Human Genetics: Barr Bodies, Karyotyping, and Syndromes
1. Barr Body
Definition
- Barr body is the condensed, inactive X chromosome seen in the nucleus of female somatic cells.
- It is also called sex chromatin.
Formation
- In females (46,XX), one X chromosome becomes inactive.
- The inactive X chromosome becomes highly condensed to form the Barr body.
- X-inactivation occurs early in embryonic development.
Number of Barr Bodies
Formula: Barr bodies = Number of X chromosomes − 1
- Normal female (46,XX): 1 Barr body
- Normal male (46,XY): 0
- Klinefelter (47,XXY): 1
- Turner (45,X): 0
- 47,XXX: 2
Identification
Barr bodies can be demonstrated in:
- Buccal smear
- Neutrophils (as a drumstick appendage)
Clinical Importance
- Useful for detecting sex chromosome abnormalities.
- Historically used for sex chromatin determination.
2. Karyotyping
Definition
Karyotyping is the process of arranging and studying chromosomes of a cell according to their number, size, shape, and centromere position.
Principle
Chromosomes are best studied during metaphase, when they are maximally condensed.
Steps
- Sample collection: Peripheral blood lymphocytes are commonly used.
- Cell culture: Lymphocytes are stimulated to divide.
- Metaphase arrest: Colchicine or colcemid is used.
- Hypotonic treatment: Causes cells to swell and chromosomes to spread.
- Fixation: Cells are fixed.
- Staining: Usually G-banding is used.
- Imaging: Chromosomes are photographed.
- Arrangement: Chromosomes are arranged in homologous pairs (1–22 + sex chromosomes).
Uses
- Detection of Down syndrome (trisomy 21)
- Turner syndrome (45,X)
- Klinefelter syndrome (47,XXY)
- Detection of structural chromosomal abnormalities
- Investigation of infertility and recurrent abortions
- Prenatal diagnosis
3. Klinefelter Syndrome
Definition
A sex chromosome abnormality in males, usually due to an extra X chromosome.
- Karyotype: 47,XXY
- Cause: Usually due to nondisjunction during meiosis.
Clinical Features
- Tall stature
- Long limbs
- Small, firm testes
- Hypogonadism
- Reduced testosterone
- Infertility (azoospermia)
- Sparse facial and body hair
- Gynecomastia
- Female-type body habitus
- Learning difficulties
Hormonal Findings
- ↓ Testosterone
- ↑ FSH
- ↑ LH
Treatment
- Testosterone replacement
- Management of gynecomastia
- Fertility counseling
4. Turner Syndrome
Definition
A chromosomal disorder affecting females due to complete or partial absence of one X chromosome.
- Karyotype: 45,X
- Cause: Usually due to chromosomal nondisjunction.
Clinical Features
- Short stature
- Webbed neck
- Broad/shield-shaped chest
- Widely spaced nipples
- Streak ovaries
- Primary amenorrhea
- Infertility
- Poor development of secondary sexual characteristics
- Lymphedema of hands and feet
- Congenital heart abnormalities
Hormonal Findings
- ↓ Estrogen
- ↑ FSH
- ↑ LH
5. Lyon’s Hypothesis
Definition
States that in female mammals, one of the two X chromosomes becomes functionally inactive early in embryonic development.
Main Features
- Occurs in female cells with more than one X chromosome.
- X-inactivation is early, random, and generally permanent.
- Either paternal or maternal X can be inactivated.
- The inactive X forms the Barr body.
- Provides dosage compensation between males and females.
6. X-Linked Recessive Inheritance
Definition
A condition where the abnormal gene is located on the X chromosome and its expression is recessive.
Pedigree Characteristics
- More common in males than females.
- Males are hemizygous.
- No male-to-male transmission.
- Affected male transmits the abnormal X to all daughters (carriers) and Y to all sons (unaffected).
- Carrier mother × normal father: 50% of sons affected, 50% of daughters are carriers.
- Disease may skip generations.
Examples
- Hemophilia A and B
- Duchenne muscular dystrophy
- Red-green color blindness