This document summarizes several chromosomal abnormalities including Trisomy 13, 18, and 21 as well as disorders of sex chromosomes. Trisomy 13 (Patau syndrome) involves an extra chromosome 13 and has a high mortality rate in the first year with developmental delays and physical defects. Trisomy 18 (Edwards syndrome) also has a high infant mortality rate and involves physical abnormalities. Trisomy 21 (Down syndrome) has an incidence of 1 in 700 births and causes developmental delays. Disorders of sex chromosomes include Turner syndrome, which affects females missing an X chromosome, and Klinefelter syndrome, which affects males with an extra X chromosome.

1. CHROMOSOMAL
ABNORMALITIES
Nsikak-Abasi David

2. Trisomy 13
• Referred to as Patau syndrome
• All cells in the body contain an extra genetic material from
chromosome 13
• Incidence is 1/20000
• 90% are dead in the 1st year
• Common signs includes: severe developmental retardation, midline
brain defect, malformed ear, micropthalmos, and coloboma, scalp
defect

5. Trisomy 18
• Referred to as Edwards Syndrome
• Overlaps with Trisomy 13 and the incidence rate is 1/8000
• >90% dead in the 1st year
• Common presentations include: small face with prominent occiput,
small sternum and pelvis, flexion deformity of the finger, VSD and
horseshoe kidney, severe mental retardation.

8. Trisomy 21
• Referred to as Down Syndrome
• Incidence is 1 in 700
• 95% have trisomy 21 , chromosome no 47
• 1% cases are mosaics, mitotic nondisjunction
• 4% cases- extra chromosomal material derives from presence of
robertsonian translocation of long arm of chromosome 21 to
acrocentric chromosome
• 10*20 fold increased risk of developing leukemia

12. Disorders affecting Sex Chromosomes
• More common than autosomal aberrations
• Lyon Hypothesis plays a big role here:
“doseage compensation in mammals is by inactivation of all but one X
chromosome in cells with more than one X chromosome”

13. Turner Syndrome
• Complete or partial monosomy of X chromosome
• Referred to as 45,X. or monosomy X or TS
• Occurs in 1/2000 live born females
• 57% missing an entire X chromosome – 45,X Karyotype
• 14% have structural abnormalities of X chromosomes
• 29% are mosaics. Mosaic patterns includes:
- 45,X/46.XX
- 45,X/46.XY
- 45,X/47.XXX

16. Klinefelter Syndrome
• Referred to as 47,XXY or XXY,
• is the set of symptoms that result from two or more X chromosomes
in males
• Occurs in 1/660 live male births
• 15% cases are mosaic
• Treating adolescents with implants of controlled-release testosterone
has shown good results when appropriately monitored