A designer baby is a baby genetically modified in its embryonic stage of development or before to that. It is done before implanting the baby into the mother's womb by the scientists/embryologists. The purpose of this is to create babies that can be free of some undesired genetic traits such as diseases or specifically to provide the parent with the physical traits they desire in their kid, including its sex.
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This is not to be mistaken with Eugenics which is supposedly a more natural way of matching individuals so they could produce offspring with desired qualities. In the case of designer baby, genetic engineering is involved. Much like modifying James "Bucky" Barnes into becoming Captain America (cool right?), well, there are no evidences that a fully grown man's genes could be modified to become a super soldier but what we are saying is that, it is quite possible to set the traits in motion before he is even a fully developed baby in Mrs Barnes womb. Get the picture?
In a very basic term, evolution teaches that in order to adapt to their environments, organisms (biological cells) change. They become new organisms, similar but entirely different from the previous one. It is worthy to note that evolution is a slow process taking millions (sometimes, hundred millions) of years.
So scientists have finally found a way to by-pass the millions of years that could possibly allow them to provide us with humans with some nice traits (not just humans becoming 9ft average height but sure we could have that) but picture this: A baby whose sex is chosen as a female by the parents, who would not inherit a cancer-gene, asthma, Alzheimer, not prone to HIV, super intelligent as a child, without the parents having to clean her nose time and time again like a regular kid, perfect (or whatever is defined by the parent as that) facial features with no need for plastic surgery.
The way I see it; being a fan of technology, sci-fi and comic, I think it is a brilliant technology. As there is the many good side, sadly, there is going to be a reverse side, which means the it could be used to achieve the opposite. This means that with this kind of ability, the elites could decide to create a dystopian society for all. Only thing that comes to my mind is the the divergent series (book and movie).
Note: I am not an expert, this is only my opinion after browsing a few random pages on the topic. lol.
Interesting Read: https://www.newscientist.com/article/2143534-crispr-makes-piglets-that-may-be-better-organ-donors-for-humans/