Flash cloning

Flash Cloning is a medical process used by the UNSC whereby human body parts are cloned. This usually involved a sample of a person's DNA which is used to grow a clone of an organ or body part, which can then be transplanted into the person the DNA came from. This way, organ transplants are faster and easier and the rate of success is far higher. Usually the organ is programmed to grow at an accelerated rate then cease rapid development when transplanted into the person. That way, the organ grows quickly.

Human Cloning
A flash clone is a quick and incomplete clone. An embryo is taken and developed a hundred times faster than it would have naturally, becoming something different than it would have been. Many anomalies appear because flash clones are forced to develop too quickly. Flash clones have none of the memories of their hosts, including the natural behavioral training most humans get observing people while growing up. They can be trained with intensive therapy, but after a month or two, they start to degenerate from metabolic instability in a process called metabolic cascade failure until they die from various neurological diseases.

After the future SPARTAN-IIs were kidnapped from their homes, they were replaced by flash clones. Their purpose is to keep parents from looking for their children, as this would be most troublesome for the SPARTAN-II project and the UNSC.