMOSOMAL DISORDERS
Errors that occur in meiosis during the production of
gametes can lead to abnormalities of chromosome
structure or number.
Syndromes caused by chromosomal abnormalities
include:
1- trisomy 21 (Down syndrome or DS).
2-trisomy 18 (Edwards syndrome).
3- trisomy 13 (Patau syndrome ).
4-Turner syndrome (TS).
5- Klinefelter syndrome (KS).

3. Down Syndrome
DS is the most common
abnormality of chromosomal
number, occurs in 1 of every 1000
births.
This syndrome caused by:
1-Most cases (92.5%) are due to
nondisjunction.
As a result of nondisjunction, there
are three copies of chromosome
21 (trisomy 21 ).

4. 2-In (4.5%) of cases, the extra chromosome is part of a
robertsonian translocation.

5. 3- (1% to 2%) of children with DS, mosaicism occurs.
These individuals have two populations of cells: one
with trisomy 21 and one with a normal chromosome
complement .

6. Clinical features
1-Children with DS are most
likely diagnosed in the
neonatal Period, they tend to
have normal birth weight and
length, but are hypotonic with
the characteristic facial
appearance( brachycephaly,
flattened occiput, hypoplastic
midface, flattened nasal
bridge, upslanting palpebral
fissures, epicanthal folds, and
large protruding tongue).

7. 2-Infants also have short broad hands, often with a single
transverse palmar crease and a wide gap between the first and
second toes(sandal gap) with the severe hypotonia that may
cause feeding problems and decreased activity.
This video showing the examination of hypotonia in
Down syndrome.

8. Associated medical problems
1-Approximately( 50%) of children with DS have
congenital heart disease, including
(atrioventricular canal { Endocardial cushion
defect}, ventriculoseptal or atrioseptal defects,
and valvular disease).
2-(4% to 18%) of infants with DS are found to
have congenital hypothyroidism, which is
identified as part of the newborn screening
program.
Acquired hypothyroidism is a more common
problem.
Thyroid function testing must be monitored
periodically during the child’s life.

9. 3-Approximately (10%) of newborns with DS have
gastrointestinal tract anomalies( duodenal atresia,
annular pancreas, hirschsprung disease and imperforate
anus).

10. Gastrointestinal tract anomalies in Down
syndrome
Duodenal atresia with double
bubble sing

11. 4-Polycythemia at birth (hematocrit levels >70%)
is common and may require treatment.
5-Some infants with DS show a leukemoid
reaction, with markedly elevated white blood cell
counts.
Although this resembles congenital leukemia, it
is a self-limited condition, resolving on its own
over the first month of life.

12. Nonetheless, children with DS also have an
increased risk of leukemia, with a 10- to 20-fold
increase in risk compared with individuals without
DS.
In children with DS younger than 2 year of age, the
type is generally acute megakaryoblastic leukemia.
In acute megakaryoblastic
leukemia , the bone marrow
aspirate smear showing 3
megakayroblasts with
cytoplasmic pseudopod
formation and granular
basophilic area in the
cytoplasm.

13. In children with DS older than 3 years of age, the types of
leukemia are similar to those of other children, with acute
lymphoblastic leukemia being the predominant type.
In acute lymphoblastic
leukemia, the bone marrow
aspirate smear reveal increased
blasts with high nuclear to
cytoplasmic ratios and round to
irregular nuclei.

14. 6-Children with DS are more
susceptible to infection.
7-Children with DS more likely to
develop cataracts .
8-approximately (10%) have
atlantoaxial instability( an increased
distance between the first and
second cervical vertebrae that may
predispose to spinal cord injury).
9-moderate to severe learning
difficulties .

15. 10-epilepsy .
11-short stature.
12-hearing impairment due to secretory otitis media.
13-Many individuals older than 35 years of age
develop Alzheimer-like disease .

16. The recurrence risk for parents who have had a child
with DS depends on the child’s cytogenetic findings:
1- If the child has trisomy 21, the empiric recurrence risk
is 1% (added to the age-specific risk for women up to 40
years of age; after 40, the age-specific risk alone is used
for subsequent pregnancies).

17. 2- If the child has a robertsonian translocation,
chromosomal analysis of both parents must be
performed.
A: In approximately 65% of cases, the translocation is
found to have arisen de novo (i.e., spontaneously, with
both parents having normal karyotypes),
B: in 35% of cases, one parent has a balanced
translocation.
The recurrence risk depends on which parent is the
carrier: if the mother is the carrier, the risk is 10% to 15%;
if the father is the carrier, the recurrence risk is 2% to 5%.
3- If the child has a mosaicism, the recurrence risk is less
than 1% until the maternal age of 40.
After 40,the risk based on the maternal age at the time of
delivery.

18. Edwards syndrome(Trisomy 18)
Trisomy 18 is the second most
common autosomal trisomy, More than
95% of conspectuses with trisomy 18
are spontaneously aborted in the first
trimester. Trisomy 18 is usually lethal;
fewer than 10% of affected infants
survive until their first birthday, clinical
features include :
1-low birth weight.
2-hypertonia.
3- prominent occiput, Micrognathia,
low-set and malformed ears.
4- short sternum, rocker-bottom feet,
hypoplastic nails, and characteristic
clenching of fists(the second and fifth
digits overlap the third and fourth digits).
5-Congenital heart disease (e.g., VSD,
PDA, and ASD)

19. Patau syndrome (Trisomy 13)
1-low birth weight.
2-punched-out scalp lesion over the occiput called
aplasia cutis congenita; this finding is essentially
pathognomonic for the diagnosis of trisomy 13.
3-cleft lip and palate.
4-central nervous system anomalies.
5- microphthalmia or anophthalmia may occur.
6- polydactyly of the hands.
7-congenital heart disease( e.g., VSD, PDA, and
ASD).

20. Turner Syndrome( 45, X)
It is believed that the 45,X karyotype results
from a loss of either an X or a Y chromosome
after conception.
Clinical findings :
1-At birth presented with puffiness of the
hands and feet due to lymphedema .
2-characteristic facial appearance with low-
set, mildly malformed ears, triangular face,
flattened nasal bridge, and epicanthal folds.
3- webbing of the neck, with or without cystic
hygroma.
4-a shield-like chest with widened inter nipple
distance.
5-wide carrying angle (cubitus valgus) .
6-congenital heart defect ( coarctation of the
aorta and bicuspid aortic valve) .
7-Renal anomalies, including horseshoe
kidney.

21. 8-Short stature which is the cardinal feature of this
condition .
9- delayed puberty due to the presence of streak
gonads (gonadal dysgenesis) instead of well
developed ovaries leads to estrogen deficiency, which
prevents these women from developing secondary
sexual characteristics and results in amenorrhea.
Although 10% of women with TS may have normal
pubertal development and are even fertile, most
affected women require estrogen replacement
to complete secondary sexual development.
10-normal intelligence and life expectancy.
11-acquired hypothyroidism .

22. Klinefelter Syndrome
KS is the most common genetic cause of
hypogonadism and infertility in men. It is
caused by the presence of an extra X
chromosome (47,XXY).
Before puberty, boys with KS are
phenotypically indistinguishable from the
rest of the population.
The diagnosis is often made when the boy
is15 or 16 years of age.
At that point, the finding of the progressive
development of pubic and axillary hair in the
presence of testes that remain infantile in
volume should alert the clinician to the
disorder.

23. Clinical finding :
1-tall, with long limbs.
2- gynecomastia .
3-failure to develop later secondary sexual
characteristics, due to Low production of testicular
testosterone .
Because of these findings, testosterone
supplementation is indicated.
4-Most men with KS are infertile because they
produce few viable sperm.