The karyotype is used to look for abnormal numbers or structures of chromosomes. When I hear the word "karyotype", I think about a picture of chromosomes. When somebody has their blood studied to look at how many chromosomes they have and whether the chromosomes are complete, we come up with a picture in which we can line up all the chromosomes and count them.
For example, when you have a sunburn and your skin peels, then you are shedding skin cells. In the center of each cell is an area called the nucleus. Human chromosomes are located inside the nucleus of the cell. Using this naming system, locations on chromosomes can be described consistently in the scientific literature.
One such powerful cytological technique is karyotyping, a method in which traits characterized by chromosomal abnormalities can be identified from a single cell. In the laboratory, the isolated cells are stimulated to begin actively dividing. A chemical called colchicine is then applied to cells to arrest condensed chromosomes in metaphase. Cells are then made to swell using a hypotonic solution so the chromosomes spread apart.
Finally, the sample is preserved in a fixative and applied to a slide. The geneticist then stains chromosomes with one of several dyes to better visualize the distinct and reproducible banding patterns of each chromosome pair. Following staining, the chromosomes are viewed using bright-field microscopy.
A common stain choice is the Giemsa stain. Giemsa staining results in approximately — bands of tightly coiled DNA and condensed proteins arranged along all of the 23 chromosome pairs. Once formed the testes produce sex hormones that direct the rest of the developing embryo to become male, while the ovaries make different sex hormones that promote female development.
The testes and ovaries are also the organs where gametes sperm or eggs are produced. How do the sex chromosome behave during meiosis? Well, in those individuals with two of the same chromosome i. In mammals the consequence of this is that all egg cells will carry an X chromosome while the sperm cells will carry either an X or a Y chromosome. Half of the offspring will receive two X chromosomes and become female while half will receive an X and a Y and become male.
The stages shown are anaphase I, anaphase II, and mature sperm. Note how half of the sperm contain Y chromosomes and half contain X chromosomes.
Analysis of karyotypes can identify chromosomal abnormalities, including aneuploidy , which is the addition or subtraction of a chromosome from a pair of homologs. More specifically, the absence of one member of a pair of homologous chromosomes is called monosomy only one remains. On the other hand, in a trisomy , there are three, rather than two disomy , homologs of a particular chromosome. The addition or loss of a whole chromosome is a mutation, a change in the genotype of a cell or organism.
The most familiar human aneuploidy is trisomy i. Most but not all other human aneuploidies are lethal at an early stage of embryonic development. Note that aneuploidy usually affects only one set of homologs within a karyotype, and is therefore distinct from polyploidy , in which the entire chromosome set is duplicated see below.
Aneuploidy is almost always deleterious, whereas polyploidy appears to be beneficial in some organisms, particularly many species of food plants.
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