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By the end of this section, you will be able to:
  • Describe how a karyogram is created
  • Explain how nondisjunction leads to disorders in chromosome number
  • Compare disorders caused by aneuploidy
  • Describe how errors in chromosome structure occur through inversions and translocations

Inherited disorders can arise when chromosomes behave abnormally during meiosis. Chromosome disorders can be divided into two categories: abnormalities in chromosome number and chromosomal structural rearrangements. Because even small segments of chromosomes can span many genes, chromosomal disorders are characteristically dramatic and often fatal.

Identification of chromosomes

The isolation and microscopic observation of chromosomes forms the basis of cytogenetics and is the primary method by which clinicians detect chromosomal abnormalities in humans. A karyotype    is the number and appearance of chromosomes, and includes their length, banding pattern, and centromere position. To obtain a view of an individual’s karyotype, cytologists photograph the chromosomes and then cut and paste each chromosome into a chart, or karyogram    , also known as an ideogram ( [link] ).

This is a karyotype of a human female. There are 22 homologous pairs of chromosomes and an X chromosome.
This karyotype is of a female human. Notice that homologous chromosomes are the same size, and have the same centromere positions and banding patterns. A human male would have an XY chromosome pair instead of the XX pair shown. (credit: Andreas Blozer et al)

In a given species, chromosomes can be identified by their number, size, centromere position, and banding pattern. In a human karyotype, autosomes    or “body chromosomes” (all of the non–sex chromosomes) are generally organized in approximate order of size from largest (chromosome 1) to smallest (chromosome 22). The X and Y chromosomes are not autosomes. However, chromosome 21 is actually shorter than chromosome 22. This was discovered after the naming of Down syndrome as trisomy 21, reflecting how this disease results from possessing one extra chromosome 21 (three total). Not wanting to change the name of this important disease, chromosome 21 retained its numbering, despite describing the shortest set of chromosomes. The chromosome “arms” projecting from either end of the centromere may be designated as 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 system, locations on chromosomes can be described consistently in the scientific literature.

Career connection

Geneticists use karyograms to identify chromosomal aberrations

Although Mendel is referred to as the “father 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 individual’s karyotype, a person’s cells (like white blood cells) are first collected from a blood sample or other tissue. 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 400–800 bands (of tightly coiled DNA and condensed proteins) arranged along all of the 23 chromosome pairs; an experienced geneticist can identify each band. 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 image, identifies each chromosome, and manually arranges the chromosomes into this pattern ( [link] ).

At its most basic, the karyogram may reveal genetic abnormalities in which an individual has too many or too few chromosomes per cell. Examples of this are Down Syndrome, which is identified by a third copy of chromosome 21, and Turner Syndrome, which is characterized by the presence of only one X chromosome in women instead of the normal two. Geneticists can also identify large deletions or insertions of DNA. For instance, Jacobsen Syndrome—which involves distinctive facial features as well as heart and bleeding defects—is identified by a deletion on chromosome 11. Finally, the karyotype can pinpoint translocations , which occur when a segment of genetic material breaks from one chromosome and reattaches to another chromosome or to a different part of the same chromosome. Translocations are implicated in certain cancers, including chronic myelogenous leukemia.

During Mendel’s lifetime, inheritance was an abstract concept that could only be inferred by performing crosses and observing the traits expressed by offspring. By observing a karyogram, today’s geneticists can actually visualize the chromosomal composition of an individual to confirm or predict genetic abnormalities in offspring, even before birth.

Questions & Answers

why drinking excess alcohol causes thirst and dehydration
uwikuzo Reply
what is reproduction
smart Reply
it is d act of bringing young ones to life
to ensure survival of a species🚴‍♀️
what is a genotype
what is hazardous
a cell is the smallest unit of a living thing. so we all have cell
It is the formation of a zygote resulting from the fusion of the sperm cell with the ovum.Thus,this results in the production of new species which are genetically dissimilar from their parent cells.
what is size of cell
Mohd Reply
what is size of Hart
nanometers=um sign thingie
monomers and polymers of nucleic acids?
Jyrl Reply
dna and rna involvement
give me the elements of the soil
Iguma Reply
Air, water, organic matter, inorganic matter
soil water humus air
silica, iron
potassium, sulfur, calcium, carbon
what is cell
iyaji Reply
A cell is a smallest fundamental unit of a living organisms.
the basic structural and functional unit of life
what is size of cell
all things are made up of.....all things cannot exist without pre-exisiting cells...check out the 16th ce tury to learn more about microscope use and cells. i will give a hint: Mr. L.
we are all made of cells
Nutrition - sensitive intervention
Therowda Reply
what does the sori In fern mean
arhin Reply
biology is the study .exactly what is life?
i'm sorry , the study of life
how can U identify a person through his blood
Frankyx Reply
through genetic fingerprinting where specific DNA sequences in a person genome can be identified.
by help of genetics and DNA test
can it also be detected using an RNA test
environmental biology
Ojesola Reply
what is phytoplankton
What is anabolism
Treasure Reply
the break down of substance
how many teeth has an adult person
32 teeth
32 (or 36 including the wisdom teeth)
what is homoestastis
Ayo Reply
hypothalamus negative feedback vs. postive feedback systems.
it is the maintenance of a steady internal environment.it is controlled largely by the brain especially the hypothalamus.
What is a cell
Eric Reply
A cell is the building block of all organisms
yes it is , it also helps in functions .
cell is structural & functional unit of oraganisms
it is a building blocks of a living organism
it is the basic unit of life in living organisms
it is the basic unite of life in living organisms
it is the fundamental unit of life
it is the structural and functional unit of life
Why cells are classify as living and non living things
Millicent Reply
due to such as virus which acts as a living cell inside the cell of an organisms and out side of cell in form of crystals
because there are living cells which performs complex processes such as reproduction and metabolism then there are dead cells like the top most part of the epidermis or the bark
because there are living cells which performs complex function such as reproduction and metabolism
cause otherwise you could call fire alive

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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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