C. Gametes are the sex cells, and each (sperm and ovum) contain half of the parent's genetic material. These cells will fuse to produce a zygote, which will usually contain the full number of genes required by an organism to live. If there is a chromosomal abnormality, such as a gamete with an extra chromosome, the offspring can either die in development or sometimes be born with physiological and/or developmental difficulties. An example of this phenomenon is trisomy 21, also known as Down Syndrome, in which the 21st chromosome bears 3 copies instead of the regular 2.