Refers to the number and appearance of chromosomes in the nucleus of an eukaryotic cell.

Karyotypes describe the chromosome count and what these chromosomes look like under a light microscope.

Karyotypes describe chromosome length, the position of the centromeres, banding pattern, any differences between the sex chromosomes, and any other physical characteristics.

The study of karyotypes is part of cytogenetics.

Karyogram using Giemsa staining depicts chromosomes by rearranging a photomicrograph in a standard format known as a karyogram or idiogram, which is in pairs, ordered by size and position of centromere for chromosomes of the same size.

Karyotyping can be used for many purposes; such as to study chromosomal aberrations, cellular function, taxonomic relationships, and to understand past evolutionary events.

Human chromosome 2 is the result of an end-to-end fusion of two ancestral ape chromosomes.

The study of karyotypes is made possible by staining, usually with dye, such as Giemsa, and is applied after cells have been arrested during cell division by a solution of colchicine usually in metaphase or prometaphase when most condensed.

White blood cells are used most frequently because they are easily induced to divide and grow in tissue culture.

The sex of an unborn fetus can be determined by observation of interphase cells.

Chromosomes can vary in absolute size reflecting different amounts of DNA duplication, and segmental interchange of unequal lengths.

Differences in the position of centromeres are a result of translocations.

Humans have one pair fewer chromosomes than the great apes.

Chromosome variations are often found:

between the sexes,

between gametes and the rest of the body

between members of a population

in mosaics

Variations from the standard karyotype may lead to developmental abnormalities.

The normal human karyotype contains 22 pairs of autosomal chromosomes and one pair of sex chromosomes.

Normal karyotypes for females contain two X chromosomes and are denoted 46,XX.

Males have both an X and a Y chromosome denoted 46XY.

The inactivation of one X chromosome takes place during early development.

In females some 15% of somatic cells escape inactivation, and the number of genes affected on the inactivated X chromosome varies between cells.

In fibroblast cells up about 25% of genes on the Barr body escape inactivation.

The existence of supernumerary or B chromosomes means that chromosome number can vary even within one interbreeding population; and aneuploids are another example, though in this case they would not be regarded as normal members of the population.

Ploidy refers to the the number of complete sets of chromosomes in a cell.

Aneuploidy refers to the condition in which the chromosome number in the cells is not the typical number for the species: giving rise to a chromosome abnormality such as an extra chromosome or one or more chromosomes lost.

Abnormalities in chromosome number cause Down syndrome and Turner syndrome.

Chromosomes display a banded pattern when treated with some stains manifesting as alternating light and dark stripes that appear along the lengths of chromosomes.

Unique banding patterns can identify chromosomes and to diagnose chromosomal ab2242ations, including chromosome breakage, loss, duplication, translocation or inverted segments.

There is a range of banding patterns: G-bands, R-bands, C-bands, Q-bands, T-bands and NOR-bands.

G-banding is obtained with Giemsa stain following digestion of chromosomes with trypsin, and will normally produce 300–400 bands in a normal, human genome.

R-banding is the reverse of G-banding.

C-banding: Giemsa stains centromeres.

Q-banding is a fluorescent pattern obtained using quinacrine for staining, and the pattern of bands is very similar to that seen in G-banding.

T-banding: visualize telomeres.

Each chromosome has a characteristic banding pattern that helps to identify them; both chromosomes in a pair will have the same banding pattern.

Karyotypes are arranged with the short arm of the chromosome on top, and the long arm on the bottom.

Some karyotypes call the short and long arms p and q, respectively.

Differently stained regions and sub-regions are given numerical designations from proximal to distal on the chromosome arms.

Cri du chat syndrome involves a deletion on the short arm of chromosome 5, written as 46,XX,5p-, and region for this syndrome is deletion of p15.2, the locus on the chromosome, which is written as 46,XX,del(5)(p15.2).

Multicolor FISH is used to identify structural chromosome aberrations in cancer cells and other disease conditions when Giemsa banding or other techniques are not accurate enough.

Chromosome abnormalities can be numerical, as in the presence of extra or missing chromosomes, or structural, as in derivative chromosome, translocations, inversions, large-scale deletions or duplications.

Numerical abnormalities, also known as aneuploidy, often occur as a result of nondisjunction during meiosis in the formation of a gamete.

Trisomies, refers to three copies of a chromosome being present instead of the usual two, and are common numerical abnormalities.

Structural abnormalities can arise from errors in homologous recombination, and those occurring in gametes will be present in all cells of an affected person.

If abnormalities occur during mitosis they can give rise to a genetic mosaic individual who has some normal and some abnormal cells.

Chromosomal abnormalities that lead to disease in humans include

Turner syndrome results from a single X chromosome (45,X or 45,X0).

Klinefelter syndrome, the most common male chromosomal disease, otherwise known as 47,XXY, is caused by an extra X chromosome.

Edwards syndrome is caused by trisomy of chromosome 18.

Down syndrome, a common chromosomal disease, is caused by trisomy of chromosome 21.

Patau syndrome is caused by trisomy of chromosome 13.

Trisomy 9, the 4th most common trisomy, has many long lived affected individuals but only in a form other than a full trisomy, such as trisomy 9p syndrome or mosaic trisomy 9.

Trisomy 8 and trisomy 16, patients generally do not survive to birth.

Disorders a result from loss of just a piece of one chromosome, including:

Cri du chat-from a truncated short arm on chromosome 5.

1p36 Deletion syndrome, from the loss of part of the short arm of chromosome 1.

Angelman syndrome – 50% of cases have a segment of the long arm of chromosome 15 missing; a deletion of the maternal genes, example of imprinting disorder.

Prader-Willi syndrome – 50% of cases have a segment of the long arm of chromosome 15 missing. It is a deletion of the paternal genes, example of imprinting disorder.

Chromosomal abnormalities can also occur in cancerous cells of an otherwise genetically normal individual; the Philadelphia chromosome, a translocation mutation commonly associated with chronic myelogenous leukemia and less often with acute lymphoblastic leukemia.

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