2. Karyotype Fundamentals

Definition

A karyotype is a laboratory-produced image of an individual's chromosomes, isolated from a single cell and arranged in numerical order according to specific structural features.

Criteria for Arranging Chromosomes

1. Size / Length: Chromosomes are ordered from largest to smallest.

2. Centromere Location: Position of the primary constriction.

3. Banding Pattern: Distribution of light and dark bands after staining.

Medical Uses & Diagnostic Benefits

Sex Determination: Determines whether an individual is male (XY) or female (XX).

Numerical Abnormalities: Identifies conditions like Down syndrome (Trisomy 21) or Klinefelter syndrome (XXY).

Structural Abnormalities: Detects deletions, duplications, inversions, and translocations.

Clinical Diagnosis: Used in prenatal screening and diagnosing genetic disorders.

3. Diploid vs. Haploid Karyotypes

Feature

Diploid Cells (Somatic & Stem Cells)

Haploid Cells (Gametes: Sperm / Egg)

Chromosome Layout

Displayed in homologous pairs (1 maternal + 1 paternal per pair).

Displayed as a single set (no pairs).

Autosomes

22 pairs (numbered 1 through 22).

22 single autosomes (numbered 1 through 22).

Sex Chromosomes

1 pair (23rd pair): XX (female) or XY (male).

1 single sex chromosome: X or Y (sperm); X only (egg).

Total Chromosome Count

2n = 46 chromosomes

n = 23 chromosomes

📌 CORE RULE: Humans have 46 chromosomes organized into 23 pairs. Autosomes (pairs 1–22) govern general traits, while sex chromosomes (pair 23) determine biological sex.

4. The Karyotyping Protocol (8 Sequential Steps)

1. Blood Sampling: A blood sample is drawn, and white blood cells (WBCs) are isolated using centrifugation. (Red blood cells are excluded because they lack nuclei).

2. Culturing: WBCs are grown in a nutritive medium to stimulate growth and cell division via mitosis, making chromosomes condensed and visible.

3. Adding Colchicine: Colchicine is added to arrest mitosis at metaphase, the phase where chromosomes reach maximum condensation.

4. Hypotonic Treatment: Cells are placed in a hypotonic solution (e.g., distilled water). Water enters via osmosis, bursting the cell membrane and freeing the intact chromosomes.

5. Slide Preparation: The freed chromosomes are spread evenly onto a microscopic slide.

6. Staining: Specific dyes are applied to reveal distinct light and dark banding patterns.

7. Microphotography: The magnified chromosomal spread is observed under a microscope and photographed.

8. Cut & Arrange: Images of individual chromosomes are cut out and systematically paired/ordered by size, centromere location, and banding pattern.

5. Chromosomal Abnormalities

A. Numerical Abnormalities

Numerical anomalies typically result from nondisjunction (failure of homologous chromosomes or sister chromatids to separate during meiosis).

Trisomy: Presence of an extra chromosome in a set (3 copies instead of 2). Total chromosome count = 47.

Monosomy: Loss of one chromosome from a pair (1 copy instead of 2). Total chromosome count = 45.

B. Structural Abnormalities

Deletion: A chromosome segment is lost, resulting in missing genes and loss of genetic information.

Duplication: A chromosome segment is repeated, leading to extra copies of genes and potential gene overexpression.

Inversion: A chromosome segment breaks off, rotates 180°, and reattaches in reverse order. Gene order changes, which can disrupt function.

Translocation: A segment breaks off and attaches to a non-homologous chromosome. Can be balanced (no lost DNA) or unbalanced.

6. Karyotype Analysis & Worked Examples

Standard Method for Karyotype Analysis:

State the total autosome count / pairs.

Describe the 23rd pair (sex chromosomes: XX or XY).

Conclude the sex of the individual (male or female).

Identify any numerical or structural abnormalities present.

Case Study

Chromosomal Formula

Analysis & Conclusion

Normal Male

2n = 44 autosomes + XY

(or 46, XY)

The karyotype consists of 22 normal autosome pairs and a non-homologous XY sex pair. Belongs to a normal male with no chromosomal abnormalities.

Normal Female

2n = 44 autosomes + XX

(or 46, XX)

The karyotype consists of 22 normal autosome pairs and a homologous XX sex pair. Belongs to a normal female with no chromosomal abnormalities.

Klinefelter Syndrome

2n = 44 autosomes + XXY

(or 47, XXY)

The karyotype consists of 22 normal autosome pairs and 3 sex chromosomes (XXY). Belongs to a male with a numerical abnormality (Trisomy of sex chromosomes).

Down Syndrome (Female)

2n = 45 autosomes + XX

(or 47, XX, +21)

The karyotype consists of 22 autosome pairs with an extra chromosome 21 (45 autosomes total) and an XX sex pair. Belongs to a female with Down syndrome (Trisomy 21).

7. Nondisjunction

Definition: Nondisjunction is the failure of chromosomes to separate correctly during cell division, which produces gametes with an abnormal number of chromosomes.

Consequences after fertilization (with a normal gamete, n = 23):

Gamete n+1 (24) + normal gamete (23) → zygote with 47 chromosomes (Trisomy)

Gamete n−1 (22) + normal gamete (23) → zygote with 45 chromosomes (Monosomy)

Examples:

Down syndrome: 47, XX or XY, +21 (Trisomy 21)

Klinefelter syndrome: 47, XXY (extra sex chromosome)

Turner syndrome: 45, X (Monosomy X, a female with only one X)

📌 CORE RULE: Nondisjunction in meiosis I gives 100% abnormal gametes, while in meiosis II it gives 50% abnormal gametes. Its risk increases with advanced maternal age.

8. Critical Conceptual Deductions

📌 RED BLOOD CELLS & KARYOTYPING: Human red blood cells (RBCs) lose their nucleus as they mature to maximize room for hemoglobin. Since chromosomes are located within the nucleus, mature RBCs contain zero DNA/chromosomes and cannot be used for karyotyping.

Species Complexity: Chromosome count does NOT indicate evolutionary complexity (e.g., some simple organisms have more chromosomes than humans).

Chromosome Number vs. Identity: Two distinct species can have the same number of chromosomes. What differentiates species is the specific genetic code on the DNA.