Duplications occur when part of a chromosome is copied (duplicated) too many times. This type of chromosomal change results in extra copies of genetic material from the duplicated segment.

1) Single Sperm Cell - A single sperm cell (length is about a third the diameter of the egg). 2) Unfertilized Human egg Of the approximately 300 million sperm cells released in an ejaculation, only 1% will reach the egg and only a single sperm will penetrate the protective layers and successfully fertilize the egg. After the union of sperm and egg, the fusion of genetic material takes place. The fertilized egg, now called a zygote, then divides into two cells after about 30 hours and four cells after 40 hours. As it divides, it is slowly carried down the fallopian tube. When it reaches the 16-cell stage, it is called a morula, and approximately 72 hours after fertilization, it reaches the uterus.

There are 23 pairs of homologous chromosomes in a female human somatic cell. The condensed chromosomes are viewed within the nucleus (top), removed from a cell in mitosis and spread out on a slide (right), and artificially arranged according to length (left); an arrangement like this is called a karyotype. In this image, the chromosomes were exposed to fluorescent stains for differentiation of the different chromosomes. A method of staining called “chromosome painting” employs fluorescent dyes that highlight chromosomes in different colors. (credit: National Human Genome Project/NIH)