The genetic engineering of humans is not yet a reality. But, with advancements in gene therapy and cloning, it will be and it is critical that Catholics be ahead of the rhetorical curve on this one, instead of behind.
Now is the time to look at the genetic engineering of humans and what the Church says on the issue. Now is the time to understand what we as Catholics can embrace and what we should reject.
First, under the umbrella of “genetic engineering” we must make a strong distinction between gene therapy and genetic enhancement. These concepts are often confused and lumped together, but there are important moral differences.
For many years scientists have envisioned using gene therapy to cure devastating disease. Gene therapy would deliver a copy of a normal gene into the cells of a patient with defective genes to cure or slow the progress of disease. The added gene would produce a protein that is missing or defective in the diseased patient. A good example would be Duchenne Muscular Dystrophy  or DMD. DMD is an inherited disorder where a patient cannot make the protein dystrophin which supports muscle tissue. DMD strikes in early childhood and slowly degrades all muscle tissue, including heart muscle. Average life expectancy is only 30 years.
Researchers have recently been able to introduce the normal gene for dystrophin in mice with DMD. They achieved this by inserting the dystrophin gene into the DNA of the mice. The genetically modified mice were then able to produce eight times more dystrophin  than DMD-mice without the modification.
More dystrophin means more muscle which, in this case of a devastating muscle-wasting disease, is good. But apply this technology to a normal man who wants more muscle to improve his athletic ability, and you have entered the world of genetic enhancement. Genetic enhancement would take a otherwise normal individual and genetically modify them to be more than human in intelligence, strength or beauty.
Both are technically genetic engineering, but they have different intent and very different outcomes. Gene therapy seeks to cure disease. Genetic enhancement seeks to change the very nature of man: to make him “super-human.”
Those that that look forward to an age of human genetic enhancement are transhumanists. Humanity Plus  is a transhumanist organization that wants everyone to “enjoy better minds, better bodies and better lives” and be “better than well.”
It is with this distinction between genetic engineering as therapy and genetic engineering as enhancement that we must approach the advent of this technology.
Confusion on this issue is common. For example, in his piece “The Vanishing Republican”  in the New York Times, David Frum writes:
It is probable that the trend to inequality will grow even stronger in the years ahead, if new genetic techniques offer those with sufficient resources the possibility of enhancing the intelligence, health, beauty and strength of children in the womb. How should conservatives respond to such new technologies? The anti-abortion instincts of many conservatives naturally incline them to look at such techniques with suspicion — and indeed it is certainly easy to imagine how they might be abused. Yet in an important address delivered as long ago as 1983, Pope John Paul II argued that genetic enhancement was permissible — indeed, laudable — even from a Catholic point of view, as long as it met certain basic moral rules. Among those rules: that these therapies be available to all. Ensuring equality of care may become inseparable from ensuring equality of opportunity.
Frum believes that the Catholic Church would find genetic enhancement “laudable” as long as it was available to everyone. Is that true? Is that what the Church is really saying regarding genetic engineering?
Let us take a closer look. From Donum Vitae :
A strictly therapeutic intervention whose explicit objective is the healing of various maladies such as those stemming from chromosomal defects will, in principle, be considered desirable, provided it is directed to the true promotion of the personal well-being of the individual without doing harm to his integrity or worsening his conditions of life. Such an intervention would indeed fall within the logic of the Christian moral tradition.
This passage clearly says that genetic engineering as a “strictly therapeutic intervention” is moral. So gene therapy is acceptable. But is genetic enhancement? This passage from the Charter for Health Care Workers  sheds some light on genetic enhancement:
“In moral evaluation a distinction must be made between strictly <therapeutic> manipulation, which aims to cure illnesses caused by genetic or chromosome anomalies (genetic therapy), from manipulation <altering> the human genetic patrimony. A curative intervention, which is also called “genetic surgery,” “will be considered desirable in principle. provided its purpose is the real promotion of the personal well-being of the individual, without damaging his integrity or worsening his condition of life.
On the other hand, interventions which are not directly curative, the purpose of which is ‘the production of human beings selected according to sex or other predetermined qualities,’ which change the genotype of the individual and of the human species, ‘are contrary to the personal dignity of the human being, to his integrity and to his identity. Therefore they can be in no way justified on the pretext that they will produce some beneficial results for humanity in the future,’ ‘no social or scientific usefulness and no ideological purpose could ever justify an intervention on the human genome unless it be therapeutic, that is its finality must be the natural development of the human being.“
Once again this passage clearly states that gene therapy is morally acceptable, but it distinguishes between therapeutic manipulation and manipulation that simply alters the human genome for purposes other than curing genetic disease. Gene therapy is a good, while genetic enhancement is morally troubled.
So David Frum is wrong. The Catholic Church would never embrace genetic enhancement even if it was available to everyone.
The Church also makes a distinction between gene therapy that is done at the somatic level, or just in the diseased cells, and gene therapy that changes the genetics of the person so that it is a germ-line, or inheritable change. A germ-line modification would occur at the level of egg or sperm so that it would be passed on to future generations. Gene therapy at the tissue or organ level is acceptable. Gene therapy that changes the sperm or egg, forcing the change on offspring is not currently ethical. From the Instruction Dignitas Personae on Certain Bioethical Questions: 
In theory, it is possible to use gene therapy on two levels: somatic cell gene therapy and germ line cell therapy. Somatic cell gene therapy seeks to eliminate or reduce genetic defects on the level of somatic cells, that is, cells other than the reproductive cells, but which make up the tissue and organs of the body. It involves procedures aimed at certain individual cells with effects that are limited to a single person. Germ line cell therapy aims instead at correcting genetic defects present in germ line cells with the purpose of transmitting the therapeutic effects to the offspring of the individual….
Procedures used on somatic cells for strictly therapeutic purposes are in principle morally licit. Such actions seek to restore the normal genetic configuration of the patient or to counter damage caused by genetic anomalies or those related to other pathologies….
The moral evaluation of germ line cell therapy is different. Whatever genetic modifications are effected on the germ cells of a person will be transmitted to any potential offspring. Because the risks connected to any genetic manipulation are considerable and as yet not fully controllable, in the present state of research, it is not morally permissible to act in a way that may cause possible harm to the resulting progeny.
Because the effects of a germ line modification, even if it was for a therapeutic reason, could cause unforeseeable and permanent side effects, in the current state of genetic engineering research, a germ-line modification is unethical. In the future, if the technology can prove to be safe and can be done without creating or manipulating human life in a dish, then germ-line gene therapy may be ethical.
A germ-line modification for genetic enhancement reasons would be genetic engineering at its worst. Not only would the child have no choice in being genetically modified, but also the modification would extend to his or her eggs or sperm. The modification would be “permanent.” These genetically modified human beings would have no choice but to pass the enhancement for intelligence or beauty or strength on to their children and their children’s children. Germ-line genetic enhancement would force upon future generations the genetic preferences of the previous. This is exactly what the Church is referring to when it talks about manipulating the “the human genetic patrimony.”
Many people argue that genetically enhancing a child is no different than getting them the best tutors or making them go to piano lessons. Genetic enhancement, especially a germ-line enhancement is nothing like piano lessons. When a child turns 18, she can stop playing piano if she so chooses. Not so with a genetic enhancement. Genetic enhancement will force her to live with her parents choices for her whole life. If it is a germ-line enhancement, parents will also be forcing their choice on their grandchildren and great grandchildren. Germ-line genetic enhancement of children would be like forcing them and their children, and their grandchildren, into piano lessons for their entire lives.
Some would argue there is only a hair’s difference between gene therapy and genetic enhancement and with one will come the other. We must fight this thinking for two reasons. One, because genetic engineering will no doubt have unintended consequences and unforeseen side effects. It should only be under taken in cases where the benefits will outweigh the risks, as in the treatment of life-threatening illness. Genetic engineering should never be used on an otherwise healthy person because the risk is not worth the so-called “reward.”
Secondly, because it is important to embrace ethical technology whenever we can. Catholics often knee-jerk against any biotechnology painting it all as unethical. This not only ignorant, but it allows others to label us as uncompassionate Luddites. We must make a distinction between gene therapy and genetic enhancement so we can reap the rewards of genetic research while rejecting the push to fundamentally change humanity.