Sex chromosome


The sex chromosome differs from an ordinary chromosome in form, size, and behavior.

The human sex chromosomes, a typical pair of chromosomes, determine the sex of an individual created in sexual reproduction.

Each cell nucleus contains 23 pairs of chromosomes, a total of 46 chromosomes.

The first 22 pairs are called autosomes.

Autosomes are homologous chromosomes, indicating chromosomes which contain the same genes in the same order along their chromosomal arms.

The chromosomes of the 23rd pair are called allosomes consisting of two X chromosomes in most females, and an X chromosome and a Y chromosome in most males.

Females have 23 homologous chromosome pairs, while males have 22.

The X and Y chromosomes have small regions of homology called pseudoautosomal regions.

The X chromosome is always present as the 23rd chromosome in the ovum, while either an X or a Y chromosomes is present in an individual sperm.

Early in embryonic development of the female , in cells other than egg cells, one of the X chromosomes is randomly and permanently partially deactivated.

In some cells the X chromosome inherited from the mother is deactivated.

In some cells the X chromosome from the father is deactivated.

This process ensures that both sexes always have exactly one functional copy of the X chromosome in each body cell.

The deactivated X-chromosome is silenced by repressive heterochromatin that compacts the DNA and prevents expression of most genes.

This compaction is regulated by PRC2 (Polycomb Repressive Complex 2) gene.

To be chromosomally female, the individual must receive an X chromosome from both parents, whereas to be chromosomally male, the individual must receive a X chromosome from their mother and a Y chromosome from their father.

It is the male’s sperm that determines the sex of each offspring in humans.

A divergent sexual development can result from allosomes that are neither XX nor XY.

Allosomes carry the genes that determine male and female traits, but also contain those for some other characteristics as well: sex linked.

Sex linked diseases are passed down through families through one of the X or Y chromosomes.

Only men inherit Y chromosomes, they are the only ones to inherit Y-linked traits.

Men and women can get the X-linked ones since both inherit X chromosomes.

An allele is either said to be dominant or recessive.

Dominant inheritance occurs when an abnormal gene from one parent causes disease even though the matching gene from the other parent is normal.

The abnormal allele dominates.

Recessive inheritance occurs when both matching genes must be abnormal to cause disease.

If only one gene in the pair is abnormal, the disease does not occur, or is mild.

One abnormal gene without symptoms is called a carrier.

A carrier can pass this abnormal gene to his or her children.

The X chromosome carries about 1500 genes, more than any other chromosome in the human body.

Most of the genes code for something other than female anatomical traits.

Many of the non-sex determining X-linked genes are responsible for abnormal conditions.

The Y chromosome carries about 78 genes.

Most of the Y chromosome genes are involved with essential cell keeping activities and sperm production.

The SRY gene is the only Y chromosome gene responsible for anatomical male traits.

If any of the 9 Y genes involved in sperm production are missing or defective very low sperm counts and infertility result.

Mutations on the X chromosome include more common diseases such as color blindness, hemophilia, and fragile-X syndrome.

Color blindness is the inability or decreased ability to see color, or perceive color differences.

The cause of color blindness is a fault in the development of one or more sets of retinal cones perceiving color in light and transmit that information to the optic nerve

Color blindness is usually a sex-linked condition, because the genes that produce photopigments are carried on the X chromosome.

If some of these C-related genes are missing or damaged, color blindness will be expressed in males with a higher probability than in females because males only have one X chromosome.

Hemophilia is a X-Linked recessive disorder, which is much more common in males than females because males are hemizygous, expressing the trait when they inherit one mutant allele.

A female must inherit two mutant alleles, a less frequent event.

X-linked traits are maternally inherited from carrier mothers or from an affected father.

Each son born to a carrier mother has a 50% probability of inheriting the X-chromosome carrying the mutant allele.

46,XX testicular disorder of sex development, also called XX male syndrome, is a condition in which individuals with two X chromosomes in each cell, the pattern normally found in females, but have a male appearance.

The xx male syndrome is associated with male external genitalia.

In most people with 46,XX testicular disorder of sex development, the condition results from an abnormal exchange of genetic material between chromosomes.

The SRY gene on the Y chromosome is misplaced in this disorder, almost always onto an X chromosome.

Anyone with an X chromosome that carries the SRY gene will develop male characteristics despite not having a Y chromosome.

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