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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

what is metabolism
Mlungisi Reply
pls what is the chemical symbol for Methane
Afia Reply
CH4
Francis
ch4
Paolo
what is life?
Josephus
What do you think is life?
Isala
Hi
Isaac
hey
Isala
what is DNA
Machuol
all living things have certain characteristics in common which are referred to as dash
IBUKUN Reply
classifying living things in the world in two major groups
IBUKUN
mention the seven characteristics that distinguish living things from nonliving things
IBUKUN
which of the characteristics of living things involved taking in the use of seed by animals as well as the taking in of mineral sentences and their uses by plants
IBUKUN
Life process
Afia
Growth, Respiration, Excretion, Movement, Sensitivity, Nutrition,and Reproductive
Afia
Nutrition
Afia
nutrition
Lawrence
Mendel experiment when years ago in work
Biruk Reply
what is the smallest unit in an organism
Neimar Reply
what is endoplasmic
Fatou Reply
If Jane was in room(B) while her mother Stella was in room (Y). Jane was cooking fish, her mother came to smell the good scent, By what process did her mother came to smell the scent
Neimar Reply
Diffusion
Afia
Is it a serious question?
Ehsan Reply
what's the question
Joseph
how many days do a bean seed take to germinate
Nando
no idea
Afia
what is DNA
Yahaya Reply
genetic information
MG
Deoxyribonucliec acid (DNA) is the cell's hereditary material that contains instructions for growth, development and reproduction.
Joseph
ok
oly
what's different between sex and gender
oly
Are there differences between sex and gender?
Theo
lol
Andrew
hi Yahaya its Deoxyribonucleic acid
Neimar
describe an experiment to show that plants require light for photosynthesis
Diyara Reply
uuh... putting a plant in a dark closet... and another in a light enviroment?
Anastasiya
wow! awesome explanation Anastasiya.
Joseph
please can you explain why air is homogenous
Joseph
Because each layer of the Earth's atmosphere has a different density, each layer of air is it's own homogenous.
MG
tell me about big bang
Mustafa
what's DNA
Mustafa
What's an amoeba
Bigger Reply
An amoeba is a cell or an organism that has the ability to ulter it's shape.
Joseph
An amoeba has an irregular shape and it changes constantly,it's a unicellular organism belong to a group called protists it has a pseudopodia used for locomotion and ingestion...
Emmanuel
amoeba is an organism that has an inregular shape which changes constantly
Cashizz
amoeba is an unicellular organism that uses pseudopodia,it does not have a constant shape
Alohan
what is the difference between DNA and RNA
Alohan
DNA with oxygenated but RNA without oxygenated
qax
what is the mode of nutrition of fungi
Survive
asexual
qax
amoeba are protozoa with one cell and no fixed shape
James
heterotrophic and outrotrophic
qax
what is the between arteries and capillary
qax
arteries>arterioles>capillaries. decrease in size and thus pressure
Anastasiya
DNA deossoribonucleic acid. RNA ribonucleic acid. The difference between the two lies in a lack of one oxygen on the sugar in DNA. Also: in DNA the azotate bases are Guanine, Citosine, Adenine and TIMINE base, the latter is replaced by URACILE in RNA. DNA formes a double helix structure...
Anastasiya
... while RNA form is usually a single stand line, but it can form loops etc
Anastasiya
what is a zygote?
Darius Reply
zygote is a eukaryotic cell formed by a fertilization event between two gametes. The zygote's genome is a combination of the DNA in each gamete, and contains all of the genetic information necessary to form a new individual.
MG
A zygote is an organism formed after fertilization
Neimar
how do plants form a zygote
Paclina Reply
What is zygote
Van
what is zygote
Darius
Zygote is located inside the ovule, which is present in the ovary.
MG
zygote is a eukaryotic cell formed by a fertilization event between two gametes. The zygote's genome is a combination of the DNA in each gamete, and contains all of the genetic information necessary to form a new individual.
MG
Okay
Van
what is biology?
Aadan Reply
biology is the study of living n non living organism
Kelsia
what is procotist?
Kelsia
Biology is the branch of science that deals with the study of living and non living things
Neimar
Describe at least two major changes to the animal phylogenetic tree that have come about due to molecular or genetic findings.
Tamala Reply

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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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