compensation). in placental mammals, the inactivation is random as between the two xs; thus the mammalian female is a mosaic in respect of her x chromosomes. in marsupials it is always the paternal x which is inactivated. in human 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. number of chromosomes in a set a spectacular example of variability between closely related species is the muntjac, which was investigated by kurt benirschke and doris wurster. the diploid number of the chinese muntjac, muntiacus reevesi, was found to be 46, all telocentric. when they looked at the karyotype of the closely related indian muntjac, muntiacus muntjak, they were astonished to find it had female = 6, male = 7 chromosomes. they simply could not believe what they saw ... they kept quiet for two or three years because they thought something was wrong with their tissue culture ... but when they obtained a couple more specimens they confirmed .— hsu p. 73-4 the number of chromosomes in the karyotype between (relatively) unrelated species is hugely variable. the low record is held by the nematode parascaris univalens, where the haploid n = 1; and an ant: myrmecia pilosula. the high record would be somewhere amongst the ferns, with the adder's tongue fern ophioglossum ahead with an average of 1262 chromosomes. top score for animals might be the shortnose sturgeon acipenser brevirostrum at 372 chromosomes. 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. fundamental number the fundamental number, fn, of a karyotype is the number of visible major chromosomal arms per set of chromosomes. thus, fn ≤ 2 x 2n, the difference depending on the number of chromosomes considered single-armed (acrocentric or telocentric) present. humans have fn = 82, due to the presence of five acrocentric chromosome pairs: 13, 14, 15, 21, and 22 (the human y chromosome is also acrocentric). the fundamental autosomal number or autosomal fundamental number, fna or an, of a karyotype is the number of visible major chromosomal arms per set of autosomes (non-sex-linked chromosomes). ploidy ploidy is the number of complete sets of chromosomes in a cell. polyploidy, where there are more than two sets of homologous chromosomes in the cells, occurs mainly in plants. it has been of major significance in plant evolution according to stebbins. the proportion of flowering plants which are polyploid was estimated by stebbins to be 30–35%, but in grasses the average is much higher, about 70%. polyploidy in lower plants (ferns, horsetails and psilotales) is also common, and some species of ferns have reached levels of polyploidy far in excess of the highest levels known in flowering plants. polyploidy in animals is much less common, but it has been significant in some groups. polyploid series in related species which consist entirely of multiples of a single basic number are known as euploid. haplo-diploidy, where one sex is diploid, and the other haploid. it is a common arrangement in the hymenoptera, and in some other groups. endopolyploidy occurs when in adult differentiated tissues the cells have ceased to divide by mitosis, but the nuclei contain more than the original somatic number of chromosomes. in the endocycle (endomitosis or endoreduplication) chromosomes in a 'resting' nucleus undergo reduplication, the daughter chromosomes separating from each other inside an intact nuclear membrane. in many instances, endopolyploid nuclei contain tens of thousands of chromosomes (which cannot be exactly counted). the cells do not always contain exact multiples (powers of two), which is why the simple definition 'an increase in the number of chromosome sets caused by replication without cell division' is not quite accurate.this process (especially studied in insects and some higher plants such as maize) may be a developmental strategy for increasing the productivity of tissues which are highly active in biosynthesis. the phenomenon occurs sporadically throughout the eukaryote kingdom from protozoa to humans; it is diverse and complex, and serves differentiation and morphogenesis in many ways. see palaeopolyploidy for the investigation of ancient karyotype duplications. aneuploidy aneuploidy is the condition in which the chromosome number in the cells is not the typical number for the species. this would give rise to a chromosome abnormality such as an extra chromosome or one or more chromosomes lost. abnormalities in chromosome number usually cause a defect in development. down syndrome and turner syndrome are examples of this. aneuploidy may also occur within a group of closely related species. classic examples in plants are the genus crepis, where the gametic (= haploid) numbers form the series x = 3, 4, 5, 6, and 7; and crocus, where every number from x = 3 to x = 15 is represented by at least one species. evidence of various kinds shows that trends of evolution have gone in different directions in different groups. in primates, the great apes have 24x2 chromosomes whereas humans have 23x2. human chromosome 2 was formed by a merger of ancestral chromosomes, reducing the number. chromosomal polymorphism some species are polymorphic for different chromosome structural forms. the structural variation may be associated with different numbers of chromosomes in different individuals, which occurs in the ladybird beetle chilocorus stigma, some mantids of the genus ameles, the european shrew sorex araneus. there is some evidence from the case of the mollusc thais lapillus (the dog whelk) on the brittany coast, that the two chromosome morphs are adapted to different habitats. species trees the detailed study of chromosome banding in insects with polytene chromosomes can reveal relationships between closely related species: the classic example is the study of chromosome banding in hawaiian drosophilids by hampton l. carson. in about 6,500 sq mi (17,000 km2), the hawaiian islands have the most diverse collection of drosophilid flies in the world, living from rainforests to subalpine meadows. these roughly 800 hawaiian drosophilid species are usually assigned to two genera, drosophila and scaptomyza, in the family drosophilidae. the polytene banding of the 'picture wing' group, the best-studied group of hawaiian drosophilids, enabled carson to work out the evolutionary tree long before genome analysis was practicable. in a sense, gene arrangements are visible in the banding patterns of each chromosome. chromosome rearrangements, especially inversions, make it possible to see which species are closely related. the results are clear. the inversions, when plotted in tree form (and independent of all other information), show a clear "flow" of species from older to newer islands. there are also cases of colonization back to older islands, and skipping of islands, but these are much less frequent. using k-ar dating, the present islands date from 0.4 million years ago (mya) (mauna kea) to 10mya (necker). the oldest member of the hawaiian archipelago still above the sea is kure atoll, which can be dated to 30 mya. the archipelago itself (produced by the pacific plate moving over a hot spot) has existed for far longer, at least into the cretaceous. previous islands now beneath the sea (guyots) form the emperor seamount chain. all of the native drosophila and scaptomyza species in hawaiʻi have apparently descended from a single ancestral species that colonized the islands, probably 20 million years ago. the subsequent adaptive radiation was spurred by a lack of competition and a wide variety of niches. although it would be possible for a single gravid female to colonise an island, it is more likely to have been a group from the same species. there are other animals and plants on the hawaiian archipelago which have undergone similar, if less spectacular, adaptive radiations. chromosome banding chromosomes display a banded pattern when treated with some stains. bands are alternating light and dark stripes that appear along the lengths of chromosomes. unique banding patterns are used to identify chromosomes and to diagnose chromosomal aberrations, including chromosome breakage, loss, duplication, translocation or inverted segments. a range of different chromosome treatments produce a range of banding patterns: g-bands, r-bands, c-bands, q-bands, t-bands and nor-bands. depiction of karyotypes types of banding cytogenetics employs several techniques to visualize different aspects of chromosomes: g-banding is obtained with giemsa stain following digestion of chromosomes with trypsin. it yields a series of lightly and darkly stained bands — the dark regions tend to be heterochromatic, late-replicating and at rich. the light regions tend to be euchromatic, early-replicating and gc rich. this method will normally produce 300–400 bands in a normal, human genome. it is the most common chromosome banding method. r-banding is the reverse of g-banding (the r stands for "reverse"). the dark regions are euchromatic (guanine-cytosine rich regions) and the bright regions are heterochromatic (thymine-adenine rich regions). c-banding: giemsa binds to constitutive heterochromatin, so it stains centromeres. the name is derived from centromeric or constitutive heterochromatin. the preparations undergo alkaline denaturation prior to staining leading to an almost complete depurination of the dna. after washing the probe the remaining dna is renatured again and stained with giemsa solution consisting of methylene azure, methylene violet, methylene blue, and eosin. heterochromatin binds a lot of the dye, while the rest of the chromosomes absorb only little of it. the c-bonding proved to be especially well-suited for the characterization of plant chromosomes. q-banding is a fluorescent pattern obtained using quinacrine for staining. the pattern of bands is very similar to that seen in g-banding. they can be recognized by a yellow fluorescence of differing intensity. most part of the stained dna is heterochromatin. quinacrin (atebrin) binds both regions rich in at and in gc, but only the at-quinacrin-complex fluoresces. since regions rich in at are more common in heterochromatin than in euchromatin, these regions are labelled preferentially. the different intensities of the single bands mirror the different contents of at. other fluorochromes like dapi or hoechst 33258 lead also to characteristic, reproducible patterns. each of them produces its specific pattern. in other words: the properties of the bonds and the specificity of the fluorochromes are not exclusively based on their affinity to regions rich in at. rather, the distribution of at and the association of at with other molecules like histones, for example, influences the binding properties of the fluorochromes. t-banding: visualize telomeres. silver staining: silver nitrate stains the nucleolar organization region-associated protein. this yields a dark region where the silver is deposited, denoting the activity of rrna genes within the nor. classic karyotype cytogenetics karyogram from a human female lymphocyte probed for the alu sequence using fish. in the "classic" (depicted) karyotype, a dye, often giemsa (g-banding), less frequently mepacrine (quinacrine), is used to stain bands on the chromosomes. giemsa is specific for the phosphate groups of dna. quinacrine binds to the adenine-thymine-rich regions. 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. in addition, the differently stained regions and sub-regions are given numerical designations from proximal to distal on the chromosome arms. for example, cri du chat syndrome involves a deletion on the short arm of chromosome 5. it is written as 46,xx,5p-. the critical 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 (mfish) and spectral karyotype (sky technique) spectral karyogram of a human female multicolor fish and the older spectral karyotyping are molecular cytogenetic techniques used to simultaneously visualize all the pairs of chromosomes in an organism in different colors. fluorescently labeled probes for each chromosome are made by labeling chromosome-specific dna with different fluorophores. because there are a limited number of spectrally distinct fluorophores, a combinatorial labeling method is used to generate many different colors. fluorophore combinations are captured and analyzed by a fluorescence microscope using up to 7 narrow-banded fluorescence filters or, in the case of spectral karyotyping, by using an interferometer attached to a fluorescence microscope. in the case of an mfish image, every combination of fluorochromes from the resulting original images