Cytogenetics

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Each human nucleated cell contains 46 chromosomes (22 pairs of homologous chromosomes and a pair of sex chromosomes).  Mature germ cells (sperms and eggs) are the only exception. They contain 23 chromosomes, i.e. only one of each chromosome. 

DNA, our genetic blue-print, and various proteins make up each chromosome.

Chromosomes can be visualized during cell division (metaphase), stained, and evaluated under a microscope. This facilitates analysis of number and gross structure of chromosomes.  The chromosome complement is called karyotype. The karyotype is depicted as karyogram.

 
                
A: Karyogram of a                                           B: Karyogram of a normal male (46,XY).
normal female (46,XX). “46” refers                   “46” refers to the number of chromosomes,
to the number of chromosomes                         including a male (Y) and a female (X) sex
including the two female sex                             chromosome
chromosomes (XX).