Philadelphia chromosome Health Dictionary

Philadelphia Chromosome: From 1 Different Sources


an abnormal form of chromosome 22 that has a foreshortened long arm due to a reciprocal *translocation with chromosome 9. It is most commonly seen in the marrow cells of patients with chronic *myeloid leukaemia.
Health Source: Oxford | Concise Colour Medical Dictionary
Author: Jonathan Law, Elizabeth Martin

X Chromosome

One of two SEX CHROMOSOMES. Every normal female body cell has a pair of X chromosomes. Men have only one X chromosome and this is paired with a Y chromosome. The sex cells in men and women each have one X and one Y chromosome. Certain diseases are linked to the presence of an X chromosome: these include HAEMOPHILIA (see GENETIC DISORDERS). (See also GENES.)... x chromosome

Y Chromosome

One of two SEX CHROMOSOMES that is present in every male body cell where it is paired with an X CHROMOSOME. The sex or germ cells in women as well as men contain one X and one Y chromosome (see also GENES).... y chromosome

Chromosomes

The rod-shaped bodies to be found in the nucleus of every cell in the body. They contain the GENES, or hereditary elements, which establish the characteristics of an individual. Composed of a long double-coiled ?lament of DNA, they occur in pairs – one from the maternal, the other from the paternal – and human beings possess 46, made up of 23 pairs. The number of chromosomes is speci?c for each species of animal. Each chromosone can duplicate an exact copy of itself between each cell division. (See GENETIC CODE; GENETICS; HEREDITY; MEIOSIS; SEX CHROMOSOMES.)... chromosomes

Sex Chromosomes

In humans there are 23 pairs of CHROMOSOMES. Male and female di?er in respect of one pair. In the nucleus of female cells, the two members of the pair are identical and are called X chromosomes. In the male nucleus there is one X chromosome paired with a dissimilar, di?erently sized chromosome called the Y chromosome. In the sex cells, after MEIOSIS, all cells in the female contain a single X chromosome. In the male, half will contain an X chromosome and half a Y chromosome. If a sperm with an X chromosome fertilises an ovum (which, as stated, must have an X chromosome) the o?spring will be female; if a sperm with a Y chromosome fertilises the ovum the o?spring will be male. It is the sex chromosomes which determine the sex of an individual.

Sometimes during cell division chromosomes may be lost or duplicated, or abnormalities in the structure of individual chromosomes may occur. The surprising fact is the infrequency of such errors. About one in 200 live-born babies has an abnormality of development caused by a chromosome, and two-thirds of these involve the sex chromosomes. There is little doubt that the frequency of these abnormalities in the early embryo is much higher, but because of the serious nature of the defect, early spontaneous ABORTION occurs.

Chromosome studies on such early abortions show that half have chromosome abnormalities, with errors of autosomes being three times as common as sex chromosome anomalies. Two of the most common abnormalities in such fetuses are triploidy with 69 chromosomes and trisomy of chromosome 16. These two anomalies almost always cause spontaneous abortion. Abnormalities of chromosome structure may arise because of:

Deletion Where a segment of a chromosome is lost.

Inversion Where a segment of a chromosome becomes detached and re-attached the other way around. GENES will then appear in the wrong order and thus will not correspond with their opposite numbers on homologous chromosomes.

Duplication Where a segment of a chromosome is included twice over. One chromosome will have too little nuclear material and one too much. The individual inheriting too little may be non-viable and the one with too much may be abnormal.

Translocation Where chromosomes of different pairs exchange segments.

Errors in division of centromere Sometimes the centromere divides transversely instead of longitudinally. If the centromere is not central, one of the daughter chromosomes will arise from the two short arms of the parent chromosome and the other from the two long arms. These abnormal daughter chromosomes are called isochromosomes.

These changes have important bearings on heredity, as the e?ect of a gene depends not only upon its nature but also upon its position on the chromosome with reference to other genes. Genes do not act in isolation but against the background of other genes. Each gene normally has its own position on the chromosome, and this corresponds precisely with the positon of its allele on the homologous chromosome of the pair. Each member of a pair of chromosomes will normally carry precisely the same number of genes in exactly the same order. Characteristic clinical syndromes, due to abnormalities of chromosome structure, are less constant than those due to loss or gain of a complete chromosome. This is because the degree of deletion, inversion and duplication is inconstant. However, translocation between chromosomes 15 and 21 of the parent is associated with a familial form of mongolism (see DOWN’S (DOWN) SYNDROME) in the o?spring, and deletion of part of an X chromosome may result in TURNER’S SYNDROME.

Non-disjunction Whilst alterations in the structure of chromosomes arise as a result of deletion or translocation, alterations in the number of chromosomes usually arise as a result of non-disjunction occurring during maturation of the parental gametes (germ cells). The two chromosomes of each pair (homologous chromosomes) may fail to come together at the beginning of meiosis and continue to lie free. If one chromosome then passes to each pole of the spindle, normal gametes may result; but if both chromosomes pass to one pole and neither to the other, two kinds of abnormal gametes will be produced. One kind of gamete will contain both chromosomes of the pair, and the other gamete will contain neither. Whilst this results in serious disease when the autosomes are involved, the loss or gain of sex chromosomes seems to be well tolerated. The loss of an autosome is incompatible with life and the malformation produced by a gain of an autosome is proportional to the size of the extra chromosome carried.

Only a few instances of a gain of an autosome are known. An additional chromosome 21 (one of the smallest autosomes) results in mongolism, and trisomy of chromosome 13 and 18 is associated with severe mental, skeletal and congenital cardiac defects. Diseases resulting from a gain of a sex chromosome are not as severe. A normal ovum contains 22 autosomes and an X sex chromosome. A normal sperm contains 22 autosomes and either an X or a Y sex chromosome. Thus, as a result of nondisjunction of the X chromosome at the ?rst meiotic division during the formation of female gametes, the ovum may contain two X chromosomes or none at all, whilst in the male the sperm may contain both X and Y chromosomes (XY) or none at all. (See also CHROMOSOMES; GENES.)... sex chromosomes

Philadelphia

(Greek) One who offers sisterly love

Philly, Phillie, Philli, Philley, Phillee, Phillea... philadelphia

Sex Chromosome

a chromosome that is involved in the determination of the sex of the individual. Women have two *X chromosomes; men have one X chromosome and one *Y chromosome. Compare autosome.... sex chromosome

Chromosome Analysis

Study of the chromosomes in body cells to discover whether a chromosomal abnormality is present or to establish its nature. Fetal cells for analysis can be obtained in the uterus by amniocentesis or chorionic villus sampling. If a serious abnormality such as Down’s syndrome is identified, termination of the pregnancy and genetic counselling is offered. Chromosome analysis is also carried out when a baby is stillborn without an obvious cause, or is born with abnormal physical characteristics that suggest a chromosomal defect, such as Turner’s sydrome.

Chromosome analysis in children and adults uses white blood cells taken from a blood sample. Analysis of the sex chromosomes may be carried out to establish the chromosomal sex of a child in cases where the genitals have an ambiguous appearance (see genitalia, ambiguous); to confirm or exclude the diagnosis of chromosomal abnormalities; or to investigate infertility.... chromosome analysis

Chromosome

n. one of the threadlike structures in a cell nucleus that carry the genetic information in the form of *genes. It is composed of a long double filament of *DNA coiled into a helix together with associated proteins, with the genes arranged in a linear manner along its length. It stains deeply with basic dyes during cell division (see meiosis; mitosis). The nucleus of each human somatic cell contains 46 chromosomes, 23 of which are of maternal and 23 of paternal origin (see illustration). Each chromosome can duplicate an exact copy of itself between each cell division (see interphase) so that each new cell formed receives a full set of chromosomes. See also chromatid; centromere; sex chromosome. —chromosomal adj.... chromosome



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