How Can Karyotype Analysis Detect Genetic Disorders Answer Key

13.1C: Identification of Chromosomes and Karyotypes

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A karyotype depicts the number, size, and any abnormalities of the chromosomes in an organism.

Learning Objectives

• Describe a normal man karyotype and discuss the various abnormalities that tin be detected using this technique

Key Points

• A normal human being karyotype contains 23 pairs of chromosomes: 22 pairs of autosomes and ane pair of sexual activity chromosomes, generally arranged in order from largest to smallest.
• The short arm of a chromosome is referred to every bit the p arm, while the long arm is designated the q arm.
• To notice a karyotype, cells are collected from a claret or tissue sample and stimulated to begin dividing; the chromosomes are arrested in metaphase, preserved in a fixative and applied to a slide where they are stained with a dye to visualize the distinct banding patterns of each chromosome pair.
• A karyotype can be used to visualize abnormalities in the chromosomes, such as an incorrect number of chromosomes, deletions, insertions, or translocations of DNA.

Cardinal Terms

• autosome: whatsoever chromosome other than sex chromosomes
• karyotype: the observed characteristics (number, blazon, shape etc) of the chromosomes of an individual or species
• translocation: a transfer of a chromosomal segment to a new position, specially on a nonhomologous chromosome

Identification of Chromosomes

The isolation and microscopic observation of chromosomes forms the footing of cytogenetics and is the chief method by which clinicians discover chromosomal abnormalities in humans. A karyotype is the number and appearance of chromosomes. To obtain a view of an individual's karyotype, cytologists photo the chromosomes and so cut and paste each chromosome into a chart, or karyogram, also known as an ideogram.

In a given species, chromosomes can exist identified by their number, size, centromere position, and banding design. In a human karyotype, autosomes or "torso chromosomes" (all of the non–sex chromosomes) are mostly organized in approximate order of size from largest (chromosome 1) to smallest (chromosome 22). Withal, chromosome 21 is actually shorter than chromosome 22. This was discovered after the naming of Down's syndrome as trisomy 21, reflecting how this illness results from possessing ane actress chromosome 21 (three total). Non wanting to change the name of this important affliction, chromosome 21 retained its numbering, despite describing the shortest set of chromosomes. The Ten and Y chromosomes are not autosomes and are referred to every bit the sex chromosomes.

The chromosome "arms" projecting from either finish of the centromere may be designated every bit short or long, depending on their relative lengths. The short arm is abbreviated p (for "petite"), whereas the long arm is abbreviated q (because it follows "p" alphabetically). Each arm is further subdivided and denoted by a number. Using this naming organisation, locations on chromosomes can be described consistently in the scientific literature.

Although Mendel is referred to every bit the "male parent of modern genetics," he performed his experiments with none of the tools that the geneticists of today routinely employ. One such powerful cytological technique is karyotyping, a method in which traits characterized by chromosomal abnormalities can be identified from a single cell. To observe an private's karyotype, a person's cells (like white blood cells) are first collected from a claret sample or other tissue. In the laboratory, the isolated cells are stimulated to begin actively dividing. A chemic called colchicine is then applied to cells to arrest condensed chromosomes in metaphase. Cells are so fabricated to swell using a hypotonic solution so the chromosomes spread autonomously. Finally, the sample is preserved in a fixative and practical to a slide.

The geneticist then stains chromosomes with i of several dyes to meliorate visualize the distinct and reproducible banding patterns of each chromosome pair. Following staining, the chromosomes are viewed using vivid-field microscopy. A mutual stain choice is the Giemsa stain. Giemsa staining results in approximately 400–800 bands (of tightly coiled Deoxyribonucleic acid and condensed proteins) arranged along all of the 23 chromosome pairs. An experienced geneticist tin can identify each chromosome based on its characteristic banding pattern. In addition to the banding patterns, chromosomes are further identified on the basis of size and centromere location. To obtain the classic depiction of the karyotype in which homologous pairs of chromosomes are aligned in numerical order from longest to shortest, the geneticist obtains a digital prototype, identifies each chromosome, and manually arranges the chromosomes into this pattern.

Figure \(\PageIndex{1}\): A human karyotype: This karyotype is of a male homo. Notice that homologous chromosomes are the same size, and accept the same centromere positions and banding patterns. A human female would accept an XX chromosome pair instead of the XY pair shown.

At its nearly basic, the karyotype may reveal genetic abnormalities in which an private has as well many or too few chromosomes per cell. Examples of this are Down Syndrome, which is identified past a third re-create of chromosome 21, and Turner Syndrome, which is characterized by the presence of only one X chromosome in women instead of the normal ii. Geneticists tin too identify large deletions or insertions of DNA. For case, Jacobsen Syndrome, which involves distinctive facial features too as center and haemorrhage defects, is identified by a deletion on chromosome eleven. Finally, the karyotype can pinpoint translocations, which occur when a segment of genetic textile breaks from ane chromosome and reattaches to another chromosome or to a dissimilar office of the same chromosome. Translocations are implicated in certain cancers, including chronic myelogenous leukemia.

During Mendel's lifetime, inheritance was an abstruse concept that could only be inferred past performing crosses and observing the traits expressed by offspring. By observing a karyotype, today's geneticists can actually visualize the chromosomal composition of an individual to confirm or predict genetic abnormalities in offspring, fifty-fifty earlier nascency.

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