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The fundamental number, FN, of a karyotype is the number of visible major chromosomal arms per set of chromosomes.
Aneuploidy is the condition in which the chromosome number in the cells is not the typical number for the species.
The sex of an unborn fetus can be determined by observation of interphase cells (see amniotic centesis and Barr body).
Six different characteristics of karyotypes are usually observed and compared: The normal human karyotypes contain 22 pairs of autosomal chromosomes and one pair of sex chromosomes (allosomes).
The low record is held by the nematode Parascaris univalens, where the haploid n = 1; and an ant: Myrmecia pilosula.
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.
But when they obtained a couple more specimens they confirmed [their findings].
A spectacular example of variability between closely related species is the muntjac, which was investigated by Kurt Benirschke and his colleague Doris Wurster.
In the germ-line (the sex cells) the chromosome number is n (humans: n = 23). So, in normal diploid organisms, autosomal chromosomes are present in two copies. Polyploid cells have multiple copies of chromosomes and haploid cells have single copies.
The study of karyotypes is important for cell biology and genetics, and the results may be used in evolutionary biology (karyosystematics) and medicine.
and for a test that detects this complement or measures the number.
Karyotypes describe the chromosome count of an organism and what these chromosomes look like under a light microscope.
Karyotypes can be used for many purposes; such as to study chromosomal aberrations, cellular function, taxonomic relationships, and to gather information about past evolutionary events.