Human Anatomy
Chromosomes
Chromosome anatomy for MBBS: structure, classification, karyotyping, Barr body and the Lyon hypothesis of X inactivation, mapped to NMC codes AN73.1 to AN73.3.
MedNext Academy | 3 min read
Clinically reviewed by Dr Shameer Deen, MBBS, MS, MRCS
Chromosomes
Chromosome anatomy for MBBS: structure, classification, karyotyping, Barr body and the Lyon hypothesis of X inactivation, mapped to NMC codes AN73.1 to AN73.3.
This chapter describes the structure and classification of human chromosomes and the technique of karyotyping. It explains the Lyon hypothesis of X inactivation and the basis of the Barr body.
High-yield: Chromosomes
- A normal human somatic cell contains 46 chromosomes, arranged as 22 pairs of autosomes and one pair of sex chromosomes.
- Females are 46,XX and males are 46,XY, so the Y chromosome determines the male sex.
- Each chromosome has a short p arm and a long q arm joined at the centromere.
- Chromosomes are classified by centromere position as metacentric, submetacentric and acrocentric.
- Acrocentric chromosomes carry satellites on their short arms bearing the genes for ribosomal RNA.
- Karyotyping arranges the stained chromosomes in order of size to detect numerical and structural abnormalities.
- Cells for karyotyping are arrested at metaphase, when the chromosomes are most condensed and visible.
- Giemsa or G banding produces a reproducible pattern of light and dark bands that identifies each chromosome.
- Barr body is the inactivated X chromosome seen as a dark mass at the nuclear margin in female cells.
- The number of Barr bodies equals the number of X chromosomes minus one.
- The Lyon hypothesis states that one X chromosome is randomly inactivated in each female somatic cell early in development.
- X inactivation makes females mosaics for X-linked genes and explains dosage compensation between the sexes.
- Prenatal karyotyping can be performed on cells from amniocentesis or chorionic villus sampling.
- Fluorescence in situ hybridisation uses labelled probes to detect specific chromosome regions.
- Chromosomal analysis confirms conditions such as Down syndrome, Turner syndrome and Klinefelter syndrome.
Chromosome facts at a glance
- Human chromosome number: 46 in somatic cells: 22 pairs of autosomes plus XX in females or XY in males.
- Classification by centromere: Metacentric, submetacentric and acrocentric. Acrocentric chromosomes carry satellites.
- Karyotyping: Chromosomes arrested at metaphase, banded with Giemsa and arranged by size.
- Lyon hypothesis: One X randomly inactivated per female cell. Barr bodies equal X chromosomes minus one.
NMC competencies in this chapter
- AN73.1: Structure and classification of chromosomes
- AN73.2: Technique of karyotyping and its applications
- AN73.3: The Lyon hypothesis
Frequently Asked Questions
How many chromosomes does a human cell have?
A normal somatic cell has 46 chromosomes: 22 pairs of autosomes and one pair of sex chromosomes, which are XX in females and XY in males.
What is a Barr body?
It is the inactivated X chromosome seen as a dense mass at the edge of the nucleus. The number of Barr bodies equals the number of X chromosomes minus one.
What does the Lyon hypothesis state?
It states that one X chromosome in each female somatic cell is randomly and permanently inactivated early in development, providing dosage compensation and making females mosaics for X-linked genes.
Why are chromosomes studied at metaphase?
At metaphase the chromosomes are maximally condensed and clearly separated, so they can be stained, banded and arranged into a karyotype for analysis.
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