SEX CHROMOSOME

🔬 What is sex chromatin?

Sex chromatin is condensed chromatin observed in interphase nuclei, differing in males and females due to sex chromosome composition. It includes:
X chromatin (Barr body) in females — inactive X chromosome
Y chromatin (F body) in males — constitutive heterochromatin of Y's long arm

📍 What is a Barr body?

A Barr body is an inactivated X chromosome in female cells, seen as a dense chromatin mass near the nuclear membrane. It forms due to dosage compensation via facultative heterochromatin.

🧬 What is the Lyon Hypothesis?

It states that in somatic cells of female mammals, only one X chromosome remains active while the other is inactivated early in embryogenesis → ensures dosage compensation.

📊 How is the number of sex chromatin calculated?

Number of Barr bodies = Number of X chromosomes - 1

👩‍⚕ What does "chromatin-positive" mean?

"Chromatin-positive" = Barr body is visible
"Chromatin-negative" = Barr body not visible

🧪 Which sex-linked gene escapes dosage compensation?

Genes that escape inactivation:
Pseudoautosomal region genes
Xg blood group gene
Steroid sulfatase gene (deficient in X-linked ichthyosis).

🧬 What are the 3 main genetic consequences of X-inactivation?

1. Dosage compensation
2. Variable expression in heterozygous females
3. Mosaicism: females have cells with either maternal or paternal X active.

📉 Conditions that reduce Barr body detection?

First 2 weeks after birth
Antibiotic treatment
Malignancy (before chemotherapy)
Metabolic dysfunctions

🧬 What happens in selective X-inactivation?

Structural abnormal X → that X is preferentially inactivated (if XIC intact)
X-autosome translocation → normal X is inactivated to avoid silencing autosomal genes

🧪 Steps for sex chromatin test on blood smear?

1. Make smear
2. Fix with methanol (5 mins)
3. Stain with Giemsa (3 drops/1 mL water, 25–30 mins)
4. Wash, dry
5. Count 400 neutrophils; ≥6 drumsticks → female

Steps for sex chromatin test using buccal smear?

1. Scrape cheek lining
2. Fix in 95% ethanol
3. Air dry
4. Stain with 1–2 drops aceto-orcein
5. Score 100 cells under microscope

Indications for Barr body test?

Ambiguous genitalia
Delayed puberty
Suspected Turner or Klinefelter syndrome
Forensic sex determination
Screening before karyotyping

What is Y chromatin (F body)?

Represents constitutive heterochromatin on long arm of Y
Seen with quinacrine stain under UV light
Appears as fluorescent body
1 F body = 1 Y chromosome
Females = 0 F bodies

🧬 Klinefelter syndrome (47,XXY): Key features?

Male with 1 Barr body
Underdeveloped testes, prostate
Tall stature, long limbs
May have learning difficulties
Considered chromatin-positive

Turner syndrome (45,X): Key features?

Female with no Barr body
Webbed neck, short stature, broad chest
Underdeveloped ovaries, uterus
No puberty or menstruation
Normal intelligence