X chromosome

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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 sex chromosome that in humans and many other animals is present in both sexes, appearing singly in the cells of normal males and in duplicate in the cells of normal females. The chromosome is present in all of the female gametes and in half of the male gametes, is much larger than the Y chromosome, and has many sex-linked genes associated with clinically significant disorders, such as hemophilia, Duchenne's muscular dystrophy, and Hunter's syndrome. Compare Y chromosome.

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.

X chromosome,

n a sex chromosome that in humans and many other species is present in both male and female. The male somatic cell consists of one X chromosome and one Y chromosome; the female somatic cell carries two X chromosomes. All female gametes carry the X chromosome, whereas half of the male gametes possess the X chromosome and the other half the Y chromosome.

chromosome

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.
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. The diploid numbers (number of total chromosomes per cell) are cattle—60, sheep—54, horse—64, donkey—62, pig—38, dog—78, cat—38, human—46. The chromosomes are arranged in pairs and one of the pairs is the sex chromosomes (XX or XY), which determines the sex of the organism. See also heredity.

compound chromosome
a genetic engineering procedure which produces two chromosomes in one of which the left arms of the two original chromosomes are joined together and the two original right arms are also joined together; used in genetic control of insect populations.
homologous c's
the chromosomes of a matching pair in the diploid complement that contain alleles of specific genes.
lampbrush chromosome
so named because of the bristling appearance given them by many open loops of chromatin along the extended chromosome.
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 constituting an unequal pair, the X and the Y chromosome.
somatic chromosome
autosome.
submetacentric chromosome
W chromosome
sex chromosome in animals such as poultry in which the female is the heterogametic state, the male has the ZZ genotype and the female the ZW genotype.
X chromosome
the female sex chromosome, being carried by half the male gametes and all female gametes; female diploid cells have two X chromosomes, the male has the XY genotype.
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; females carry the XX genotype.
Z chromosome
sex chromosome in animals, such as poultry, in which the female is the heterogametic sex; the male has the ZZ genotype and the female the ZW genotype.