12.2

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Sex Chromosomes and Chromosomal Basis of Inheritance

Introduction to Sex Chromosomes

The discovery of sex-linked traits, such as the white eye mutation in Drosophila, was pivotal in developing the chromosomal theory of inheritance. Sex chromosomes (X and Y) determine the biological sex of an organism and play a crucial role in inheritance patterns.

Sex Chromosomes: In mammals, females have two X chromosomes (XX), while males have one X and one Y chromosome (XY).
Autosomes: All other chromosomes that are not sex chromosomes.
Chromosomal Theory of Inheritance: Genes are located on chromosomes, and the behavior of chromosomes during meiosis explains inheritance patterns.
Example: The inheritance of eye color in Drosophila is linked to the X chromosome.

Sex Determination Systems

Sex determination varies among species and is based on the combination of sex chromosomes inherited from the parents.

Mammalian System: Females are XX, males are XY. The sex of offspring depends on whether the sperm carries an X or Y chromosome.
Other Systems: Birds, some insects, and other animals have different sex determination systems (e.g., ZW in birds, XO in some insects).

System	Female	Male
Mammals (XY)	XX	XY
Birds (ZW)	ZW	ZZ
Grasshoppers (XO)	XX	XO
Bees (haplo-diploid)	Diploid	Haploid
Additional info: Some reptiles use temperature-dependent sex determination.

X-Linked Genes and Inheritance Patterns

X-Linked Recessive Traits

X-linked genes are located on the X chromosome. Inheritance patterns differ between males and females due to the presence of only one X chromosome in males.

X-Linked Recessive: Traits are expressed in males if they inherit the mutant allele, while females must inherit two copies to express the trait.
Carrier Females: Females with one mutant and one normal allele are carriers but do not express the trait.
Example: Red-green color blindness and hemophilia are common X-linked recessive disorders.

Parent Genotype	Possible Offspring
Carrier Female (XNXn) × Normal Male (XNY)	50% carrier daughters, 50% affected sons
Affected Male (XnY) × Normal Female (XNXN)	All daughters are carriers, all sons are normal

Pedigree Analysis of X-Linked Traits

Pedigrees can be used to trace the inheritance of X-linked traits through families. Affected males often have carrier mothers, and the trait can skip generations.

Example: In a pedigree for color blindness, affected males are often born to carrier mothers.

X Inactivation and Dosage Compensation

Mechanism of X Inactivation

In female mammals, one of the two X chromosomes in each cell is randomly inactivated during early embryonic development, forming a Barr body. This process ensures dosage compensation between males and females.

Barr Body: The inactivated X chromosome condenses into a compact structure called a Barr body.
Random Inactivation: Inactivation is random in each cell, leading to mosaic expression of X-linked genes.
Example: Tortoiseshell cats display mosaic fur coloration due to X inactivation.

Genetic and Molecular Basis of X Inactivation

X inactivation is controlled by the XIST gene, which produces RNA that coats the X chromosome and initiates inactivation.

XIST Gene: Produces RNA that binds to the X chromosome, leading to its inactivation.
Regulation: RNA products from nearby genes help regulate the process.

Applications and Implications

X inactivation explains the patchy expression of X-linked traits in females and is important in understanding genetic disorders and inheritance patterns.

Example: Female carriers of X-linked disorders may show mild symptoms due to mosaicism.
Additional info: X inactivation is also relevant in cloning and stem cell research.

Summary Table: Key Concepts

Concept	Description	Example
Sex Chromosomes	Chromosomes that determine sex (X and Y in mammals)	XX (female), XY (male)
X-Linked Genes	Genes located on the X chromosome	Color blindness, hemophilia
X Inactivation	Random inactivation of one X chromosome in females	Tortoiseshell cats
Barr Body	Condensed, inactivated X chromosome	Seen in female somatic cells

Key Equations and Genetic Principles

Probability of X-linked recessive trait in offspring:

Dosage Compensation: