General ---------Biology----------------------------------------------------- ------------------Online--------------------------------------- Ch. 13 Meiosis and Sexual Life Cycles

A karyotype is a photograph of an individuals chromosomes.  The methods box in Ch. 13 describes how karyotypes are prepared. 

Basically, a cell is interrupted during mitosis and the contents spread out on a slide and stained.  This slide is called a chromosome smear.  The figure below is a photograph of a chromosome smear.  These images are from the University of Texas Web site:http://www.ece.utexas.edu/~weston/Jonas3.html#1

To make a karyotype, you first identify homologous chromosomes, i.e. chromosomes pairs .  Each member of the pair, each homologue, has  the same the same shape and staining pattern. Once you have identified a homologous pair, you cut each member of the pair out of the photograph with a pair of scissors, and lay them side by side on a sheet of paper.    You repeat this process until you have each chromosome cut out and paired.  Then you photograph your chromosome collage, and you have a karyotype.  See the image below.

A karyotype allows inspection of the chromosomes for abnormalities, most of which cause a disease. 

Metaphase chromosomes stained red and black and white karyotype below.

Genetic Disease.  In addition to abnormalities resulting from too many, to few, or misshapen chromosomes, the actual gene nucleotide sequence (ATTCGTT....etc.) can contain an error that causes the ribosome to build a protein that does not function appropriately.   There are many diseases caused by chromosome abnormalities.

Down syndrome is one of the most common.  It is caused by an error during meiosis that results in one of the gametes produced having two chromosomes of pair number 22.  When this gamete joins with another gamete in fertilization, the resulting zygote will have three of chromosome 22.