Genetics of Sex Chromosomes

The sex chromosomes determine an individual's gender in mammals. Females have two X chromosomes (XX) and are considered homogametic, while males have one X and one Y chromosome (XY) and are considered heterogametic.

The pseudoautosomal region (PAR) is a small area on the short arms of the X and Y chromosomes that contains similar DNA sequences. This region allows for crossing-over between the X and Y chromosomes, similar to what happens with autosomes.

The X chromosome is larger than the Y chromosome and has significantly more genes. The X chromosome carries genes essential for both sexes, while the Y chromosome carries genes primarily responsible for male development.

The SRY gene, located on the Y chromosome, plays a crucial role in male sex determination. It encodes the testis-determining factor (TDF) that triggers testes formation, leading to the production of testosterone and the development of male secondary sex characteristics.

In females, the absence of the SRY gene leads to the development of ovaries and female characteristics.

X-inactivation is a process that happens in female mammals where one of the two X chromosomes is inactivated to ensure equal expression of X-linked genes between males and females.

The X-inactivation center (XIC) is a region on the X chromosome that plays a key role in X-inactivation. It acts as a control center that ensures only one X chromosome is inactivated.

Chromosome counting is the first step of X-inactivation. It involves determining the number of X chromosomes in a cell.

The X-controlling element (XCE) is responsible for selecting which of the two X chromosomes will be inactivated. Different alleles of XCE have different probabilities of being inactivated.

The inactivation of an X chromosome is mediated by the XIST gene. It produces Xist RNA, a long non-coding RNA that coats and silences the inactive X chromosome.

About 15% of genes on the X chromosome escape inactivation and remain active on both X chromosomes in females. This means these genes are expressed equally in both sexes.

Some genes on the inactive X chromosome are expressed at different levels, meaning they may be partially active. This contributes to the diversity of gene expression in females.

A condition where one X chromosome is missing or partially missing, leading to a variety of physical, developmental, and health challenges.

Meiotic nondisjunction during gamete formation, resulting in an extra X chromosome in males (XXY).

A pattern of inheritance where genes carried on the X chromosome are expressed differently in males and females due to the different number of X chromosomes. This can result in a variety of X-linked disorders.

An error in cell division that occurs during meiosis, leading to an abnormal number of chromosomes in the resulting gametes.

An individual with one X chromosome and no Y chromosome. Also referred to as monosomy X.

An individual with two X chromosomes and one Y chromosome.

A condition affecting males with an extra X chromosome, leading to developmental delays and physical features like tall stature, small testes, and reduced testosterone levels.

A condition affecting females with a missing X chromosome, causing a range of physical challenges like short stature, webbed necks, and heart defects.

The Y chromosome lacks many genes present on the X chromosome, leading to different expression patterns of genes on the X chromosome in males and females.

A type of genomic instability where chromosomes or parts of chromosomes are missegregated during cell division, leading to changes in both chromosome number and structure (numerical & structural chromosome abnormalities).

In normal cells, about 1% of cell divisions have errors in chromosome segregation, whereas cells with CIN have a significantly increased error rate of around 20%.

The process by which chromosomes are accurately separated and distributed to daughter cells during cell division.

A checkpoint mechanism that ensures proper attachment of chromosomes to the spindle fibers before the start of anaphase (cell division). It prevents premature separation of sister chromatids to maintain genome stability.

A proper attachment of all chromosomes to the spindle microtubules, where both sister chromatids are connected to opposite poles of the spindle. This ensures equal distribution of genetic material.

Errors in chromosome segregation can occur when chromosomes are not properly attached to the spindle microtubules, leading to unequal distribution of genetic material.

Processes that lead to chromosome instability, including errors in kinetochore-microtubule attachment, abnormal sister chromatid cohesion, centrosome replication, and telomere attrition.

Errors in the attachment between kinetochores (protein structures on chromosomes) and microtubules (fibers of the spindle) during mitosis.

Aberrant (abnormal) cohesion between sister chromatids, which are identical copies of a chromosome, can lead to missegregation during mitosis.