Download Appendix A: Analyzing Chromosomes through Karyotyping

Appendix A: Analyzing Chromosomes through Karyotyping
Diseases that run in families are called “genetic diseases”. What is the risk of inheriting a genetic disease? Why
do some diseases appear more often in males than in females? Scientists use family histories, called “pedigrees”, as
well as images of chromosomes and molecular studies of DNA, to answer these questions.
Until recently doctors could not tell whether someone had a genetic disease until symptoms appeared. However,
gene-screening techniques have now made it possible to determine whether a person is predisposed to a certain
disease. These tests can also confirm the presence of a specific gene defect or mutation in an individual or a family.
Genetic screening involves examining a person’s DNA in order to detect an abnormality that signals a disease or
disorder. The defect may be extremely small. In some cases, a genetic disease is caused by a change in or deletion of
one nucleotide base. Other genetic disorders are caused by large abnormalities. “Translocations” result from a
segment of DNA detaching from one chromosome and reattaching to another. “Deletions” occur when large
segments of DNA are missing from a chromosome altogether. Another type of large genetic error is known as an
“inversion” Inversions result from a segment of DNA becoming detached from a chromosome, rotating 180 degrees
and reattaching itself to the same chromosome. Still another abnormal condition is a “duplication” in which a
segment of DNA is copied more than once, end to end, resulting in more than one gene of the same type on the same
chromosome.
The normal number of chromosomes in humans is 46, meaning that you have a total of 23 homologous pairs of
chromosomes. You inherited one set of 23 chromosomes from your mother and a corresponding set of 23
chromosomes from your father. Chromosomes 1 through 22 are called "autosomes”, because they have genes which
code for traits other than the sex of the individual. The sex chromosomes (#23) come in two forms, X and Y. In
humans, inheriting two X chromosomes results in a female child while inheriting one X and one Y results in a male.
Other genes are located on the sex chromosomes also, with the X chromosome having more genes than the Y
chromosome. Those traits coded for by genes on the sex chromosomes are called "sex-linked" traits.
Meiosis is the process by which eggs or sperm are produced. In order to keep the chromosome number constant
at 46 from generation to generation, each egg or sperm must contain only 23 chromosomes. At fertilization, the total
number of 46 chromosomes is restored and the embryo has a complete set of genetic instructions from its parents.
During meiosis, chromosome pairs line up and separate into daughter cells. Sometimes, this separation doesn't occur
normally and a daughter cell with either too many or too few chromosomes can result. This process is called "nondisjunction" and can occur during the production of either eggs or sperm. These defective cells may still participate
in fertilization. A daughter cell which receives the extra chromosome copy will end up with a total of 3 copies of that
chromosome after fertilization. This is called a "trisomy”. The most common human trisomy is trisomy 21, which
results in Down syndrome. The incidence of this disorder is approximately 1 in 600 live births. It causes mental
retardation, distinct facial features, and heart defects. The daughter cell receiving no copies of a chromosome during
meiosis will end up with only a single copy of that chromosome following fertilization. This is called a
"monosomy”. Most trisomies and monosomies are lethal to the embryo.
Another common chromosomal disorder is Turner's syndrome. Individuals with this disorder have only one X
chromosome and no Y chromosome. This results from a non-disjunction of the sex chromosomes during meiosis.
The incidence of this disorder is approximately 1 in 5,000 live female births. Because these individuals have no Y
chromosome, they are females. However, they lack functional reproductive organs and have a range of
abnormalities, including short stature, renal and cardiovascular anomalies and lower-than-average IQ.
Predictive gene tests can now identify people at risk for a disease before any symptoms appear. These tests
identify disorders that run in families because a defective gene is passed from one generation to the next. More than
two dozen of these tests are currently available for diseases such as Tay-Sachs, cystic fibrosis and Huntington's.
Scientists are in the process of developing similar tests for Lou Gehrig's disease, some forms of Alzheimer's,
extremely high cholesterol levels, and some cancers, such as colon and breast.
To develop predictive gene tests, scientists study DNA samples from members of families which, over several
generations, have experienced a high incidence of a particular condition. If the gene itself cannot be studied, they
look for easily identifiable segments of DNA, known as "genetic markers” that are consistently inherited by family
members with the disease but are not found in relatives who are disease-free.
An accurate gene test can tell whether an individual has the mutation associated with a particular disease, but it
cannot determine whether the person will actually develop the disease. That's because many factors influence the
gene's expression. Therefore, predictive gene tests deal in probabilities, not certainties. In some cases, people who
know they are predisposed to a particular disease can modify their diet, behavior or lifestyle to decrease their risk of
developing the disease. In other cases, they can undergo regular screening for the disease to aid early treatment. For
instance, many cancers can be successfully treated if they are caught early enough.
The DNA contained in a single chromosome is a long, highly compact chain made up of 50 million to 250
million base pairs. Each chromosome contains several thousand genes. In 1989, scientists from allover the world
began collecting data on the sequence of nucleotide bases in human chromosomes. This effort is known as the
"Human Genome Project”. Their goal is to identify the function and map the location of all human genes. This
knowledge may help pinpoint genes responsible for many serious human diseases and disorders. One possible
benefit of the Human Genome Project is the development of gene therapy to treat certain types of inherited
disorders. Gene therapy involves using retroviruses to insert a normal human gene into a defective chromosome.
In order to identify defective, extra, or missing chromosomes, scientists prepare a karyotype -- a visual
representation of an individual's chromosomes arranged in a specific way. This standard arrangement shows
homologous pairs ordered according to their size, shape, banding pattern and centromere position. Karyotypes have
become increasingly important as more diseases are linked to chromosomal abnormalities.
Scientists who prepare and study karyotypes are called “cytogeneticists”. First, they extract chromosomes from
white blood cells and expose them to a variety of chemicals and stains to make the chromosomes easier to see. Next,
they photograph the cells in the metaphase stage of mitosis (e.g. division) through a microscope. This is done
because the characteristics of chromosomes are easiest to see during metaphase. The cytogeneticists then use
computer equipment to arrange the chromosome spread into a karyotype. The following activity will show you what
cytogeneticists do every day. You will be given an unknown chromosome spread which may be from a normal male,
normal female, someone with Down syndrome or someone with Turner's syndrome.
Source:
Neo/Sci (1999). Neo/Sci student’s guide: Karyotyping preparation and analysis. Retrieved from
http://www.neosci.com/catalog.asp?sid=5256552&content=cn_results&hp=lab.