Flash cloning

Summary
A piece of UNSC technology, a flash clone is a quick, 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 go through time 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 year 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 avoid parents from looking for their children, as this would be most troublesome for the SPARTAN-II project and the UNSC.

Also, if a DNA sample is available, flash cloning can be used to clone replacement organs (or maybe even limbs) that don't suffer from the same problems as complete clones. This was seen when Spartan Kelly received a new lung during the Battle of Reach. The MC's AI Cortana is also a product of Doctor Halsey's flash-cloned brain