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1998 Jason Anderson, Chris Wedge & Ania Zaremba.  Send your comments to 18258@advanced.org

 

Genetic Analysis:  Karyotypes and Pedigrees

What's my Karyotype?   I drive a Chevy Van, thank you very much.

No, no, no... a karyotype does not refer to the type of car you drive, but to your Define... chromosomes instead.  A karyotype shows the pairs of Define... metaphase chromosomes of an individual cell, arranged in pairs and sorted according to size.   That's all!  Below are standard karyotypes:   

Male:   46 XY
Male Karyotype: 46 XY

 

Female:   46 XX
Female Karyotype: 46 XY

There is a specific way to label a karyotype.  A human has 23 sets of chromosomes, and thus, a total of 46 chromosomes.  Twenty-two of these pairs are autosomal chromosomes, and the last pair is comprised of the sex chromosomes, which don't have to be identical.  A karyotype is labeled in the following fashion:   number of chromosomes, sex chromosomes (by listing XX for female, or XY for male), any genetic malfunctions.  For example, a normal male would be 46, XY, and a normal female would be 46, XX.

Analyzing a Human Karyotype

The first thing to do is to count the chromosomes.  If you have the standard number of chromosomes for a human, 46, you aren't missing any chromosomes, nor have any extras.  Then, group the pairs of Define... homologous chromosomes.   If you are analyzing a male karyotype, the last set of chromosomes (the sex chromosomes), should not match, since males have an X and Y chromosome.  After pairing the chromosomes, look for sets that do not have two chromosomes:  there's your problem!

Having too many or too few chromosomes is a result of Define... nondisjunctionDuring the process of meiosis, which is a type of cell division that produces sex cells (sperm and egg), each sex cell normally receives one of each chromosome.   When an egg and sperm fuse in fertilization, the resulting zygote has the two copies of each chromosome, the normal number for proper development. Sometimes during meiosis, chromosomes are not correctly distributed into the two daughter cells: this is called nondisjunction. When this happens, one daughter cell has twice the number of copies of that particular chromosome, while the other daughter cell is left with no copies.  If these cells later are fertilized, the zygote will have either a total of one of that type of chromosome (monosomy) or have three of that type of chromosome (trisomy). Generally, humans can tolerate excess genetic material more easily than it can tolerate missing genetic information, but there may be severe side effects, as seen in Down's syndrome.
MutationsHere's a brief outline of common malformations:

  • Not Enough Chromosomes
    • Turner’s Syndrome: 45, X.  The individual is missing another X or Y chromosome.  The person develops as a girl, is sterile, small in stature, and may have some major and minor malformations.
  • Too Many Chromosomes
    • Klinefelter’s Syndrome: 47, XXY.  The individual has two or more X chromosomes, as well as a Y chromosome.  The person develops as a male and has subtle physical defects.  Usually, a Y Chromosome defines a male's outward appearance, no matter how many X chromosomes one may have.  Below is a karyotype showing someone with Klinefelter's Syndrome.  Please note the extra X chromosome:

 

Klinefelter's Syndrome:   47 XXY
Kleinfelter's Syndrome: 47XXY
    • Down's Syndrome:  47, T21.  This defect is caused by trisomy 21, which means having 3 chromosomes #21.  It occurs in one in 800 births; symptoms include mental retardation, congenital organ defects, respiratory infections, leukemia, and a Mongoloid appearance.  Below is a karyotype for someone with Down's Syndrome.   Note the extra 21st chromosome:
Down's Syndrome:   47 XY, t21
Down's Syndrome: 47 XY, t21
  • Translocation diagramMixed up pieces
    • These are called translocations, and involve pieces of non-homologous chromosomes swapping locations. They are the most common chromosomal abnormalities in humans—1 in 500 people have a translocation.  See diagram to the right:
    • Robertsonian translocations involve the acrocentric (chromosomes with the centromeres very near to the end) chromosomes (13, 14, 15, 21, and 22), and involves the entire chromosomes fusing together so that you are down one chromosome
    • Philadelphia Chromosome: A translocation from chromosome 22 to the long arm of chromosome 9.  This can lead to cancer and the abl gene is mutated
  • Missing Pieces or Deletions
    • Cri-du-chat Syndrome: "Cry of the Cat" Syndrome;  This is a terminal deletion: a part of the short arm near the end of chromosome 5 is deleted.  The name of this deletion refers to cry of children who have this defect; they suffer from mental retardation, a shortened life span, and a distinctive facial appearance.  It occurs one in 50,000 births.
    • Williams Syndrome: this deletion is too small to be seen in a karyotype.  You must use Fluorescence in Situ Hybridization (FISH) to find the deletion.   This process works by labeling the gene in question with a fluorescent chemical (called probing).  The elastin gene on chromosome 7 is deleted in this interstitial deletion (a deletion in the middle of the chromosome).  This mutation brings about heart disease, since elastin is the protein that contributes to the elasticity in blood vessels.
  • Other errors
    • Fragile X syndrome: Look for a break, or weakness, on the long arm of the X chromosome.  This causes mental retardation.  It is a sex-linked factor, so more boys than girls are affected by fragile X.  Since males get their X chromosome from their mother, t is not transmitted from father to son.

Learn more about another important tool used in genetic analysis: pedigrees!

DNADNADNA

 

Permissions

1.  Normal Female 46 XX Karyotype at http://library.advanced.org/18258/noframes/ped-karyo2.htm from Karyotype Analysis http://raven.umnh.utah.edu/units/karyotyping/index.html, Ellen Wilson, wilson@genetics.utah.edu , August 18, 1998.  Email message.

2.  Normal Male 46 XY Karyotype at http://library.advanced.org/18258/noframes/ped-karyo2.htm from Karyotype Analysis http://raven.umnh.utah.edu/units/karyotyping/index.html, Ellen Wilson, wilson@genetics.utah.edu , August 18, 1998.  Email message.

3.  Klinefelter's Syndrome 47 XXY Karyotype at http://library.advanced.org/18258/noframes/ped-karyo2.htm from Karyotype Analysis http://raven.umnh.utah.edu/units/karyotyping/index.html, Ellen Wilson, wilson@genetics.utah.edu , August 18, 1998.  Email message.

4.  Down's Syndrome 47 XY, t21 Karyotype at http://library.advanced.org/18258/noframes/ped-karyo2.htm from Karyotype Analysis http://raven.umnh.utah.edu/units/karyotyping/index.html, Ellen Wilson, wilson@genetics.utah.edu , August 18, 1998.  Email message.

References

1.   The University of Utah School of Medicine, The Eccles Institute of Human Genetics, and The Utah Museum of Natural History.  "Karyotype Analysis."  1996-1997.  < http://raven.umnh.utah.edu/units/karyotyping/index.html >

2. Magloire, Kim.  Cracking the AP Biology.  New York: Random House, Inc., 1997.

3. Curtis, Helena and Barnes, N. Sue.  Biology.  New York: Worth Publishers, Inc., 1989.