Genetic engineering is the process of transferring genes between species. The aim of this process is to transfer desirable traits from one organism to another. For example a gene from an Arctic fish has been transferred to a tomato to try to make it more frost resistant. In Australia, a part of a human gene has been put in a pig to make the it grow larger with more lean meat. In practice there are side effects, the pigs end up with arthritis and tomato crops have failed to perform.
Side effects arise because genes function in a coordinated manner with other genes.
The popular press promotes a picture of genes as separate entities in a chain each performing an individual task. This picture is misleading. In actuality, there are 'families' of genes that work together. Families in turn work in a coordinated manner within the whole genome involving hundreds of thousands of genes. Other cellular components also play a role in controlling the expression of genes.