As we understand genetics better the possibility of altering or engineering the genetics of ourselves or, more easily, our children, has moved out of the realm of science fiction into a real possibility. There are a number of different types of genetic engineering, from the most basic which humans have practiced on plants and animals for millennia, to genetic screening, which is becoming more common even among humans, to splicing, which is common among plants and limited in other areas primarily due to concerns of morality.
The most basic type of genetic engineering is that of selective breeding. Humans began doing this long before the idea of genetics was understood. Animals that have desirable traits are allowed to have offspring while those who do not are not allowed. Every breed of dog, cow, horse and other domesticated animals are genetically engineered by humans through this form of artificial selection, often creating animals which would be far less able to survive in the wild.
While artificial selection is genetic engineering in the strict sense of the world, it is so common that it is not generally what people mean by genetic engineering. This leads to the more controversial questions of gene splicing in general and genetically altered foods specifically. In the case of gene splicing, a scientist will take the genes from one type of plant or animal and insert them in another. The first of these created plants which were resistant to herbicides, allowing farmers to kill weeds without killing their crop. Among the questions that this has created are those of the effects of genetically altered food on humans, the migration of the newly spliced genes into other plants and potentially even the weeds that are designed to be easier to kill.
For humans the more likely form of genetic engineering is going to come, at least at first, in the form of genetic screening. Genetic screening involves a scientist discovering genes in sperm that would pass on a negative trait and remove that sperm from the fertilization process. This is meant to be used to help those with genetic disorders or genes that make specific disorders likely from passing them on to their children. For example, there is a set of genes which is found in nearly everyone who gets Alzheimer’s disease, which can potentially be screened out. The potential danger is that nearly anything could be screened out, including things like brown eyes, male pattern baldness or light skin.
In the future, genetic engineering is likely to grow even more specific as we understand genes better. It is possible that you could create plants, animals or even humans with any traits you wanted. A tail on a person would likely not be difficult, but it is unclear whether a third arm or gills would be any more difficult.
Genetic engineering, at its most basic, is the alteration of the DNA of a living creature to make it into something else. It has generally been done slowly and through the more natural processes of birth, but even now humans are speeding up that process. The more that the process is sped up, the more questions it will lead to and the more accurately it can be called genetic engineering.