X chromosome


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Related to X chromosome: Y chromosome

chromosome

 [kro´mo-sōm]
in animal cells, a structure in the nucleus, containing a linear thread of deoxyribonucleic acid (DNA), which transmits genetic information and is associated with ribonucleic acid and histones. In bacterial genetics, a closed circle of double-stranded DNA that contains the genetic material of the cell and is attached to the cell membrane; the bulk of this material forms a compact bacterial nucleus. adj., adj chromoso´mal.

During cell division the material composing the chromosome is compactly coiled, making it visible with appropriate staining and permitting its movement in the cell with minimal entanglement. Each organism of a species is normally characterized by the same number of chromosomes in its somatic cells, 46 being the number normally present in humans, including 22 pairs of autosomes and the two sex chromosomes (XX or XY), which determine the sex of the organism. (See also heredity.)
Chromosome Analysis. This can be done on fetal cells obtained by amniocentesis or chorionic villus sampling, on lymphocytes from a blood sample, on skin cells from a biopsy, or on cells from products of conception such as an aborted fetus. The cells are then cultured in the laboratory until they divide. Cell division is arrested in mid-metaphase by the drug Colcemid. The chromosomes can be stained by one of several techniques that produce a distinct pattern of light and dark bands along the chromosomes, and each chromosome can be recognized by its size and banding pattern. The chromosomal characteristics of an individual are referred to as the karyotype. It is also possible to make a photomicrograph of a cell nucleus, cut it apart, and rearrange it so that the individual chromosomes are in order and labeled. The autosomes are numbered 1–22, roughly in order of decreasing length. The sex chromosomes are labeled X and Y. Karyotyping is useful in determining the presence of chromosome defects.

Before the chromosomes could be precisely identified they were placed in seven groups: A (chromosomes 1–3), B (4–5), C (6–12 and X), D (13–15), E (16–18), F (19–20), and G (21–22 and Y).
Chromosomal Abnormalities. The prevalence of chromosomal disorders cannot be fully and accurately determined because many of these disorders do not permit full embryonic and fetal development and therefore end in spontaneous abortion. About one in every 100 newborn infants do, however, have a gross demonstrable chromosomal abnormality. A large majority of cytogenetic abnormalities can be identified by cytogenetic analysis either before birth, by means of chorionic villus sampling or amniocentesis, or after birth.

Cytogenetic disorders with visible chromosomal abnormalities are evidenced by either an abnormal number of chromosomes or some alteration in the structure of one or more chromosomes. In the language of the geneticist, trisomy refers to the presence of an additional chromosome that is homologous with one of the existing pairs so that that particular chromosome is present in triplicate. An example of this type of disorder is a form of down syndrome (trisomy 21). Another example is patau's syndrome (trisomy 13), which produces severe anatomical malformations and profound mental retardation.

The term monosomy refers to the absence of one of a pair of homologous chromosomes. Monosomy involving an autosome usually results in the loss of too much genetic information to permit sufficient fetal development for a live birth. Either trisomy or monosomy involving the sex chromosomes yields relatively mild abnormalities.

A condition known as mosaicism results from an error in the distribution of chromosomes between daughter cells during an early embryonic cell division, producing two and sometimes three populations of cells with different chromosome numbers in the same individual. Mosaicism involving the sex chromosomes is not uncommon.

Other abnormal structural changes in the chromosome are consequences of some kind of chromosomal breakage, with either the loss or rearrangement of genetic material. translocation involves the transfer of a segment of one chromosome to another. inversion refers to a change in the sequence of genes along the chromosome, which occurs when there are two breaks in a chromosome and the segment between the breaks is reversed and reattached to the wrong ends. deletion occurs when a portion of a chromosome is lost. An example of this type of chromosomal abnormality is cri du chat syndrome, a deletion in the short arm of chromosome 5, marked by mental retardation and sometimes congenital heart defects. When deletion occurs at both ends of the chromosome, the two damaged ends can unite to form a circle and the rearrangement produces a ring chromosome. isochromosomes form when the centromere divides along the transverse plane rather than the normal long axis of the chromosome so that both arms are identical. All of the previously described structural abnormalities can affect both autosomal and sex chromosomes.

The causes of chromosomal errors are not completely understood. In some conditions such as Down syndrome, late maternal age seems to be a factor. Other factors may include the predisposition of chromosomes to nondisjunction (failure to separate during meiosis), exposure to radiation, and viruses.
homologous c's the chromosomes of a matching pair in the diploid complement that contain alleles of specific genes.
Ph1 chromosome (Philadelphia chromosome) an abnormality of chromosome 22, characterized by the translocation of genetic material from its long arm to chromosome 9, seen in the marrow cells of most patients with chronic myelogenous leukemia.
ring chromosome a chromosome in which both ends have been lost (deletion) and the two broken ends have reunited to form a ring-shaped figure.
sex c's the chromosomes responsible for determination of the sex of the individual that develops from a zygote; in mammals they are an unequal pair, the X and Y chromosomes.
somatic chromosome autosome.
X chromosome the female sex chromosome, being carried by half the male gametes and all female gametes; female diploid cells have two X chromosomes.
Y chromosome the male sex chromosome, being carried by half the male gametes and none of the female gametes; male diploid cells have an X and a Y chromosome.

sex chro·mo·somes

the pair of chromosomes responsible for sex determination. In humans and most animals, the sex chromosomes are designated X and Y; females have two X chromosomes, males have one X and one Y chromosome In certain birds, insects, and fish the sex chromosomes are designated Z and W; males have two Z chromosomes, females may have one Z and one W chromosome, or one Z and no W chromosome.
Synonym(s): gonosome

X chromosome

or

X-chromosome

(ĕks′krō′mə-sōm′)
n.
The sex chromosome associated with female characteristics in mammals, occurring paired in the female and single in the male.

X chromosome

A chromosome that contains the genetic information needed to produce a female. All eggs (ova) contain one X chromosome; half of all sperm carry an X chromosome, while the other half have a Y chromosome.

X chromosome

A sex chromosome with 2 copies in normal ♀ and one copy in normal ♂. Cf Y chromosome.

sex chro·mo·somes

(seks krō'mŏ-sōmz)
The pair of chromosomes responsible for sex determination. In humans and most animals, the sex chromosomes are designated X and Y; females have two X chromosomes, males have one X and one Y chromosome.

X chromosome

The CHROMOSOME which, with the Y chromosome, determines the sex of the individual. About 50% of sperms carry an X chromosome and 50% a Y. The sex of the future child is determined by whether an X-carrying or a Y-carrying sperm happens to fertilize the ovum. The ovum carries only an X chromosome. Females have two X chromosomes in each body cell, males have one X and one Y. The X chromosome is large and contains about 6% of the genomic DNA. The Y is about half the size. Well over a hundred disorders are known to be determined by genes on the X chromosome. These are called X-linked conditions. See also X-INACTIVATION.

X chromosome

One of the two sex chromosomes (the other is Y) that determine a person's gender. Normal males have both an X and a Y chromosome, and normal females have two X chromosomes.
References in periodicals archive ?
Washington, March 17 ( ANI ): Males with an extra "girly" sex chromosome two (X chromosomes) seemed to have an insatiable appetite for sex, at least in mice.
Female mammals have two X chromosomes, but males only have one.
Scientists understand the off--but not the on--half of a molecular switch that controls X chromosomes' fates in mice and people (SN: 8/05/00, p.
In one recent study, for example, biologists found that many more X chromosome genes than expected escape inactivation.
Supporting a theory proposed 2 years ago, a research team has uncovered evidence that DNA sequences usually dismissed as junk DNA without any function actually help determine what genes on the X chromosome become suppressed.
Marked social difficulties and academic failure occurred far more often in the group with a maternally inherited X chromosome, the researchers report in the June 12 Nature.
Finally, Skuse's team found that another eight Turner's syndrome volunteers, each lacking much of the short arm of the paternally inherited X chromosome, performed as well on social and verbal tests as the other group of paternal-X females.
Several clues point to Xist's importance in silencing one copy of a female's X chromosome. Xist is active on the inactivated X chromosome but inactive on the active copy.
Furthermore, the RNA strands made by Xist cover the inactivated X chromosome "like a sheath," says Rudolf Jaenisch of the Whitehead Institute for Biomedical Research in Cambridge, Mass.
"The case of human DAZ challenges the prevailing view that most, if not all, Y chromosomal genes were once shared with the X chromosome....
Two-thirds of the 32 pairs of homosexual brothers included in the final analysis share the same markers for one portion of their X chromosome and thus inherited the same span, Hamer and his colleagues at the University of Colorado in Boulder and the Whitehead Institute for Biomedical Research in Cambridge, Mass., report in the November Nature Genetics.
The new study is not likely to settle whether the X chromosome holds a gene that sways sexual orientation.