Karyotypes

The complete set of chromosomes in the cells of an organism is its karyotype. It is most often studied when the cell is at metaphase of mitosis and all the chromosomes are present as dyads.

The karyotype of the human female contains 23 pairs of homologous chromosomes:

• 22 pairs of autosomes
• 1 pair of X chromosomes

The karyotype of the human male contains:

• the same 22 pairs of autosomes
• one X chromosome
• one Y chromosome

(A gene on the Y chromosome designated SRY is the master switch for making a male.)

The X and Y chromosomes are called the sex chromosomes.

Above is a human karyotype (of which sex?). It differs from a normal human karyotype in having an extra #21 dyad. As a result, this individual suffered from a developmental disorder called Down Syndrome. The inheritance of an extra chromosome, is called trisomy, in this case trisomy 21. It is an example of aneuploidy

http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/Chromosomes.html