Table of Contents
Chromosomal Basis of Dosage Compensation
In humans and many other mammals, females have two X chromosomes (XX) and males have one X and one Y chromosome (XY). Because the X chromosome carries many genes essential for basic cell function, there is a potential problem: without some kind of regulation, females would have roughly twice the “dose” of X‑linked gene products as males.
The Barr body and the Lyon hypothesis describe how mammals solve this problem: by inactivating (silencing) one of the two X chromosomes in each somatic cell of the female. This process is called X‑inactivation and is a form of dosage compensation.
Discovery of the Barr Body
Morphological Observation
In the 1940s, Murray Barr and Ewart Bertram examined nerve cells from male and female cats. They noticed that cells from females contained a small, darkly staining mass at the inner edge of the cell nucleus, visible with a light microscope after special staining. This dense structure was absent in male cells.
This structure was later called the Barr body (or sex chromatin). Cytogenetically, a Barr body is:
- A highly condensed, transcriptionally largely inactive X chromosome.
- Located at the nuclear periphery.
- Visible in interphase nuclei (i.e., when chromosomes are not individually condensed as in mitosis).
In normal human cells:
- Typical female karyotype (46,XX) → 1 Barr body per somatic cell.
- Typical male karyotype (46,XY) → 0 Barr bodies per somatic cell.
Use of Barr Body Counting
Before the routine use of chromosomal banding and DNA‑based tests, counting Barr bodies was used as a quick indirect method to:
- Distinguish genetic males from genetic females.
- Screen for some sex chromosome abnormalities (e.g., extra X chromosomes).
A practical rule that emerged:
- Number of Barr bodies = (Number of X chromosomes) − 1
Examples:
- 46,XX female → 1 Barr body.
- 46,XY male → 0 Barr bodies.
- 47,XXX female → 2 Barr bodies.
- 47,XXY male (Klinefelter syndrome) → 1 Barr body.
- 45,X female (Turner syndrome) → 0 Barr bodies.
This rule reflects the inactivation of all but one X chromosome in each somatic cell.
Lyon Hypothesis: Concept of X-Inactivation
Mary Lyon’s Proposal
In 1961, geneticist Mary Lyon proposed a model to explain the Barr body and patterns of X‑linked inheritance in mammals, especially in mice. This model is known as the Lyon hypothesis or Lyonization.
Key points of the Lyon hypothesis:
- Early in embryonic development (in humans, around the blastocyst stage), in each somatic cell of a female, one of the two X chromosomes is inactivated.
- The choice of which X (maternal or paternal) is inactivated in any particular cell is random.
- After this choice is made, the inactivated state is stably transmitted to all daughter cells derived from that cell (mitotic inheritance).
- The inactivated X chromosome becomes highly condensed and forms the Barr body.
This process ensures that:
- Females (XX) and males (XY) have the same effective dose of most X‑linked genes: one functional X chromosome per somatic cell.
Mosaicism in Females
Because the choice of X to inactivate is random and occurs independently in many embryonic cells, female mammals become genetic mosaics with respect to X‑linked genes:
- Some cell lineages express genes from the maternal X (paternal X inactivated).
- Others express genes from the paternal X (maternal X inactivated).
Thus, adult females consist of patches (clones) of cells with different active X chromosomes.
A classic visible example (especially in cats) is explained below.
Classic Example: Calico and Tortoiseshell Cats
Female cats with two different alleles for a fur color gene on the X chromosome can exhibit patchy coat coloration:
- Suppose allele
X^B→ black fur, and alleleX^O→ orange fur. - A female cat with genotype
X^B X^Ohas both alleles, one on each X chromosome.
Due to random X‑inactivation:
- In some skin cell clones, the
X^Bchromosome remains active → black fur patches. - In others, the
X^Ochromosome remains active → orange fur patches.
The result is:
- Calico or tortoiseshell pattern: distinct areas of different colors reflecting mosaic expression of X‑linked coat color genes.
Male cats (usually XY) have only one X chromosome. Unless they have a sex chromosome abnormality (e.g., XXY), they do not typically show such mosaic coat patterns.
This feline example visually demonstrates the consequences of the Lyon hypothesis on phenotype.
Mechanism and Timing of X-Inactivation (Overview)
Without going into molecular detail (covered elsewhere), some features are important in this context:
- Timing: In humans, X‑inactivation is initiated early in embryogenesis, after fertilization but before organ formation. Once established in a cell, the same X stays inactive in all its descendants.
- Cis‑acting control: Inactivation affects the X chromosome that carries specific regulatory elements (e.g., the X‑inactivation center).
- Histone modification and DNA methylation: The inactive X becomes heavily methylated and packed into heterochromatin, contributing to its condensation into a Barr body.
- Not all genes are silenced: Some genes escape inactivation and are expressed from both X chromosomes, which is important in explaining certain clinical features in sex chromosome aneuploidies.
Clinical and Genetic Consequences
Skewed X-Inactivation
In an idealized case, approximately 50% of cells inactivate the maternal X and 50% inactivate the paternal X. However, in reality, the ratio can deviate significantly. This is called skewed (non‑random) X‑inactivation.
Causes may include:
- Chance effects in small early cell populations.
- Selection for cells with inactivation of one particular X (e.g., if one X carries a deleterious mutation).
Consequences:
- Females heterozygous for an X‑linked mutation may show:
- Very mild symptoms if mostly the normal X is active.
- More pronounced symptoms if mostly the mutant X is active.
Skewed inactivation can therefore modify the expressivity and even the penetrance of X‑linked disorders in females.
X-Linked Disorders and Lyonization
In X‑linked recessive diseases (e.g., hemophilia A, Duchenne muscular dystrophy):
- Males (XY) with a mutant allele on their single X often show full manifestations of the disease.
- Heterozygous females (XX) typically have one normal and one mutant allele:
- Due to random X‑inactivation, some cells express the normal allele, others the mutant allele.
- Overall, this mosaicism often provides enough normal gene product to prevent or reduce symptoms → females are often “carriers” with little or no clinical signs.
- However, skewed inactivation can make some heterozygous females clinically affected.
In X‑linked dominant diseases, both males and females with one mutant allele can be affected, but the mosaicism in females again influences severity.
Sex Chromosome Aneuploidies
The rule “all but one X are inactivated” applies regardless of the total number of X chromosomes, but inactivation is not a perfect cure for having extra or fewer X chromosomes.
Klinefelter Syndrome (47,XXY and variants)
- Genotype: typically 47,XXY (genetic male).
- X‑inactivation: one X is inactivated in each somatic cell → 1 Barr body.
- Despite inactivation, affected individuals show characteristic features (e.g., tall stature, infertility) because:
- Some genes escape X‑inactivation.
- The Y chromosome is present and active.
Triple X Syndrome (47,XXX) and Higher-Order Polysomies
- Genotype: 47,XXX (genetic female) or higher numbers of X chromosomes.
- X‑inactivation: all but one X are inactivated:
- 47,XXX → 2 Barr bodies.
- 48,XXXX → 3 Barr bodies.
- Phenotype is often relatively mild but can include learning difficulties and variable physical traits.
- Again, genes that escape inactivation contribute to the phenotype.
Turner Syndrome (45,X)
- Genotype: 45,X (monosomy X, genetic female).
- X‑inactivation: there is only one X chromosome → no Barr body.
- The absence of a second X (even though one would be inactivated) leads to characteristic clinical features (e.g., short stature, infertility), partly because:
- Some genes are normally expressed from both X chromosomes or from the X and the Y; their reduced dosage causes problems.
These examples show that the Lyon hypothesis explains why extra X chromosomes do not simply double or triple gene dosage, but it also explains why abnormalities in the number of sex chromosomes still have significant clinical consequences.
X-Inactivation and Genetic Counseling
Knowledge of the Barr body and Lyon hypothesis is important in genetic counseling and interpretation of pedigrees involving X‑linked traits:
- Helps explain why some women with a mutant X‑linked allele are healthy, mildly affected, or clearly affected.
- Informs risk assessments for offspring when a parent carries an X‑linked mutation.
- Provides a conceptual framework for understanding mosaicism in diseases such as:
- X‑linked muscular dystrophies.
- Certain forms of X‑linked immunodeficiencies.
- X‑linked metabolic disorders.
Understanding that female cells represent a patchwork of different active X chromosomes is also crucial for interpreting results from:
- Biochemical tests (enzyme activities may be intermediate).
- Molecular tests (mosaic patterns in DNA methylation or expression).
Summary
- The Barr body is a condensed, largely inactive X chromosome found in somatic cells with more than one X chromosome.
- The Lyon hypothesis states that, in female mammals, one of the two X chromosomes in each somatic cell is randomly inactivated early in development, and this inactivation is stably inherited by daughter cells.
- As a result, females are mosaics for X‑linked gene expression, which is visually evident in calico/tortoiseshell cats and functionally important in human X‑linked diseases.
- In humans, number of Barr bodies = number of X chromosomes − 1, and X‑inactivation acts as a dosage compensation mechanism.
- X‑inactivation is incomplete: some genes escape inactivation, which helps explain the clinical manifestations of sex chromosome aneuploidies such as Turner, Klinefelter, and triple X syndromes.
- Variability in which X is inactivated (random vs. skewed) influences the phenotype of carriers of X‑linked mutations and is a key consideration in human medical genetics.